Re: Copy constructor not called.

[Sven Eschenberg <eschenb at cs dot uni-frankfurt dot de>]

As far as I can tell:

Your assignment Operator does not return a reference, that's why the 
compiler complains about not finding a matching copy contructor. Afaik 
semantically ( b = a ) would yield a copy constructor to create a 
temporary for the RV, which is A and not A & in your case. I don't know 
though, why a const in the copy constructor fixes the compile error as well.


And I want to quote from the man page:
       -fno-elide-constructors
           The C++ standard allows an implementation to omit creating a 
tempo-
           rary which is only used to initialize another object of the same
           type.  Specifying this option disables that optimization, and
           forces G++ to call the copy constructor in all cases.


As I read this this only relates to temporarys used for initialization. 
I don't see any initialization, only a RV temporary passed to the 
assignment operator. Thus the RVO will still be done and not be disabled 
by that option, as I read it.

Regards

-Sven


Mustafa4LP schrieb:
> Hi all,
>
> Please consider the following program:
>
> #include <iostream>
> using namespace std;
> class A
> {
> public:
>    A ()
>    {
>       cout << "constructor" << endl;
>    }
>    A (A &a)
>    {
>       cout << "copy constructor" << endl;
>    }
>    A operator = (const A &a)
>    {
>       cout << "= operator" << endl;
>    }
> };
> int main()
> {
>    A a;
>    A b;
>    b = ( b = a );
> }
>
>
> Following error is issued while running the above test case on gcc 3.4.6
>
> test007.cpp: In function `int main()':
> test007.cpp:23: error: no matching function for call to `A::A(A)'
> test007.cpp:11: note: candidates are: A::A(A&)
>
> However, if I change the argument of copy constructor to "const A &", the
> program compiles finely.
> I could not understand why const is required in copy constructor for running
> the test case.
>
> Also note that in the output of the modified program (which compiled
> finely), no copy constructor is called.
> I am running the test case with --no-elide-constructors option to disable
> the RVO (so that I can get all copy constructor calls).
>
> Thanks in advance for any help.
>
> Regards,
> Mustafa