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From: aejeet on 2 Jul 2010 06:16
Hi,
The following code samples must give the same output but are
giving different output.

class A{
public:
A(){}
A(const A&){cout<<"Copying myself"<<endl;}
};

A func(A a)
{
return a;
}

int main()
{
A a;
func(a);
}

output:
Copying myself
Copying myself

**********************


class A{
public:
A(){}
A(const A&){cout<<"Copying myself"<<endl;}
};

A func(A a)
{
A obj;
return obj;
}

int main()
{
A a;
func(a);
}

output:
Copying myself

DOUBT: In the second sample why is the copy constructor not being
called while returning the value ?


-Aejeet

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From: Peter C. Chapin on 2 Jul 2010 09:34
On 2010-07-02 17:16, aejeet wrote:

> DOUBT: In the second sample why is the copy constructor not being
> called while returning the value ?

This is most likely due to the "return value optimization." The local
object is actually constructed in the caller's address space where the
return value is going to ultimately be placed. Thus there is no need to
copy it when the function returns. If you Google for "RVO" or similar
things, you should find a lot about it.

Peter

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