Is there a difference between copy initialization and direct initialization?

Question

Suppose I have this function:

void my_test()
{
    A a1 = A_factory_func();
    A a2(A_factory_func());

    double b1 = 0.5;
    double b2(0.5);

    A c1;
         


        

Answer 1

Assignment is different from initialization.

Both of the following lines do initialization. A single constructor call is done:

A a1 = A_factory_func();  // calls copy constructor
A a1(A_factory_func());   // calls copy constructor

but it's not equivalent to:

A a1;                     // calls default constructor
a1 = A_factory_func();    // (assignment) calls operator =

I don't have a text at the moment to prove this but it's very easy to experiment:

#include 
using namespace std;

class A {
public:
    A() { 
        cout << "default constructor" << endl;
    }

    A(const A& x) { 
        cout << "copy constructor" << endl;
    }

    const A& operator = (const A& x) {
        cout << "operator =" << endl;
        return *this;
    }
};

int main() {
    A a;       // default constructor
    A b(a);    // copy constructor
    A c = a;   // copy constructor
    c = b;     // operator =
    return 0;
}