Using malloc/realloc for array of classes/structs including std vector

Question

I have a question wrt malloc/realloc memory that will contain an array of class/struct (i tried both struct and class the issue remains) members that include std vectors. I

Answer 1

If you use malloc() with non-POD classes, you must call constructors (via placement new) and destructors manually.

Using an object which was not constructed properly results in undefined behavior, which often means a crash when it comes to pointers.

Obviously, freeing a memory for object without a proper destruction of it causes UB too.

Your code must look like this:

MyClass *arr = (MyClass *) malloc(10 * sizeof (MyClass));

for (int i = 0; i < 10; i++)
    new (arr + i) MyClass; // This line calls constructors

// Do something with the array here

for (int i = 0; i < 10; i++)
    arr[i].~MyClass(); // This line calls destructors.

free(arr);

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This requirement also means that you can't use realloc() with non-POD types, because it wont call destructors for the old array and contructors for the new one for you.

Manual reallocation code might look like this:

MyClass *new_ptr = (MyClass *) malloc(new_size * sizeof (MyClass));

for (int i = 0; i < new_size; i++)
    new (new_ptr + i) MyClass((MyClass &&) old_ptr[i]);

for (int i = new_size; i < old_size; i++)
    new (new_ptr + i) MyClass;

for (int i = 0; i < old_size; i++)
    old_ptr[i].~MyClass();

free(old_ptr);

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And please keep in mind that above code is not really exception-safe. If a constructor throws an exception and you catch it, then you want to be sure that you properly destruct objects which were constructed. _{Thanks @SteveJessop.}

Now when you understand why malloc()/free() usually should be avoided in C++, I hope you'll return to a lot more safe new/delete, which do all that construction and destruction for you.