Vector preallocation does not work properly

Question

i have some trobles with the following code:

#include <iostream>
#incldue <vector>

template <typename ElemType>
class A{
private:

  std::vector<ElemType> data;

public:
  A() {};

  A(int capacity) {
    data.reserve(capacity);
  }

  int GetCapacity() {
    return data.capacity();
  }
};

int main() {
  A<int> a;
  a = A<int>(5);
  std::cout << a.GetCapacity() << std::endl; 
} 

The output is 0. What can be the problem?

Answer 1

The copy-constructor and assignment operator of std::vector<T> are not required to copy the capacity of the vector, only the elements. Because the line a = A<int>(5) indirectly invokes the assignment operator (after creating a temporary), the vector in a does not have a capacity.

Try changing the first two lines of main to just A<int> a(5) and see what the results are.

If you absolutely need the capability to transfer the capacity from one instance to another, you need to define the assignment and copy-constructor of A to both copy the data and assign the capacity.

Answer 2

Because you have not implemented the copy-assignment of A, preserving the capacity of the vector. The compiler generated copy-assignment of A, does ensure the only copying of elements of the vector. The vector's copy-constructor (and copy-assignment) is not required to preserve the capacity of the source vector object. It copies the elements, preserving the size of the vector.

That means, if you define the copy-assignment of A as:

 A& operator = (const A & a) 
 {
    data = a.data;
 }

It would still print 0.

The behaviour you want can be achieved by implementing the copy-assignment as:

 A& operator = (const A & a) 
 {
    data.reserve(a.capacity());
    data.insert(data.end(), a.begin(), a.end());
 }

Note that .insert() only copies the element, because of which .size() changes, but .capacity remains unchanged. That is why you need to explicitly call .reserve().