Constructor or Assignment Operator

Question

Can you help me is there definition in C++ standard that describes which one will be called constructor or assignment operator in this case:

#include <iostream>

using namespace std;

class CTest
{
public:

 CTest() : m_nTest(0)
 {
  cout << "Default constructor" << endl;
 }

 CTest(int a) : m_nTest(a)
 {
  cout << "Int constructor" << endl;
 }

 CTest(const CTest& obj)
 {
  m_nTest = obj.m_nTest;
  cout << "Copy constructor" << endl;
 }

 CTest& operator=(int rhs)
 {
  m_nTest = rhs;
  cout << "Assignment" << endl;
  return *this;
 }

protected:
 int m_nTest;
};

int _tmain(int argc, _TCHAR* argv[])
{
 CTest b = 5;

 return 0;
}

Or is it just a matter of compiler optimization?

Answer 1

It’s always the default constructor taking an int that’s called in this case. This is called an implicit conversion and is semantically equivalent to the following code:

CTest b(5);

The assignment operator is never invoked in an initialization. Consider the following case:

CTest b = CTest(5);

Here, we call the constructor (taking an int) explicitly and then assign the result to b. But once again, no assignment operator is ever called. Strictly speaking, both cases would call the copy constructor after creating an object of type CTest. But in fact, the standard actively encourages compilers to optimize the copy constructor call here (§12.8/15) – in practice, modern C++ compilers won’t emit a copycon call here.