Do arrays that get allocated on the stack within the scope of a function always get deallocated when leaving the function?

user_6445534 0
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Let's say I have the following code:

#include <iostream>
void f() {
  int arr[10];
}

int main() {
  f();
  f();
}

Does the array arr get deallocated once we leave the scope of f(), even though we will call it again? Also, if we only call f() once, I'm assuming it gets deallocated when leaving the scope of f(), but someone please correct me if I'm wrong.

I also realize that this may be compiler-related question so I've included g++ as a tag. Thanks.

g++ allocation c++
3 Answers

Conceptually it does indeed since arr has automatic storage duration. (For example, if you were to return a pointer to an element of that array, then the behavior on dereferencing that pointer in the caller would be undefined.)

Whether or not the compiler chooses an appropriate optimisation based on the as-if rule is another matter.

With g++ set to -O3, you can expect your entire program to be compiled to int main(){}. Check the generated assembly if you're in any doubt.

Reference: https://en.cppreference.com/w/cpp/language/as_if

user_23808300

You are not wrong. The array gets allocated every time the function is called and de-allocated every time the function ends. Although allocation/de-allocation on the stack is essentially free. Also with an int array you won't even pay for constructors to run on the elements.

user_38077290

Adding on to what @Bhathsheba said whenever you go out of the scope that memory (stack) is not automatically invalidated it's just that you give the freedom of deleting the stack based on other requirements. If there is no other process running then the stack might not get deleted. So it depends on the situation.

user_107586540
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