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.