Copy constructor not called.

Copy constructor not called.

[Mustafa4LP]

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 
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Re: Copy constructor not called.

[Sven Eschenberg]

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 

Re: Copy constructor not called.

[John (Eljay) Love-Jensen]

Hi Mustafa,
 
> 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 );
> }

Your "copy constructor" is not an appropriate copy constructor.

Change it to:

A(A const& a)
{
  cout << "copy constructor" << endl;
}

Also, your assignment operator is not an appropriate assignment operator.

Change it to:

A const& operator = (A const& a)
{
  cout << "= operator" << endl;
  return *this;
}

HTH,
--Eljay

Re: Copy constructor not called.

[Mustafa4LP]

Hi Eljay,

I know that design wise the copy constructor and assignment operator are not
correct.
Actually, I am not writing the code for application but trying some test
case to understand C++ better.

The original program that I posted was correct in terms of C++ standards, it
came out to be a bug of GCC version 3.4.6.
When I tried the same on GCC version 4.3 it executed as expected.

Thanks for the help.

Regards, 
Mustafa


John (Eljay) Love-Jensen wrote:
> 
> Hi Mustafa,
>  
>> 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 );
>> }
> 
> Your "copy constructor" is not an appropriate copy constructor.
> 
> Change it to:
> 
> A(A const& a)
> {
>   cout << "copy constructor" << endl;
> }
> 
> Also, your assignment operator is not an appropriate assignment operator.
> 
> Change it to:
> 
> A const& operator = (A const& a)
> {
>   cout << "= operator" << endl;
>   return *this;
> }
> 
> HTH,
> --Eljay
> 
> 
> 

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