Copy constructor being called after one argument constructor

[msinl]

Hi guys,
Am a newbie in C++. Can you guys please tell why the copy constructor is being called after the supplied one argument constructor is called?

class A
{
  private:
   int value;
 
  public:
    A(int val) { value = val; std::cout << "\nOne argument constructor called";}
    A() { value = 0; std::cout << "\n0 argument constructor called";}
    A(A& b) { *this = b; std::cout << "\nCopy constructor called " << this;}
 
  };
 
  int main()
  {
    int hi;
        A b = A(2);  //why is copy constructor also called after one argument constructor
    std::cin >> hi;
    return 0;
  }
 

[RRick]

In your code, you are really invoking two compiler calls.

A b = A(2);  //why is copy

First, you create a temporary object with A(2) which calls A( int) and then you assign the temporary object to another object b, which invokes the copy constructor. If you want to only call A( int) then use:

A b(2);  //  only A( int) called

You can delete the copy constructor and get the same results because the compiler will supply a default shallow copy constructor. Also, you need to add a const to your copy constructor, because my compiler (GNU g++) produces an odd error without it. The copy constructor should be:

A( const A & b) ...

[weaknessforcats]

Almose correct:

A b = A(2); //why is copy

First, you create a temporary object with A(2) which calls A( int) and then you assign the temporary object to another object b, which invokes the copy constructor.

There is not assignment here as there is no LVAL. The object b is being created and assignment cannot occur until after the object has been created.

Here A(2) calls the A(int) coinstructor to create a temp object. This temp object is used to call A(A) to intialize b.

There are just the two constructor calls.