CODEWITHSUNDEEP
c++storageheap-memorystack-memory
Denis
Denis

Reputation: 91

Creating object on stack, from a class with dynamic members

Assume we create an object on stack as

class Test{

Test(int i) {i_=i;}
Test(std::vector<int> v) {v_=v;}

int i_;
std::vector<int> v_;
};

int main() {
Test a;
// how much memory is occupied/reserved now for a?
.
.
.
return 0;
}

How compiler determines the required size for "a", when it is not yet known which constructor is going to be called? I mean, how much memory is reserved on stack? what about if i used "new" for object creation?

My example is very simplified in order to transfer my point. The aim of my question is to understand better the meaning of object creation/initialization on stack/heap.

Upvotes: 0

Views: 472

Answers (1)

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 122350

How compiler determines the required size for "a", when it is not yet known which constructor is going to be called?

This question hinges on two misunderstandings:

First, Test a; does call the default constructor. Next, the size of objects to be allocated on the stack is a constant: sizeof(Test). This size does not change when more elements are added to the vector member, because those elements to not contribute to the size of a Test, they are allocated dynamically on the heap.

what about if i used "new" for object creation?

No difference concerning the size of the object. sizeof(Test) is a compile time constant and does not change depending on how you create the object. Though with Test* p = new Test; you'll have only p on the stack and its size is sizeof(Test*), while the object is on the heap.

PS: Actually "heap" and "stack" are not terms used by the standard, but as they are so common I think its ok to use them here. More formally correct would be to talk about automatic and dynamic storage duration (https://en.cppreference.com/w/cpp/language/storage_duration).

Upvotes: 1

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