copy constructor in g++

[Sean]

the language spec tells us that a temporary class will be created when a
function's return value is an object. And the copy constructor will be
called (if it has been defined) when the temporary object is created.
But when I compile the following code with g++, there is no copy
constructor call. Why?

#include <iostream>
using namespace std;
class myclass{
int i;
public:
myclass(){ cout << "constructor ...\n";}
myclass(const myclass &ob){ cout << "copy constructor...\n";}
~myclass(){ cout << "destructor...\n";}
};
myclass foo(){ myclass temp; cout << "calling foo...\n"; return temp}
int main(){
myclass a;
a = foo();
}

Here I copy the output of the program. Supposely, there should be a
temporary object created, but I don't see its constructing/destructing
procedure, as the output shows. Why does this happen? is it a g++
implementation specific behavior? Thanks

[dash]$g++ copyconstructor1.cpp
[dash]$ a.out
constructor ...
constructor ...
calling foo...
destructor...
destructor...

 

[John Harrison]

the language spec tells us that a temporary class will be created when a
function's return value is an object. And the copy constructor will be
called (if it has been defined) when the temporary object is created.
But when I compile the following code with g++, there is no copy
constructor call. Why?

#include <iostream>
using namespace std;
class myclass{
int i;
public:
myclass(){ cout << "constructor ...\n";}
myclass(const myclass &ob){ cout << "copy constructor...\n";}
~myclass(){ cout << "destructor...\n";}
};
myclass foo(){ myclass temp; cout << "calling foo...\n"; return temp}
int main(){
myclass a;
a = foo();
}

C++ also allows the copy constructor call to be eliminated for
efficiency reasons. It allows this even if the copy constructor has some
side effect.

Here I copy the output of the program. Supposely, there should be a
temporary object created, but I don't see its constructing/destructing
procedure, as the output shows. Why does this happen? is it a g++
implementation specific behavior? Thanks

It is, although I imagine many other compilers would do the same. We had
a recent post here which showed that VC++ 7 eliminated the copy
constructor call only in the case where it was user defined. If it was
not defined then an extra call to the destructor proved that the default
copy constructor was getting called. Maybe you could try the same
experiment.

john