In particular, when you write a function that is not a one-off implementation detail, consider. (Not enforceable) Look at the use of standard-library container types (incl. ), Simple code can be very fast. In this case, the control block stores a pointer to the managed object. If you dont, an exception or a return might lead to a leak. can be used for a wide variety of algorithms. When the closure object goes out of scope the captures will also go out of scope. Having the same name for logically different functions is confusing and leads to errors when using generic programming. Occasionally classes will mix A and B, usually for debug reasons. using namespace can lead to name clashes, so it should be used sparingly. Flag multiple using namespace directives for different namespaces in a single source file. You can overload ordinary functions, function templates, and operators. This was primarily to avoid code of the form (a = b) = c such code is not common enough to warrant violating consistency with standard types. #elif. Sign in. To achieve that its copy operations are deleted. For example, unary_function is a bundle-of-typedefs that was never intended to be instantiated standalone. When unambiguous, the T can be left out of T{e}. Make the interface precisely specified and compile-time checkable in the (not so distant) future. Including entities subject to the one-definition rule leads to linkage errors. void f (const shared& s) {} shared sp; f (sp); In this sample the performance is penalized because of the reference counting necessary to convert types. through non-const pointers. Compilers do not read comments. For example: The C++11 initializer list rule eliminates the need for many constructors. Manning Publications. That can cause confusion: An overrider does not inherit default arguments. A header should include all its dependencies. There are environments where extensions are necessary, e.g., to access system resources. To keep code simple and safe. In existing implementations, the number of weak pointers is incremented ([1], [2]) if there is a shared pointer to the same control block. The fundamental aim is to ensure that we dont leak any resources and that we dont hold a resource longer than we need to. If two concepts have exactly the same requirements, they are logically equivalent (there is no refinement). smartptr. side effects: side effects on multiple non-local variables (for some definition of non-local) can be suspect, especially if the side effects are in separate subexpressions, more than N operators (and what should N be? Check that an uninitialized buffer is written into. Consider the following advice and requirements found in the C++ Standard: If a destructor called during stack unwinding exits with an exception, terminate is called (15.5.1). Such problem can often be solved by using a recursive_mutex. This section deals with the cases where you either do not have an efficient implementation of exceptions, Flag statements that are just a temporary. To decrease code size and run time. Because private data members participate in class layout and private member functions participate in overload resolution, changes to those (Simple) Warn if a unique_ptr is constructed from the result of new rather than make_unique. The Microsoft compilers C++ code analysis contains a set of rules specifically aimed at enforcement of the C++ Core Guidelines. This needlessly adds and maintains a reference count. are no longer protected by an enforced invariant. Some people found the idea that the Link no longer was hidden inside the list scary, so we named the technique Immutable objects are easier to reason about, so make objects non-const only when there is a need to change their value. Minimizing confusion with the declarator syntax. If multiple threads of execution access the same instance of shared_ptr without synchronization and any of those accesses uses a non-const member function of shared_ptr then a data race will occur; the shared_ptr overloads of atomic functions can be used to prevent the data race. In this article. Measure! Any programmer using these guidelines should know the guidelines support library, and use it appropriately. The C-style cast is dangerous because it can do any kind of conversion, depriving us of any protection from mistakes (now or in the future). Its the simplest and gives the cleanest semantics. but if you had seen us - (s + 2) or s += 2; ; us - s, would you reliably have suspected that the result would print as 4294967294? Too often, such claims are based on conjecture or experience with other languages. For example: Dont consider simple variables that are targets for input operations exceptions to this rule: In the not uncommon case where the input target and the input operation get separated (as they should not) the possibility of used-before-set opens up. Any threads spawned to execute the code of read_value are hidden behind That way, the conversion happens while it is still accessible. See also: C.lambdas: Function objects and lambdas. If you define any of the copy constructor, copy assignment operator, or destructor, you probably should define the others. Flag any indexing expression on an expression or variable of array type (either static array or. Pointers should only refer to single objects. The compiler does not read comments, and without reading other code you do not know whether p really points to n elements. It does so by focusing on removing the primary sources of type violations, including unsafe uses of casts and unions. What about a socket that wont close? but beyond that logical constraint is the fact that complex compile-time computation can seriously increase compile times We can of course pass the number of elements along with the pointer: Passing the number of elements as an argument is better (and far more common) than just passing the pointer and relying on some (unstated) convention for knowing or discovering the number of elements. A const member function can modify the value of an object that is mutable or accessed through a pointer member. This would not be any better had exception specifications been statically enforced. Here it is clear that there is a default action and that cases a and b are special. A rule can be part of several profiles, or none. It is not possible to recover from all errors. New derived classes are not added every year and only by library implementers. Having good (easy-to-understand, encouraging efficient use, not error-prone, supporting testing, etc.) Flag a dereference of a pointer that points to an object that has gone out of scope, Flag a dereference of a pointer that might have been invalidated by assigning a, Flag a dereference of a pointer that might have been invalidated by a, Flag a dereference to a pointer to a container element that might have been invalidated by dereference. Warn if it is not obvious whether a thread joins or detaches. You need to pass a pointer rather than an object if what you are transferring is an object from a class hierarchy that is to be used through an interface (base class). It does not necessarily lead to optimization, and it precludes further overriding. If you cannot be systematic about error handling, consider crashing as a response to any error that cannot be handled locally. Generality: using can be used for template aliases, whereas typedefs cant easily be templates. (and most likely more errors). It can be used for conventional I/O, reading and writing to memory (string streams), For example: Such off-by-one switch-statements are often the results of an added enumerator and insufficient testing. time is spent on A or B, see Per.1 and Per.2. The rules for {} initialization are simpler, more general, less ambiguous, and safer than for other forms of initialization. If present in your C library, gets_s(), scanf_s(), and printf_s() might be safer alternatives, but they are still not type safe. Each macro expands to an integer literal corresponding to the year and month when the corresponding feature has been included in the working draft. For example, a pointer might be uninitialized, the nullptr, point beyond the range of an array, or to a deleted object. This is not a language manual. ??? At the time of their release, the guidelines are in a 0.6 state, and contributions are welcome. In many cases, it can be useful to return a specific, user-defined type. Note that we (deliberately) violated the rule against uninitialized variables because this style commonly leads to that. Messing with the loop variable in the body of a loop is typically best avoided. this is an application of the Single Responsibility Principle. The need for consistency beats personal taste. Individual classes are rarely a good logical unit of maintenance and distribution. Postconditions are especially important when they relate to something that is not directly reflected in a returned result, such as a state of a data structure used. Eliminate cycles; dont just break them with #include guards. There is a lot of such code. Compilers already know and sometimes warn. a memory access, Some user-defined and standard library types, such as span or the iterators For example: Now raw_find() can scramble memory to its hearts content. It also speeds up the exit after failure. (Simple) A member initializer list should mention the members in the same order they are declared. is the standard C++ regular expression library. and complicate debugging. Note how >> and != are provided for string (as examples of useful operations) and there are no explicit that are needed to implement key abstractions, such as vector, span, lock_guard, and future, we use the libraries We try to ensure that good examples follow the Core Guidelines. However, it is preferable to have a default constructor default to a meaningful state such as std::strings "" and std::vectors {}. Such code is not uncommon in code written before there were convenient alternative ways of specifying integer constants. See the reference to more specific rules. It also avoids brittle or inefficient workarounds. (defined in namespace std::chrono) provides the notions of time_point and duration together with functions for This doesnt directly express the intent of the programmer and hides the structure of the program from the type system and optimizer. (maybe even by modifying some global state and hope something will notice and be able to take care of the problem). References: [Alexandrescu01] 3, [Boost], [Dewhurst03] 75, [Meyers97] 46, [Stroustrup00] 15.4.3, [Taligent94]. ); such classes tend to have constructors. However, converting code to follow those guidelines could be difficult and even impossible for third-party libraries. Occasionally, for very specific examples, you can get away with setting some state for future cleanup. Macros complicate tool building. You might need to cast away const when calling const-incorrect functions. Dereferencing an invalid pointer, such as nullptr, is undefined behavior, typically leading to immediate crashes, Thats what is expected from pointers. For example, if an owner is a member of a class, that class better have a destructor that deletes it. When combined with the RAII pattern, static local variables can replace the need for writing your own double-checked locking for initialization. Readability. not_null is not just for built-in pointers. all be declared to avoid unwanted effects like turning all potential moves The use of a static_cast is not pretty, but. If you need the notion of an optional value, use a pointer, std::optional, or a special value used to denote no value.. Sometimes, precision comes only with time and experience. If it is accidentally passed by value, with the implicitly generated copy constructor and assignment, we risk slicing: only the base portion of a derived object will be copied, and the polymorphic behavior will be corrupted. All static variables are (as their name indicates) statically allocated, so that pointers to them cannot dangle. Its error-prone and requires expert level knowledge of language features, machine architecture, and data structures. References: See the rules for calling functions. Macros ensure that the human reader sees something different from what the compiler sees. If there is a default constructor, compare those assignments to the initializations in the default constructor. A pure interface class is simply a set of pure virtual functions; see I.25. (Moderate) In the case of self-assignment, a move assignment operator should not leave the object holding pointer members that have been. A rule is aimed at being simple, rather than carefully phrased to mention every alternative and special case. pointer/iterator invalidation leading to dangling pointers: avoid static class members variables (race conditions, almost-global variables), individual shared_ptr objects are not thread-safe: different threads can call non-const member functions on different shared_ptrs that refer to the same shared object, but one thread cannot call a non-const member function of a shared_ptr object while another thread accesses that same shared_ptr object (if you need that, consider atomic_shared_ptr instead), read an explanation of the scope and structure of this Guide, FAQ: Answers to frequently asked questions, In.struct: The structure of this document, P.4: Ideally, a program should be statically type safe, P.5: Prefer compile-time checking to run-time checking, P.6: What cannot be checked at compile time should be checkable at run time, P.10: Prefer immutable data to mutable data, P.11: Encapsulate messy constructs, rather than spreading through the code, P.12: Use supporting tools as appropriate, P.13: Use support libraries as appropriate, I.4: Make interfaces precisely and strongly typed, I.9: If an interface is a template, document its parameters using concepts, I.10: Use exceptions to signal a failure to perform a required task, I.11: Never transfer ownership by a raw pointer (, I.12: Declare a pointer that must not be null as, I.13: Do not pass an array as a single pointer, I.22: Avoid complex initialization of global objects, I.23: Keep the number of function arguments low, I.24: Avoid adjacent parameters that can be invoked by the same arguments in either order with different meaning, I.25: Prefer empty abstract classes as interfaces to class hierarchies, I.26: If you want a cross-compiler ABI, use a C-style subset, I.27: For stable library ABI, consider the Pimpl idiom, C.over: Overloading and overloaded operators, C.con: Containers and other resource handles, F.1: Package meaningful operations as carefully named functions, F.2: A function should perform a single logical operation, F.4: If a function might have to be evaluated at compile time, declare it, F.5: If a function is very small and time-critical, declare it inline, F.6: If your function must not throw, declare it, F.10: If an operation can be reused, give it a name, F.11: Use an unnamed lambda if you need a simple function object in one place only, F.15: Prefer simple and conventional ways of passing information, F.16: For in parameters, pass cheaply-copied types by value and others by reference to, F.17: For in-out parameters, pass by reference to non-, F.18: For will-move-from parameters, pass by, F.20: For out output values, prefer return values to output parameters, F.21: To return multiple out values, prefer returning a struct or tuple, F.43: Never (directly or indirectly) return a pointer or a reference to a local object, F.50: Use a lambda when a function wont do (to capture local variables, or to write a local function), F.51: Where there is a choice, prefer default arguments over overloading, F.52: Prefer capturing by reference in lambdas that will be used locally, including passed to algorithms, F.53: Avoid capturing by reference in lambdas that will be used non-locally, including returned, stored on the heap, or passed to another thread, F.56: Avoid unnecessary condition nesting, treat it as an in/out parameter and pass by reference, if the object referred to should not change, discussion of dangling pointer prevention, C.1: Organize related data into structures (, C.3: Represent the distinction between an interface and an implementation using a class, C.4: Make a function a member only if it needs direct access to the representation of a class, C.5: Place helper functions in the same namespace as the class they support, C.7: Dont define a class or enum and declare a variable of its type in the same statement, C.ctor: Constructors, assignments, and destructors, Prefer to place the interface first in a class, C.10: Prefer concrete types over class hierarchies, C.20: If you can avoid defining any default operations, do, C.30: Define a destructor if a class needs an explicit action at object destruction, C.31: All resources acquired by a class must be released by the classs destructor, C.33: If a class has an owning pointer member, define a destructor, C.35: A base class destructor should be either public and virtual, or protected and non-virtual, C.40: Define a constructor if a class has an invariant, C.41: A constructor should create a fully initialized object, C.42: If a constructor cannot construct a valid object, throw an exception, C.43: Ensure that a copyable class has a default constructor, C.44: Prefer default constructors to be simple and non-throwing, C.45: Dont define a default constructor that only initializes data members; use member initializers instead, C.46: By default, declare single-argument constructors, C.47: Define and initialize member variables in the order of member declaration, C.48: Prefer in-class initializers to member initializers in constructors for constant initializers, C.49: Prefer initialization to assignment in constructors, C.50: Use a factory function if you need virtual behavior during initialization, C.51: Use delegating constructors to represent common actions for all constructors of a class, C.52: Use inheriting constructors to import constructors into a derived class that does not need further explicit initialization, C.62: Make copy assignment safe for self-assignment, C.64: A move operation should move and leave its source in a valid state, C.65: Make move assignment safe for self-assignment, C.67: A polymorphic class should suppress public copy/move, C.82: Dont call virtual functions in constructors and destructors, C.83: For value-like types, consider providing a, C.90: Rely on constructors and assignment operators, not memset and memcpy, not to declare a variable until it can be properly initialized, a more general way to present arguments to a function, destructors, deallocation, and swap must never fail, C.100: Follow the STL when defining a container, C.103: Give a container an initializer list constructor, C.104: Give a container a default constructor that sets it to empty, C.109: If a resource handle has pointer semantics, provide, ES.28: Use lambdas for complex initialization, especially of, C.120: Use class hierarchies to represent concepts with inherent hierarchical structure (only), C.121: If a base class is used as an interface, make it a pure abstract class, C.122: Use abstract classes as interfaces when complete separation of interface and implementation is needed, C.126: An abstract class typically doesnt need a user-written constructor, C.127: A class with a virtual function should have a virtual or protected destructor, C.128: Virtual functions should specify exactly one of, C.129: When designing a class hierarchy, distinguish between implementation inheritance and interface inheritance, C.130: For making deep copies of polymorphic classes prefer a virtual, C.135: Use multiple inheritance to represent multiple distinct interfaces, C.136: Use multiple inheritance to represent the union of implementation attributes, C.138: Create an overload set for a derived class and its bases with, C.140: Do not provide different default arguments for a virtual function and an overrider, C.145: Access polymorphic objects through pointers and references, C.152: Never assign a pointer to an array of derived class objects to a pointer to its base, C.153: Prefer virtual function to casting, keep data out of any class used as an interface, C.160: Define operators primarily to mimic conventional usage, C.161: Use non-member functions for symmetric operators, C.162: Overload operations that are roughly equivalent, C.163: Overload only for operations that are roughly equivalent, C.164: Avoid implicit conversion operators, C.167: Use an operator for an operation with its conventional meaning, C.168: Define overloaded operators in the namespace of their operands, C.170: If you feel like overloading a lambda, use a generic lambda, Binary operators should treat their operands equivalently, helper functions should be defined in the same namespace as their class, Enum.2: Use enumerations to represent sets of related named constants, Enum.4: Define operations on enumerations for safe and simple use, Enum.7: Specify the underlying type of an enumeration only when necessary, Enum.8: Specify enumerator values only when necessary, R.1: Manage resources automatically using resource handles and RAII (Resource Acquisition Is Initialization), R.2: In interfaces, use raw pointers to denote individual objects (only), R.5: Prefer scoped objects, dont heap-allocate unnecessarily, R.12: Immediately give the result of an explicit resource allocation to a manager object, R.13: Perform at most one explicit resource allocation in a single expression statement, R.15: Always overload matched allocation/deallocation pairs, R.30: Take smart pointers as parameters only to explicitly express lifetime semantics, R.37: Do not pass a pointer or reference obtained from an aliased smart pointer, ES.1: Prefer the standard library to other libraries and to handcrafted code, ES.2: Prefer suitable abstractions to direct use of language features, ES.3: Dont repeat yourself, avoid redundant code, ES.6: Declare names in for-statement initializers and conditions to limit scope, ES.7: Keep common and local names short, and keep uncommon and non-local names longer, ES.10: Declare one name (only) per declaration, ES.12: Do not reuse names in nested scopes, ES.21: Dont introduce a variable (or constant) before you need to use it, ES.22: Dont declare a variable until you have a value to initialize it with, ES.26: Dont use a variable for two unrelated purposes, ES.30: Dont use macros for program text manipulation, ES.31: Dont use macros for constants or functions, ES.33: If you must use macros, give them unique names, ES.34: Dont define a (C-style) variadic function, ES.41: If in doubt about operator precedence, parenthesize, ES.42: Keep use of pointers simple and straightforward, ES.43: Avoid expressions with undefined order of evaluation, ES.44: Dont depend on order of evaluation of function arguments, ES.45: Avoid magic constants; use symbolic constants, ES.49: If you must use a cast, use a named cast, ES.62: Dont compare pointers into different arrays, ES.65: Dont dereference an invalid pointer, ES.74: Prefer to declare a loop variable in the initializer part of a, ES.78: Dont rely on implicit fallthrough in, ES.84: Dont try to declare a local variable with no name, ES.86: Avoid modifying loop control variables inside the body of raw for-loops, ES.100: Dont mix signed and unsigned arithmetic, ES.101: Use unsigned types for bit manipulation, ES.106: Dont try to avoid negative values by using, Dont use a variable for two unrelated purposes, C++s model for type- and resource-safety, Per.3: Dont optimize something thats not performance critical, Per.4: Dont assume that complicated code is necessarily faster than simple code, Per.5: Dont assume that low-level code is necessarily faster than high-level code, Per.6: Dont make claims about performance without measurements, Per.11: Move computation from run time to compile time, Per.14: Minimize the number of allocations and deallocations, Per.15: Do not allocate on a critical branch, Per.17: Declare the most used member of a time-critical struct first, Per.30: Avoid context switches on the critical path, CP.1: Assume that your code will run as part of a multi-threaded program, CP.3: Minimize explicit sharing of writable data, CP.4: Think in terms of tasks, rather than threads, CP.9: Whenever feasible use tools to validate your concurrent code, CP.22: Never call unknown code while holding a lock (e.g., a callback), CP.31: Pass small amounts of data between threads by value, rather than by reference or pointer, CP.32: To share ownership between unrelated, CP.41: Minimize thread creation and destruction, CP.43: Minimize time spent in a critical section, CP.51: Do not use capturing lambdas that are coroutines, CP.52: Do not hold locks or other synchronization primitives across suspension points, CP.53: Parameters to coroutines should not be passed by reference, CP.100: Dont use lock-free programming unless you absolutely have to, CP.101: Distrust your hardware/compiler combination, CP.110: Do not write your own double-checked locking for initialization, CP.111: Use a conventional pattern if you really need double-checked locking, E.1: Develop an error-handling strategy early in a design, E.2: Throw an exception to signal that a function cant perform its assigned task, E.3: Use exceptions for error handling only, E.4: Design your error-handling strategy around invariants, E.5: Let a constructor establish an invariant, and throw if it cannot, E.13: Never throw while being the direct owner of an object, E.14: Use purpose-designed user-defined types as exceptions (not built-in types), E.15: Throw by value, catch exceptions from a hierarchy by reference, E.17: Dont try to catch every exception in every function, E.25: If you cant throw exceptions, simulate RAII for resource management, E.26: If you cant throw exceptions, consider failing fast, E.27: If you cant throw exceptions, use error codes systematically. If that counter reaches zero, the control block calls the destructor of the managed object. Normal lambdas will have finished executing by this time so it is not a problem. To make it clear that something is being hidden/abstracted. Implicit conversions can be essential (e.g., double to int) but often cause surprises (e.g., String to C-style string). It is the fundamental building block of programs. Look for functions called with all constant-expression arguments. Use header files to represent interfaces and to emphasize logical structure. dyn_array is not resizable, and is a safe way to refer to a heap-allocated fixed-size array. Thats (obviously) a bad data race, so people trained in other languages might try to fix it like this: This has no effect on synchronization: The data race is still there! This section contains materials that have been useful for presenting the core guidelines and the ideas behind them: Note that slides for CppCon presentations are available (links with the posted videos). # define COMPILER_ATTRIBUTE_ENTRY(attr) { #attr, "_" }, Dynamic memory allocation for over-aligned data, Lambda capture of *this by value as [=,*this], Return type deduction for normal functions, Template argument deduction for class templates, Dynamic initialization and destruction with concurrency, The official document on Feature-Test Recommendations, https://en.cppreference.com/mwiki/index.php?title=cpp/feature_test&oldid=145426, Conditional trivial special member functions, Changing the active member of a union in constant evaluation, Operations for dynamic storage duration in constexpr functions, Generation of function and variable definitions when, Guaranteed copy elision through simplified, Fixing atomic initialization (value-initialize, Constexpr for all the member functions of, Ranges construction and insertion for containers, Compile-time format string checks; Reducing parameterization of, Fixing locale handling in chrono formatters; Supporting non-const-formattable types, Core Issue 1776: Replacement of class objects containing reference members (, Making non-modifying sequence operations more robust (two-range overloads for, Smart pointer creation with default initialization (, Compatibility header for C atomic operations, Utility to convert a pointer to a raw pointer (. First challenge that assumption; there are many anti-exceptions myths around. Any usage beyond that is undefined behavior which may include writing to freed memory. One declaration per line increases readability and avoids mistakes related to The latter is an inherently simpler operation that ought to be faster. This section covers answers to frequently asked questions about these guidelines. Speaking of lambdas, what would weigh in on the decision between lambdas and (local?) data isnt a better alternative to non-const global variables since that doesnt solve the issues of hidden dependencies or potential race conditions. The default operations are a set of related operations that together implement the lifecycle semantics of an object. An enumeration shows the enumerators to be related and can be a named type. ranging from the simplest (make simple things simple!) Suggest it should be a local, Flag explicit allocations used to initialize pointers (problem: how many direct resource allocations can we recognize? For containers, there is a tradition for using {} for a list of elements and () for sizes: {}-initializers do not allow narrowing conversions (and that is usually a good thing) and allow explicit constructors (which is fine, were intentionally initializing a new variable). By convention, main returns status 0 if successful, 1 or higher for errors. Messy, low-level code breeds more such code. Consider, a sort instrumented with (oversimplified) simple debug support: After all, there is nothing in sortable that requires iostream support. It also makes the intent clearer. Here is a slightly more realistic example with an explanation. Flag using namespace at global scope in a header file. A simple, common use could be expressed: Wrap traits!. For example: The best solution is to avoid explicit allocation entirely use factory functions that return owning objects: Write your own factory wrapper if there is not one already. std::string_view or gsl::span provides simple and (potentially) safe access to character sequences independently of how A lambda results in a closure object with storage, often on the stack, that will go out of scope at some point. Unless you genuinely just deal with individual characters, using character-level input leads to the user code performing potentially error-prone Readability. You must be aware of the execution environment that your code is running when Use a static analyzer. Encourages consistent definition of user types. Although cached_computation works perfectly in a single-threaded environment, in a multi-threaded environment the two static variables result in data races and thus undefined behavior. This can be useful when the implementation class has members that are not offered in the abstract interface Performance is typically dominated by memory access times. This is C++14. Avoiding inconsistent definition in different namespaces. They are not useful, and make types difficult to use by making them either uncopyable or partially uncopyable for subtle reasons. We expect to have to introduce more exceptions to better accommodate real-world needs. (e.g., lots of pointers, ill-defined ownership, and lots of unsystematic error handling based on tests of error codes) There are many ways of implementing the idea of dual hierarchies; here, we use a multiple-inheritance variant. We are considering specifying bounds-safe overloads for stdlib (especially C stdlib) functions like, For existing stdlib functions and types like. Moving an object means to transfer ownership of some resource it manages to another object. Here, we managed to get a data race on data on the stack. Windows). Following the rules will lead to code that is statically type safe, has no resource leaks, and catches many more programming logic errors than is common in code today. These types allow the user to distinguish between owning and non-owning pointers and between pointers to a single object and pointers to the first element of a sequence. That implies more memory overhead, more allocations and deallocations, and more run-time overhead to perform the resulting indirections. A few simple microbenchmarks using Unix time or the standard-library can help dispel the most obvious myths. No, i = 7 does not initialize i; it assigns to it. operator=(const T&) to perform the assignment and then return (non-const) Generic and OO techniques are complementary. The language requires operators =, (), [], and -> to be members. For example, by capitalizing type names, but not the names of functions and variables. We make this project available to friendly users to use, copy, modify, and derive from, hoping for constructive input. Checks for the presence of an attribute named by attribute-token (after macro expansion).. For standard attributes, it will expand to the year and month in which the attribute was added to the working draft (see table below), the presence of vendor-specific attributes is determined by a non-zero value. Instead, we could use vector: The standards library and the GSL are examples of this philosophy. __has_cpp_attribute can be expanded in the expression of Also, this style is a temptation to use the goto exit non-rule. You can handle self-assignment by explicitly testing for self-assignment, but often it is faster and more elegant to cope without such a test (e.g., using swap). (except for calls from a derived class member to a base class member). Assume we have a virtual class named Base and a subclass of Base named Derived. Typically, they are implicit and their correct use is very hard to ensure. Overflow usually makes your numeric algorithm meaningless. Ideally, that moved-from should be the default value of the type. Code says what is done, not what is supposed to be done. Choosing unsigned implies many changes to the usual behavior of integers, including modulo arithmetic, -preserving (D disappointed me so much when it went the Java way). This should be enforced by tooling by checking the return type (and return Breaking out of a nested loop. It is meant to articulate ideas for new code in a concrete fashion. Look at the initialization of raw member pointers and member references and see if an allocation is used. If a thread joins, we can safely pass pointers to objects in the scope of the thread and its enclosing scopes. Unfortunately this will get many false positives; the standard library violates this widely, by putting many unconstrained templates and types into the single namespace std. These require C++17 and C++20 support. Use gsl::span instead. As far as we can tell, these rules lead to code that performs as well or better than older, more conventional techniques; they are meant to follow the zero-overhead principle (what you dont use, you dont pay for or when you use an abstraction mechanism appropriately, you get at least as good performance as if you had handcoded using lower-level language constructs). On the other hand, the ifstream for another file will correctly close its file (upon destruction). Given this, the compiler cannot know if vector::sort() is called, so it must generate code for it. A virtual function can be overridden and is thus open to mistakes in a derived class. Postconditions related only to internal state belongs in the definition/implementation. Note that a function parameter is a local variable so changes to it are local. be able to destroy a cyclic structure. This state of affairs has led older coding standards to impose a blanket requirement that all base class destructors must be virtual. protected data inherently violates the guidance against putting data in base classes, which usually leads to having to deal with virtual inheritance as well. for dealing with cleanup where resource management is not systematic. There are many things that are done better by machine. For example, see std::enable_shared_from_this Unless the loop was meant to be infinite, termination is normal and expected. However, compatibility makes changes difficult even if all agree that an effort to optimize is worthwhile. Systematic use of any error-handling strategy minimizes the chance of forgetting to handle an error. Is it right? Ideally, that Ensures should be part of the interface, but thats not easily done. A human or a good static analyzer might determine that there really isnt a side effect on v in f(v, &v[i]) so that the loop can be rewritten. For a more-or-less up-to-date to-do list see: But we want lots of rules, and we want rules that everybody can use. Prefer to place the interface first in a class, see NL.16. Therefore the result of the dynamic_cast should always be treated as if it might contain a null value, and tested. Introduction to type conversion and static_cast. There are many examples of data races that exist, some of which are running in The programmer (in a library) must define is_contiguous (a trait) appropriately. Impossible and unnecessary. To satisfy thread safety requirements, the reference counters are typically incremented using an equivalent of std::atomic::fetch_add with std::memory_order_relaxed (decrementing requires stronger ordering to safely destroy the control block). This would be fine if there was a default initialization for SomeLargeType that wasnt too expensive. Problem areas: These areas are sources of serious problems (e.g., crashes and security violations). works in more scenarios where type deduction might do the wrong thing on NULL Modifying loop counters in both the iteration-expression and inside the body of the loop is a perennial source of surprises and bugs. Here, we are holding the lock for longer than necessary: Mixing a type definition and the definition of another entity in the same declaration is confusing and unnecessary. Solution: reverse the situation. Isolating less stable code facilitates its unit testing, interface improvement, refactoring, and eventual deprecation. Subsequent changes to the function may add or move suspension points which would reintroduce this class of bug. This would be a set of changes across the whole code base, but would most likely have huge benefits. { #attr, COMPILER_ATTRIBUTE_AS_NUMBER(__has_cpp_attribute(attr)) }. An API class and its members cant live in an unnamed namespace; but any helper class or function that is defined in an implementation source file should be at an unnamed namespace scope. If they are not polymorphic types only need a pointer to the derived type use static_cast and hope for the best: If you need to convert a unique_ptr containing a polymorphic type: Otherwise, the new shared_ptr will share ownership with the initial value of r, except that Consider a popular technique for providing a handle for storing small objects in the handle itself and larger ones on the heap. The valid() function could return an error_indicator (e.g. In particular, it focuses on a template definitions dependence on its context. shared. In particular, it is harder (though not impossible) to ensure that the thread completed as expected or lives for as long as expected. if the majority of your time must be spent on an implementation. Note: C++17 and C++20 also add if, switch, and range-for initializer statements. For initializers of moderate complexity, including for const variables, consider using a lambda to express the initializer; see ES.28. Avoid code bloat. If there is not, maybe there ought to be, rather than applying a local fix (cast). Recognizing that the induction variable ), uses undefined behavior (can we catch all undefined behavior?). Templates can be used to express essentially everything (they are Turing complete), but the aim of generic programming (as expressed using templates) If at all possible, reduce the conditions to a simple set of alternatives (e.g., an enum) and dont mix up selection and initialization. Here, we (incautiously) left out the precondition specification, so it is not explicit that height and width must be positive. Some styles distinguish members from local variable, and/or from global variable. We call such a set of related guidelines a profile. For now, just be explicit. The purpose of modernizing code is to simplify adding new functionality, to ease maintenance, and to increase performance (throughput or latency), and to better utilize modern hardware. By providing the factory function create(), we make construction (on the free store) convenient. Expects() can also be used to check a condition in the middle of an algorithm. Not all destructors are noexcept by default; one throwing member poisons the whole class hierarchy. Regular objects are simpler to think and reason about than irregular ones. A wait without a condition can miss a wakeup or wake up simply to find that there is no work to do. The Lifetime rules will also provide general rules that flag escaping pointers and references including via lambdas.). Compared to what? be vectorized?. This precludes consistency. Ensure that unless there is an exceptionally good reason not to. Thats OK, but say that we have checked that this cannot happen and f is changed to throw a new exception Z, To avoid a called function unexpectedly changing the value. Use the public before protected before private order. Use = only when you are sure that there can be no narrowing conversions. In particular, the compiler can interleave execution of the two expressions: Flag a declaration of a variable, function, or enumerator that hides a class or enumeration declared in the same scope. Tooling can offer rewrites of array accesses that involve dynamic index expressions to use at() instead: Turning an array into a pointer (as the language does essentially always) removes opportunities for checking, so avoid it. this is inherently error-prone, but there are ways to compensate. such as auto completers, static analyzers, and debuggers. There are a few cases where leaks can be acceptable or even optimal: Redundant virtual increases run-time and object-code size. An output parameter is one that the function writes to, invokes a non-, Identify a (single) object (not to be deleted by this function), Point to an object allocated on the free store (and delete it later), Identify a C-style string (zero-terminated array of characters), Identify an array with a length specified separately. Most works after lots of testing but in isolation it is impossible to tell whether p could be the nullptr. focus on lower-level issues, such as the spelling of identifiers, see stopping programmers from doing unusual things as their primary aim, aim at portability across many compilers (some 10 years old), are written to preserve decades old code bases, are ignored (must be ignored by programmers to get their work done well), narrowing arithmetic promotions/conversions (likely part of a separate safe-arithmetic profile), arithmetic cast from negative floating point to unsigned integral type (ditto), selected undefined behavior: Start with Gabriel Dos Reiss UB list developed for the WG21 study group. have some support for static annotation of thread safety properties. Both single-object and array allocation functions may be defined as public static member functions of a class (versions (15-18)).If defined, these allocation functions are called by new-expressions to allocate memory for single objects and arrays of this class, unless the new expression used the form :: new which bypasses class-scope lookup. You will find some of the rules contrary to your expectations or even contrary to your experience. The standard library uses unsigned types for subscripts. Consider these rules ideals for new code, opportunities to exploit when working on older code, and try to approximate these ideals as closely as feasible. Using a well-designed, well-documented, and well-supported library saves time and effort; If no exception can be thrown, use noexcept. We want owning pointers identified so that we can reliably and efficiently delete the objects pointed to by owning pointers. Statements control the flow of control (except for function calls and exception throws, which are expressions). Use span to preserve size information. The function can also be written in such a way that it will accept any time duration unit. Concepts with multiple operations have far lower chance of accidentally matching a type than a single-operation concept. In general, naked pointers can be viewed with suspicion, flagged, and/or analyzed. An entity that is responsible for releasing a resource is called an owner. Flag division by an integral value that could be zero. Computers dont tire or get bored by repetitive tasks. By stating the intent in source, implementers and tools can provide better diagnostics, such as finding some classes of errors through static analysis, and perform optimizations, such as removing branches and null tests. A union is a struct where all members start at the same address so that it can hold only one member at a time. Subscripting the resulting base pointer will lead to invalid object access and probably to memory corruption. Consider such classes suspect, but maintain a positive list of classes where a human has asserted that the semantics is correct. Its asking to return a reference to a destroyed temporary object. So I was playing around with the concept of inheritance in C++ to get a better understanding of it and I used static_cast to cast adress of a base class object to a derived class pointer. Const variables and symbolic constants. For example: Code clarity and performance. If it did, vtbls could not be generated until link time. Small simple functions are easily inlined where the cost of a function call is significant. One for derived classes (protected) and one for general users (public). expensive to move around, thus tempting people to pass pointers to it around and getting into Several shared_ptr objects may own the same object. For example, a base class should not be copyable, and so does not necessarily need a default constructor: A class that must acquire a caller-provided resource during construction often cannot have a default constructor, but it does not fall under this guideline because such a class is usually not copyable anyway: A class that has a special state that must be handled separately from other states by member functions or users causes extra work The support library facilities are designed to be extremely lightweight (zero-overhead) so that they impose no overhead compared to using conventional alternatives. The rule supports the view that a concept should reflect a (mathematically) coherent set of operations. Some languages cannot be used without exceptions, but others do not support them. There is nothing (in the C++ standard or in most code) to say otherwise and most raw references are non-owning. The errors will not be caught until link time for a program calling bar or foobar. Better performance, better compile-time checking, guaranteed compile-time evaluation, no possibility of race conditions. Catching an exception in a function that cannot take a meaningful recovery action leads to complexity and waste. The standard-library merge() is at the limit of what we can comfortably handle: Note that this is because of problem 1 above missing abstraction. are seriously overused as well as a major source of errors. it just guarantees that the function can be evaluated at compile time for constant expression arguments if the programmer requires it or the compiler decides to do so to optimize. Always suggest an alternative. From a language perspective class and struct differ only in the default visibility of their members. The call will be f(1, 2) or f(2, 1), but you dont know which. In this rare case, you could make the destructor public and non-virtual but clearly document that further-derived objects must not be used polymorphically as Bs. An unnamed local objects is a temporary that immediately goes out of scope. Avoids repetition. It is recommended to make those destructors protected and non-virtual: This simple guideline illustrates a subtle issue and reflects modern uses of inheritance and object-oriented design principles. The concurrency/parallelism rules in this document are designed with three goals If parameters are conditionally unused, declare them with the [[maybe_unused]] attribute. and bad habits die hard. Even now, mixtures are not uncommon in old code bases and in old-style teaching material. but bear with us because this is just a simple example of a technique aimed at more complex hierarchies. Non-trivially copyable types should provide a member swap or a free swap overload. For example: There is a lot of code that is non-specific about ownership. If an unconstrained template is defined in the same namespace as a type, It also makes #included headers order-dependent as they might have different meaning when included in different orders. The second version leaves the reader guessing and opens more possibilities for uncaught bugs. Making existing single-threaded code execute concurrently can be We dont know of any other good examples of returning &&. If you really have to, look at factory functions. (Simple) A function should not make control-flow decisions based on the values of variables declared at namespace scope. Types can be defined to move for logical as well as performance reasons. (and you can have switch-statement in a loop and a loop in a switch-case). Nevertheless, the guidance is to use the quoted form for including files that exist at a relative path to the file containing the #include statement (from within the same component or project) and to use the angle bracket form everywhere else, where possible. That means its constructor will attempt to use first and last too soon not just before they are set to the desired values, but before they are constructed at all. More people know the standard library. dynamic_cast ( expression)expressiontype-idType-idvoid*type-idexpressiontype-idexpressiondynamic_cast advantage of this and use different search algorithms and methods for specifying the include path. That can in itself be a problem and a source of errors: Here, the writer of g() is supplying a buffer for f() to fill, but f() simply replaces it (at a somewhat higher cost than a simple copy of the characters). By default, C++ uses this storage class for all variables. No additional initialization, such as by memcpy, should be required. by making useful operations available for implementers of related new operations (sometimes called programming by difference). How granular should namespaces be? (Simple) A function should not write to variables declared at namespace scope. A common requirement for user-defined operator< is strict weak ordering.In particular, this is required by the standard algorithms and containers that work with Compare types: std::sort, std::max_element, std::map, etc. The rules in this section are very general. Instead of passing a range (abstraction), STL passed iterator pairs (unencapsulated component values). Direct resource management in application code is error-prone and tedious. it will immediately go back to sleep, waiting. Generic lambdas offer a concise way to write function templates and so can be useful even when a normal function template would do equally well with a little more syntax. If your program spends most of its time waiting for the web or for a human, optimization of in-memory computation is probably useless. The guidelines support library offers a narrow_cast operation for specifying that narrowing is acceptable and a narrow (narrow if) that throws an exception if a narrowing would throw away legal values: We also include lossy arithmetic casts, such as from a negative floating point type to an unsigned integral type: This rule does not apply to contextual conversions to bool: A good analyzer can detect all narrowing conversions. What is cheap to copy depends on the machine architecture, but two or three words (doubles, pointers, references) are usually best passed by value. You can, of course, generalize those functions using auto or concepts. probably impossible. derived_from - specifies that a type is derived from another type. Dont gratuitously commit to details; use the most general facilities available. In this context axioms are Boolean expressions. Because that would in many cases especially simple cases be distracting clutter. Helps make style consistent and conventional. The transformation might be helped by simple program transformation. If you explicitly write the copying functions, you probably need to write the destructor: If the special work in the copy constructor is to allocate or duplicate some resource (e.g., memory, file, socket), you need to deallocate it in the destructor. We plan to modify and extend this document as our understanding improves and the language and the set of available libraries improve. but have a rough idea of the order of magnitude of cost of what you use. Better: finally from the GSL is less verbose and harder to get wrong than try/catch. However, where nullptr is a possible value, a reference might not be a reasonable alternative. It should be possible to name a function meaningfully, to specify the requirements of its argument, and clearly state the relationship between the arguments and the result. Using unique_ptr in this way both documents and enforces the function calls reseating semantics. Never cast to (void) to ignore a [[nodiscard]]return value. say from hardware failures. A destructor, close, or cleanup operation should never fail. typeid] 17.8 Source location 20.3.2.2 Class template shared_ptr [util. However, using macros obscures what is being expressed anyway. Benefit from other peoples work when they make improvements. A library could be a set of headers (a header-only library) or a set of headers plus a set of object files. Consider: std::string is safe for self-assignment and so are int. Adjacent arguments of the same type are easily swapped by mistake. Recommended information sources can be found in the references. members to enable the implementation of the policies it requires. Familiarity. You can make an argument for that abomination in C when function prototypes were rare so that banning: would have caused major problems, but not in the 21st century and in C++. Not easy. Probably, aa[0] will be a Pear (without the use of a cast!). For example: There is not a choice when a set of functions are used to do a semantically equivalent operation to a set of types. that is, make its users vulnerable to having to recompile after changes in the implementation. Also, it is a popular source of errors (buffer overflow, pointers from array decay, etc.). behaviors and edge case behavior as a direct result of not having a rigorous Sometimes such reuse of a name in an inner scope is called shadowing. This can be most confusing. Similar for vector::sort(). the including file is moved to a new location), it will now be found ahead of the previous include file and the set of includes will have been changed in an unexpected way. Declaring any copy/move/destructor function, The rule is avoid, not dont use. Of course there will be (rare) exceptions, such as cin, cout, and cerr. Minimize the chance of unintended access. If you cant measure your complete system accurately, at least try to measure a few of your key operations and algorithms. The larger and more complicated the function is, the more painful the workarounds get. If data is related (for fundamental reasons), that fact should be reflected in code. We appreciate volunteer help! Is std::async worth using in light of future (and even existing, as libraries) parallelism facilities? Pointer arithmetic is fragile and easy to get wrong, the source of many, many bad bugs and security violations. Alternatively, we can add a try-catch to use() to map Z into an acceptable exception. IDEs also tend to have defaults and a range of alternatives. The argument against is that it prevents (very frequent) use of move semantics. Be fully aware that this technique is special-purpose and error-prone. This section contains rules for people who need high performance or low-latency. other error-handling approaches, but thats not a fundamental problem with exceptions. You can put down your pitchforks. To simplify the most frequent and simplest uses, the comparison argument can be defaulted to <: This doesnt reduce the total complexity, but it reduces the surface complexity presented to many users. If modification is desired, say so: For more details about for-statements, see ES.71. This is low-level, verbose, and error-prone. It is almost always a bug to mention an unnamed namespace in a header file. A function with a not_null parameter makes it clear that the caller of the function is responsible for any nullptr checks that might be necessary. A checker probably must rely on a human-provided list of resources. Leaving behind an invalid object is asking for trouble. Look for run-time checks for range violations. That could be dangerous. The std::swap() in f1() does exactly what we asked it to do: it calls the swap() in namespace std. Otherwise, use gsl::span. The NVI pattern is a technique to avoid public virtual functions. Optimizers sometimes do marvels with high-level code. Powered by .NET 7.0 on Kubernetes, , , shared_ptr/weak_ptronwer_before22. and where consecutive values are undesirable (e.g., to get separate bits as in Base_flag). (Moderate) A move assignment operator should (implicitly or explicitly) invoke all base and member move assignment operators. We are not legislating (see the previous paragraph). outputting time in various units. In C++, generic programming is supported by the template language mechanisms. Overload instead. Read up on the ABA problem. If r is empty, so is the new shared_ptr (but its stored pointer is not necessarily null). Otherwise, if you need a read-write view that does not need guaranteed bounds-checking and you have C++20, use C++20 std::span. These should be replaced with. (e.g., look into the constructors). Because isocpp is the Standard C++ Foundation; the committees repositories are under github.com/cplusplus. these threads can be seen as just a function object called from some_fct. If the operations are virtual the use of inheritance is necessary, if not using inheritance can avoid boilerplate and forwarding. When concepts become widely available such alternatives can be distinguished directly: There are three major ways to let calling code customize a template. We try to resolve those using tools. and they need to share free store memory that needs to be deleted, a shared_ptr (or equivalent) is the only When a feature changes significantly, the macro will be updated accordingly. For further information of how to use synchronization well to eliminate data races, please consult a good book about concurrency (See Carefully study the literature). The two language mechanisms can be used effectively in combination, but a few design pitfalls must be avoided. 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