C++ deleting a pointer itself instead of deleting the pointed-to-data, pointer being freed was not allocated for pointer assignment. Ah, I see. It may be initialized to zero the first time, but you can't rely on that behavior. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Your first code snippet does indeed delete the object. Why does the USA not have a constitutional court? Context: I'm trying to wrap my head around pointers, we just saw them a couple of weeks ago in school and while practicing today I ran into a silly? It is not recommended to use delete with malloc().6. C++ C++,c++,pointers,delete-operator,forward-list,C++,Pointers,Delete Operator,Forward List, f_ Deleting pointer to object in unordered_map. Making statements based on opinion; back them up with references or personal experience. is dynamically creating a pointer, and then changing a pointer address to something else still deleting the original allocated space? Deleting a NULL pointer does not delete anything. Find centralized, trusted content and collaborate around the technologies you use most. Advantages of void pointers: 1) malloc () and calloc () return void * type and this allows these functions to be used to allocate memory of any data type (just because of . You can call memset to clear the memory returned from malloc. If the object is created using new, then we can do delete this, otherwise behavior is undefined. In a nutshell, they do what I described. In your case, you should simply remove the delete. How many transistors at minimum do you need to build a general-purpose computer? It is not safe to delete a void pointer in C/C++ because delete needs to call the destructor of whatever object it's destroying, and it is . Program3.cpp #include <iostream> using namespace std; int main () { // initialize the integer pointer as NULL int *ptr = NULL; // delete the ptr variable delete ptr; cout << " The NULL pointer is deleted."; return 0; } Output A null pointer indicates that there is "nothing there" - it points at nothing. I think you're relying on a technicality here. Online C Queue programs for computer science and information technology students pursuing BE, BTech, MCA, MTech, MCS, MSc, BCA, BSc. Maybe that pointe. Good luck with that. Your question reveals that you think that freeing up memory is as simple as releasing it when you're done with it. As we saw in the article about removing elements from a sequence container, to remove elements in a vector based on a predicate, C++ uses the erase-remove idiom: vector<int> vec {2, 3, 5, 2}; vec.erase (std::remove_if (vec.begin (), vec.end (), [] (int i) { return i % 2 == 0;}), vec.end ()); Which we can wrap in a more expressive function call: That's the kind of thinking that creates very buggy C or C++ programs. New operator is used for dynamic memory allocation which puts variables on heap memory. NULL equals 0, you delete 0, so you delete nothing. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. You are trying to delete a variable allocated on the stack. It points to some data location in storage means points to the address of variables. 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It destroys the memory block or the value pointed by the pointer. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. You can do this explicitly using std::list::iterator, but it's easier to simply use a range-based for-loop: 1 2 3 4 for (Object* obj : objList) { obj->PrintValue (); } Last edited on May 10, 2021 at 5:39am May 10, 2021 at 5:57am seeplus (5744) Delete is an operator that is used to destroy array and non-array(pointer) objects which are created by new expression. If you're using C++, do not use raw pointers. Why is the federal judiciary of the United States divided into circuits? You are not allocating a 2D array - you are making n+1 separate allocations, completely unrelated to each other as far as the compiler can tell. You can call memset to clear the memory returned from malloc. In general programs only use memset or calloc if they really need the memory buffer to be initialized to zero. The latter is more often referred to as "freeing", while the former is more often called "deleting". Answer (1 of 7): Try [code ]delete **p;[/code] Other posters are correct to suggest that naked delete is a code smell, that using a smart pointer and RAII is better than using naked pointers, and that an object-oriented approach where the pointer is a class member and deleted in the destructor i. Can a prospective pilot be negated their certification because of too big/small hands? It's simple, really - for every new, there should be a corresponding delete. Thanks for contributing an answer to Stack Overflow! The above did nothing at all. Why should I use a pointer rather than the object itself? Differences in delete and free are: In the last line above, r points to the block that was originally pointed to by q and allocated by new, so you can safely delete it. Syntax: data_type_of_pointer **name_of_variable = & normal_pointer_variable; Example: "delete this" in C++ If the object is created using new, then we can do delete this, otherwise behavior is undefined. - this memory address If you're just going to copy a string into it, you don't need to initialize it to zero first; you can just copy the string and (if necessary) append a '\0' to the end. You must delete the same block of memory that you obtained from new. You can only use delete on a pointer to memory that you have allocated using new. Ready to optimize your JavaScript with Rust? Do you need to delete pointers C++? Regardless, no clarification is necessary here, it's obvious why OP is observing this behavior. The pointer returned by new should belong to a resource handle (that can call delete ). For deleteing the pointer you need to use . So it is good practice to set a pointer to NULL (0) after deleting. The pointer itself is a local variable allocated on the stack. CPP. Delete can be used by either using Delete operator or Delete [ ] operator New operator is used for dynamic memory allocation which puts variables on heap memory. I want to just add that the allocated memory WILL be returned for other programs to use, but only AFTER your program has finished executing. Ready to optimize your JavaScript with Rust? There's no such concept as "deleting" data in C. When you allocate some memory for data, there will always be data there. Delete is an operator in C++ that can be used to free up the memory blocks that has been allocated using the new operator. @tadman . char *p = (char *) malloc (10 * sizeof (char)); . Whenever you call new, you should then 'delete' at the end of your program, because otherwise you will get a memory leak, and some allocated memory space will never be returned for other programs to use. The code presented here can be refactored in many different ways; I only focus on the smart pointer cases. You can call delete only on memory you allocated dynamically (on the heap) using the new operator. Can virent/viret mean "green" in an adjectival sense? your object that was deleted, it might still work. A void pointer is a pointer that has no associated data type with it. You pointed it at NULL, leaving behind leaked memory (the new int you allocated).You should free the memory you were pointing at. Is there a verb meaning depthify (getting more depth)? Find code solutions to questions for lab practicals and assignments. If you're just going to copy a string into it, you don't need to initialize it to zero first; you can just copy the string and (if necessary) append a '\0' to the end. The asterisk * used to declare a pointer is the same asterisk used for multiplication. But this is how it is done: // Allocate a new myClass instance and let x point at that instance myClass* x = new myClass(); // Replace the data pointed at by x to be a new myClass instance *x = myClass(); // At this point, x points at the same piece of memory // but the data stored at that memory is different. What is a smart pointer and when should I use one? If you had written. void pointer in C / C++. q still points to the block you got from new, so it's safe to delete it. True. delete is used for classes with a destructor since delete will call the destructor in addition to freeing the memory. Syntax : delete <variable name> or delete [ ] <array_name>. This Use smart pointers instead which can handle these things for you with little overhead. If you are going to use raw owning pointers, you could fix it like this: delete pearl; pearl->eat (); //Works! Useful, because the Stack has a limited size and you might want to mess about with a big load of 'ints' without a stack overflow error. Pointers are similar to normal variables in that you don't need to delete them. You can access it untill the memory is used for another variable, or otherwise manipulated. Something went wrong. Or consider using calloc, which zeros out the memory before returning the pointer to you. 2022 ITCodar.com. The behaviour of your program is undefined. Why should I use a pointer rather than the object itself? Hmm okay, I'm not sure what smart pointers are, but I'll look into it, thanks! You can not do this. In any case, free on a null pointer does nothing. In order to hold something new, you call, "On the stack" is an implementation detail -- one that C++ conspicuously avoids mentioning. Appropriate translation of "puer territus pedes nudos aspicit"? That brings up the second point--if the pointer goes out of scope before you deallocate the object you allocated on the heap, you will never be able to deallocate it, and will have a memory leak. @DarkCthulhu Thanks! How could my characters be tricked into thinking they are on Mars? Strictly speaking, the C programming language has no delete (it is a C++ keyword), it just provides the free [ ^] function. You can however use pointers to allocate a 'block' of memory, for example like this: This will allocate memory space for 20000 integers. Deleting a pointer does not destruct a pointer actually, just the memory occupied is given back to the OS. delete will deallocate the memory to which its operand points. Delete is an operator that is used to destroy array and non-array (pointer) objects which are created by new expression. I don't understand why you're accusing me of posting a "glib" answer when your own answer, posted before mine, was "put new data there." C++ Programming Foundation- Self Paced Course, Data Structures & Algorithms- Self Paced Course, Comparison of static keyword in C++ and Java, C++ Program to Show Use of This Keyword in Class, Output of Java Programs | Set 39 (throw keyword), Output of Java Programs | Set 44 (throws keyword). Received a 'behavior reminder' from manager. On the second example the error is not being triggered but doing a cout of the value of myPointer. Which means Delete operator deallocates memory from heap. The first variable was allocated on the stack. q points to statically allocated memory, not something you got from new, so you can't delete it. - not all pointers need to have their memory deleted: you only need to delete memory that was dynamically allocated (used new operator). Answer: Smart pointer in C++ can be used to delete pointers of object returned by Factory method in C++ in the client code for example in main () method. : delete ptr; @EricPostpischil but if there is old data and I put in a new char at the first position and then iterate until '\0', this will give me not the desired result, since the old data interferes with the new Glib answers before a question has been clarified risk overlooking the real problem. Step 6 Enter an element that to be deleted. It is also called general purpose pointer. You're a QPP member,aren't you? What is the difference between #include and #include "filename"? Ask genuine questions. A void pointer can hold address of any type and can be typecasted to any type. Void pointer is a pointer which is not associate with any data types. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Instead of changing where q points, though, you can copy the value you want into the space that q points to: Here you're changing the value stored in the location to which q points, but you're not changing q itself. Your program attempts to free this memory using delete, which is undefined behavior since you're calling delete for memory that wasn't allocated using new. Is it cheating if the proctor gives a student the answer key by mistake and the student doesn't report it? Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? Pointers to variables on the stack do not need to be deleted. delete pointer; does not delete the pointer itself, but the data that the pointer is pointing to. Recommended to read this post before moving forward: How to write Smart Pointer for a given class in C++? issue, it can be super straightforward to you but I have little to none programming experience. 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We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. let p point to some chars free (p); char *p = (char *) malloc (10 * sizeof (char)); Since freeing p doesn't delete the data stored at the particular memory address I'm facing the problem that for large pointers the newly allocated memory overlaps with the old one what is obviously problematic. How could my characters be tricked into thinking they are on Mars? How do I set, clear, and toggle a single bit? Pointer to object is not destroyed, value or memory block pointed by pointer is destroyed/deallocated. I believe you're not fully understanding how pointers work. What happens when you delete a pointer in C++? The above did nothing at all. delete this; Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. So it is good practice to set a pointer to NULL (0) after deleting. What are the differences between a pointer variable and a reference variable? 3. By using our site, you You should free the memory you were pointing at. Do you need your, CodeProject, Why is this usage of "I've to work" so awkward? the pointer, it was simply still a valid object. The rubber protection cover does not pass through the hole in the rim. 20 Bay Street, 11th Floor Toronto, Ontario, Canada M5J 2N8 Why does the distance from light to subject affect exposure (inverse square law) while from subject to lens does not? Deleting pointer with or without value, 5. deleting memory dynamically allocated by malloc. Although above program runs fine on GCC. Examples: You could replace your data with something like 0s or 0xFFs with memset(), which would overwrite your data. You are calling new n+1 times, so you should call delete n+1 times, or else you leak memory. You probably missed something hereCan you delete nothing? Provides the member typedef type which is the type pointed to by T, or, if T is not a pointer, then type is the same as T . When would I give a checkpoint to my D&D party that they can return to if they die? Something can be done or not a fit? Signs are OP is not initializing the memory they allocate, meaning their use of their first. It has no effect on the pointer pointing to the starting address of that memory location. You can't assume that memory returned from malloc is initialized to anything. What does "dereferencing" a pointer mean? public: void fun () {. Where does the idea of selling dragon parts come from? Thank you, I selected your answer for a) explaining what was wrong and b) giving a best practice, thanks much! A-143, 9th Floor, Sovereign Corporate Tower, We use cookies to ensure you have the best browsing experience on our website. In addition, check out this lecture on stack frames. There is no way to access that allocated new int anymore, hence memory leak. What was your intent in making that statement, if not to answer the OP's question? Step 2 Declare and read the array size at runtime. Only. 2. The error says the memory wasn't allocated but 'cout' returned an address. How many transistors at minimum do you need to build a general-purpose computer? Step 3 Input the array elements. To learn more, see our tips on writing great answers. Step 5 Allocate the memory dynamically at runtime. If you're using C++, do not use raw pointers. Can't call delete on it. You can call delete only on memory you allocated dynamically (on the heap) using the new operator. All Rights Reserved. (I literally) learn something, @AmelSalibasic The memory associated with the variable on the stack will be freed only once it goes out of scope. delete operator Since it is the programmer's responsibility to deallocate dynamically allocated memory, programmers are provided delete operator in C++ language. Or consider using calloc, which zeros out the memory before returning the pointer to you. So unless you assign another value to myPointer, the deleted address will remain. All memory on the stack is freed automatically at the end of main(): p, d and x (the variable holding the address, not the address it points to) were all allocated on the stack. various way to delete pointer to pointer morz i just search for a while in c++ groups but cannot find good answer. free (and malloc, calloc etc) is used for basic types, but in C++ new and delete can be used for them likewise, so there isn't much reason to use malloc in C++, except for compatibility reasons. I've seen quite a few questions over in SO about deleting pointers but they all seem to be related to deleting a class and not a 'simple' pointer (or whatever the proper term might be), here's the code I'm trying to run: Sorry for the long question, wanted to make this as clear as possible, also to reiterate, I have little programming experience, so if someone could answer this using layman's terms, it would be greatly appreciated! C++11 comes with several. Is there any reason on passenger airliners not to have a physical lock between throttles? Does #3 really work? How does the Chameleon's Arcane/Divine focus interact with magic item crafting? With arrays, why is it the case that a[5] == 5[a]? then you could write either delete b; or delete c; to free your memory. So the address that cout prints is the address of the memory location of myVar, or the value assigned to myPointer in this case. Answer (1 of 11): Now you understand why pointer bugs are so prevalent. You can't assume that memory returned from malloc is initialized to anything. Obtain closed paths using Tikz random decoration on circles. C Program to perform insert & delete operations on queue using pointer. I rarely, if ever, reassign individual heap allocated objects. myPointer = new int; //dynamically allocated, can call delete on it. std:: remove_pointer. Why won't the first case work? return 0; } Seeing as how no memory was allocated after that, and you didn't erase. deleting indiviual pointers from an array. Allow non-GPL plugins in a GPL main program. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. When you have a pointer pointing to some memory there are three different things you must understand: By writing: myPointer = The address of where the data in myVar is stored. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The reason you don't see the first example is because it's wrong. New operator is used for dynamic memory allocation which puts variables on heap memory while delete operator deallocates memory from heap. C++ after delete pointer int* ptr = new int(6); reserves some memory where ptr will be pointing to, that memory will be good to store one int , 6 or any other, it cannot be used to do anything else, you can reliably store the data there and access it later. The content must be between 30 and 50000 characters. You can call delete only on memory you allocated dynamically (on the heap) using the new operator. It may be initialized to zero the first time, but you can't rely on that behavior. It can be used using a Delete operator or Delete [] operator. Let's consider a program to delete NULL pointer using the delete operator in C++ programming language. In C++, the delete operator should only be used either for the pointers pointing to the memory allocated using new operator or for a NULL pointer, and free () should only be used either for the pointers pointing to the memory allocated using malloc () or for a NULL pointer. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. Since freeing p doesn't delete the data stored at the particular memory address I'm facing the problem that for large pointers the newly allocated memory overlaps with the old one what is obviously problematic. acknowledge that you have read and understood our, Data Structure & Algorithm Classes (Live), Full Stack Development with React & Node JS (Live), Fundamentals of Java Collection Framework, Full Stack Development with React & Node JS(Live), GATE CS Original Papers and Official Keys, ISRO CS Original Papers and Official Keys, ISRO CS Syllabus for Scientist/Engineer Exam, C/C++ Ternary Operator Some Interesting Observations, Pre-increment (or pre-decrement) With Reference to L-value in C++, new and delete Operators in C++ For Dynamic Memory, Pure Virtual Functions and Abstract Classes in C++, Result of comma operator as l-value in C and C++, Increment (Decrement) operators require L-value Expression, Precedence of postfix ++ and prefix ++ in C/C++, Initialize a vector in C++ (7 different ways), Map in C++ Standard Template Library (STL), Set in C++ Standard Template Library (STL). Connect and share knowledge within a single location that is structured and easy to search. i have code like this: int **ptr; ptr = new char * [2]; ptr [0] = new int (5); ptr [1] = new int (16); i know we can delete ptr like this: for (int i = 0; i <2; i++) delete ptr [i]; delete ptr; But can i delete like this? Assigning it to. This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL). Use smart pointers instead which can handle these things for you with little overhead. In this case, that happens to be memory that's allocated on the stack when the scope of the main() function is entered, and is freed automatically when the scope of main() exits. 1980s short story - disease of self absorption. How can I delete the data to which a pointer points to ? Smart pointer will handle memory deallocation itself instead of manual call. No. C++11 comes with several. Not the answer you're looking for? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Effect of coal and natural gas burning on particulate matter pollution. Delete doesn't destroy the object. With rare exceptions, C++ programmers should not have to write new or delete ever again. Not stuff that can be found one StackOverflow or even *gasp* Google lol - But here's your troll answer :-P If you meant deallocate a pointer. rev2022.12.9.43105. Solution 1 You cannot 'delete' a null pointer. The behavior of a program that adds specializations for remove_pointer is undefined. Seems the most straightforward use to use and delete a pointer? Syntax: // Release memory pointed by pointer-variable delete pointer-variable; Here, the pointer variable is the pointer that points to the data object created by new. 1. Step 7 After deletion, the elements are shifted to left by one position. new is never called. Is using ::New() allocating my smart pointer on the heap or the stack? How to solve null pointer exception in steams, list array? You didn't free anything, as the pointer pointed at NULL. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. You can access it untill the memory is used for another variable, or otherwise manipulated. myPointer = new int; delete myPointer; //freed memory myPointer = NULL; //pointed dangling ptr to NULL The better way: If you're using C++, do not use raw pointers. Asking for help, clarification, or responding to other answers. - there is "what is pointed" by the pointer (the memory) Deleting variables of User Defined data types: Exceptions:1. Did the apostolic or early church fathers acknowledge Papal infallibility? rev2022.12.9.43105. And it's logic that it prints 0 because you did: Thanks for contributing an answer to Stack Overflow! +1 (416) 849-8900. Deleting a pointer in C++ 1 & 2 myVar = 8; //not dynamically allocated. You didn't free anything, as the pointer pointed at NULL. Why Smart pointers can not be declared the usual Pointer way, c++ Deleting multiple head/tail of linked list stored in vector. Declaring Pointer to Pointer is similar to declaring a pointer in C. The difference is we have to place an additional '*' before the name of the pointer. Just use the stack Making statements based on opinion; back them up with references or personal experience. Chances are they have and don't get it. The general form of a pointer variable declaration is type *var-name; Here, type is the pointer's base type; it must be a valid C data type and var-name is the name of the pointer variable. Find centralized, trusted content and collaborate around the technologies you use most. If you meant deallocate a pointer, then the command is a keyword. User has privilege to deallocate the created pointer variable by this delete operator. They are removed from memory at the end of a functions execution and/or the end of the program. But it does not set memory before free as requested by OP. 2. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Deleting Array Objects: We delete an array using [] brackets. Even if you had allocated memory in. There is no way to access that allocated new int anymore, hence memory leak. In general programs only use memset or calloc if they really need the memory buffer to be initialized to zero. Though that might be considered extraneous and could get optimized out by your compiler. Seems to work to me, the pointer is no longer storing an address, is this the proper way to delete a pointer? Share Follow edited May 23, 2017 at 10:31 Community Bot 1 1 The first variable was allocated on the stack. What REALLY happens when you don't free after malloc before program termination? Step 4 Declare a pointer variable. C++ allows that you try to delete a pointer that points to null but it doesn't actually do anything, just doesn't give any error. Deleting NULL pointer : Deleting a NULL does not cause any change and no error. Provide an answer or move on to the next question. Motivation FactSetters have gotten a lot of mileage out of boost::shared_ptr over the years. Asking for help, clarification, or responding to other answers. 2) Once delete this is done, any member of the deleted object should not be accessed after deletion. It returns an address that points to a memory location that has been deleted. spelling and grammar. Connect and share knowledge within a single location that is structured and easy to search. Use smart pointers instead which can handle these things for you with little overhead. Posted 28-Dec-15 0:55am CPallini Solution 2 A null pointer indicates that there is "nothing there" - it points at nothing. Thanks, THIS was super helpful, I thought I HAD to delete all pointers, didn't know that was only for the ones that were new'd, thanks. Really? Deleting a pointer (or deleting what it points to, alternatively) means, p was allocated prior to that statement like, It may also refer to using other ways of dynamic memory management, like free, which was previously allocated using malloc or calloc. class A. Here is the syntax of delete operator in C++ language, delete pointer_variable; Here is the syntax to delete the block of allocated memory, delete [ ] pointer_variable; The first variable was allocated on the stack. Does balls to the wall mean full speed ahead or full speed ahead and nosedive? Does integrating PDOS give total charge of a system? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. You pointed it at NULL, leaving behind leaked memory (the new int you allocated). Did neanderthals need vitamin C from the diet? To do this: There is a rule in C++, for every new there is a delete. If the pointer returned by new is assigned to a plain/naked pointer, the object can be leaked. It will be deallocated as soon as it goes out of scope. If you see the "cross", you're on the right track, What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked, QGIS expression not working in categorized symbology. Pointer to object is not destroyed, value or memory block pointed by pointer is destroyed. Designed by Colorlib. You must do so /before/ free()ing it. C++11 comes with several. Don't attempt to derefererence either b or c after the delete call though, the behaviour on doing that is also undefined. {. Here, Below are examples where we can apply delete operator:1. What are the differences between a pointer variable and a reference variable? Step 1 Declare and read the number of elements. What is the difference between const int*, const int * const, and int const *? Question: How do I delete a pointer in C++? How is the merkle root verified if the mempools may be different? (C++11, 3.7.3). How did muzzle-loaded rifled artillery solve the problems of the hand-held rifle? Understand that English isn't everyone's first language so be lenient of bad If a question is poorly phrased then either ask for clarification, ignore it, or. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. email is in use. Why does pointer not update on empty BST tree in C when inserting node? To learn more, see our tips on writing great answers. 2 Answers Sorted by: 3 You are already deallocating what you have allocated but doublePtrNode [i]->value=i; assumes that you've allocated a Node there, but you haven't so the program has undefined behavior. Trying to delete pointer to a local stack allocated variable. How to delete the data of a pointer in C? How to sort an array of pointers by the addresses of what is inside the pointers. Deleting a pointer does not destruct a pointer actually, just the memory occupied is given back to the OS. Deleting array elements in JavaScript - delete vs splice. 3. myPointer = NULL; delete myPointer; To be even more clear, delete doesn't care about what variable it operates on, it only cares about what that variable points to. 1) delete operator works only for objects allocated using operator new (See this post ). Not the answer you're looking for? The more correct term is "with automatic storage duration". Because first you create the pointer and assign its value to myPointer, second you delete it, third you print it. Not sure if it was just me or something she sent to the whole team. Does the collective noun "parliament of owls" originate in "parliament of fowls"? The delete operator is used to deallocate the memory. Trying to delete Non-pointer object. Don't tell someone to read the manual. What is a smart pointer and when should I use one? 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