Using a constructor in a function call?

Question

I've been searching awhile for a good explanation of why/why not the following use of the struct constructor as a function argument is legal. Can someone provide one?

// Begin simple illustrative example C++ program    
#include<vector.h>

struct Item  
{  
  Item(double data, const int lead)
  : m_grid(data), m_lead(lead) {}

  double m_grid;
  int m_lead;
};

int main()
{
  double img = 0.0;
  int steps = 5;
  std::vector<Item> images;
  for (int i = 0; i < steps; i++)
  {
    img += 2.0;
    images.push_back(Item(img,i));
  }
  return 0;
}

I was under the impression a constructor has neither a return type nor statement...

Answer 1

It's not the constructor or its return value that's passed to push_back. C++ actually uses the constructor to create a nameless temporary object, which exists only for the duration of the function call; typically, on the stack. This is then passed to push_back, and push_back copies its contents into your vector.