I tested the behavior of copy constructor with functions that return an object by value, and I came across a case where copy constructor does get invoked and a case it doesn't.
Please consider the following code:
class A {
public:
A() {}
A(const A& a) {
cout << "Copy CTOR: " << "This address is " << this
<< " input address is "<< &a << "\n";
}
};
A returnMyself(A& a) {
cout<<"Myself address is: "<< &a << "\n";
return a;
}
A returnLocal(A& a) {
A local;
cout<<"local address in returnLocal is "<< &local << "\n";
return local;
}
int main () {
A a;
cout<<"Before returnMyself\n";
returnMyself(a);
cout<<"After returnMyself\n\n";
cout<<"Before returnLocal\n";
returnLocal(a);
cout<<"After returnLocal\n";
}
The output of main is:
Before returnMyself. Myself address is: 0x7fff6afd88f0. Copy CTOR Invoked: This address is 0x7fff6afd88d8. Input address is 0x7fff6afd88f0. After returnMyself. Before returnLocal. Local address in returnLocal is 0x7fff6afd88d0. After returnLocal.
As you can see, when I declare a local object and return it, copy constructor doesn't get invoked, as opposed to returning a given reference-object which does invoke copy constructor.
Does anyone have an explanation for this? In general, what are the cases of invoking copy constructor from a function that returns-by-value?
Thanks!
