Question

I’m going through C++ FAQ by Marshall Cline.

Currently playing around with code in point 10.5.

I have this:

class Fred
{
public:
  Fred();
  Fred(int x, int y)
  {
    std::cout << "Fred with two ints\n" << std::endl;
  };
};

int main()
{
  std::vector<Fred>(3, Fred(4,5));
  return 0;
}

I expected to see “Fred with two ints” printed 3 times – but it’s only printed once.
Why is that – is it not invoked 3 times?

Answer 1

This is the signature of the vector constructor you are invoking:

vector( size_type _Count, const Type& _Val, const Allocator& _Alloc = Allocator() );

Fred(4,5) calls your defined constructor once, and the created instance is passed by reference to the vector<Fred> constructor. Then, it [the created instance] is copied 3 times to the vector. The copy operation is performed by using (default) copy constructor; so your constructor is not called more than once.