Question

When I read about copy initializing vs direct initializing here. copy constructor should call in copy initializing. why here copy constructor not calling?

#include <iostream>

using namespace std;

class A{};

class B{
public:
B(const A &a){cout << "B construct from A" << endl;}
B(const B &b){cout << "B copy constructor" << endl;}
};

int main(){
A a;
B b = a;
return 0;
}

Answer 1

This is Copy Elision^{Ref 1:}.
Copy constructor calls while generating temporaries might be optimized by the compiler by creating objects inline and it is explicitly allowed by the C++ Standard.

This is nicely demonstrated in the standard with an example as well:

C++03 Standard 12.2 Temporary objects [class.temporary]
Para 2:

[Example:
class X {
    // ...
    public:
    // ...
    X(int);
    X(const X&);
    ˜X();
};
X f(X);

void g()
{
    X a(1);
    X b = f(X(2));
    a = f(a);
}

Here, an implementation might use a temporary in which to construct X(2) before passing it to f() using X’s copy-constructor; alternatively, X(2) might be constructed in the space used to hold the argument. Also, a temporary might be used to hold the result of f(X(2)) before copying it to `b usingX’s copyconstructor; alternatively,f()’s result might be constructed in b. On the other hand, the expressiona=f(a)requires a temporary for either the argument a or the result off(a)to avoid undesired aliasing ofa`. ]

^{Ref 1:}
C++03 12.8 Copying class objects [class.copy]
Para 12:

When certain criteria are met, an implementation is allowed to omit the copy construction of a class object, even if the copy constructor and/or destructor for the object have side effects…..