gcc needs copy consturctor for inplace constructed object passed to func as reference, others not

[nutty]

Hi all,

I have the following problem ( explanation below code ):

// main.cpp

class noncopyable
{
protected:
noncopyable() {}
~noncopyable() {}
private: // emphasize the following members are private
noncopyable( const noncopyable& );
const noncopyable& operator=( const noncopyable& );
};

class MyClass: noncopyable
{
public:
MyClass( ) { }
};

class IFormatter
{
public:
void func( const MyClass& ) { }
virtual ~IFormatter( ){}
};
int main()
{
MyClass mc;
IFormatter f;
f.func( MyClass( ) ); // doesn't work
f.func( mc ); // works
return 0;
}

This code is accepted by comeau in strict mode and by all MSVC that I
had used 7.0 up to 8.0
..

gcc 4.0.2 complains that the copy constructor of MyClass is private.

Now I wannted to further simplify by removing the noncopyable base and
instead making the copy and assignment private in MyClass.


class MyClass
{
public:
MyClass( ) { }
private:
MyClass( MyClass const& );
MyClass const& operator=( MyClass const& );
};

class IFormatter
{
public:
void func( MyClass const& ) { }
virtual ~IFormatter( ){}
};
int main()
{
MyClass mc;
IFormatter f;
f.func( MyClass( ) );
return 0;
}


surprisingly, now even comeau complains:

Comeau C/C++ 4.3.3 (Aug 6 2003 15:13:37) for ONLINE_EVALUATION_BETA1
Copyright 1988-2003 Comeau Computing. All rights reserved.
MODE:strict errors C++

"ComeauTest.c", line 35: error: "MyClass::MyClass(const MyClass &)",
required for
copy that was eliminated, is inaccessible
f.func( MyClass( ) );
^


Could it be possible that comeau is wrong for one or the other example?
Or did I change something significantly in my simplification?

Could someone try this with a gcc 4.1.x or 3.x.x ?

I appreciate your help and thank you in advance.

Ingo

 

[Butterfly]

when you call f.func() like this

f.func( MyClass( ) );

compiler has to create tempolary object, that means copy contructor
will be called. You declare copy constructor as a private, so that's
why compiler complains you

 

[Jakob Bieling]

when you call f.func() like this

f.func( MyClass( ) );

compiler has to create tempolary object, that means copy contructor
will be called. You declare copy constructor as a private, so that's
why compiler complains you

I do not see why the copy ctor would have to be called. Afaik, the
temporary is created using the ctor taking no arguments, and a const
reference to this object is passed. No object copying involved.

regards

 

[Markus Schoder]

Hi all,

I have the following problem ( explanation below code ):

// main.cpp

class noncopyable
{
protected:
noncopyable() {}
~noncopyable() {}
private: // emphasize the following members are private
noncopyable( const noncopyable& );
const noncopyable& operator=( const noncopyable& );
};

class MyClass: noncopyable
{
public:
MyClass( ) { }
};

class IFormatter
{
public:
void func( const MyClass& ) { }
virtual ~IFormatter( ){}
};
int main()
{
MyClass mc;
IFormatter f;
f.func( MyClass( ) ); // doesn't work
f.func( mc ); // works
return 0;
}

This code is accepted by comeau in strict mode and by all MSVC that I
had used 7.0 up to 8.0
.

gcc 4.0.2 complains that the copy constructor of MyClass is private.

Now I wannted to further simplify by removing the noncopyable base and
instead making the copy and assignment private in MyClass.


class MyClass
{
public:
MyClass( ) { }
private:
MyClass( MyClass const& );
MyClass const& operator=( MyClass const& );
};

class IFormatter
{
public:
void func( MyClass const& ) { }
virtual ~IFormatter( ){}
};
int main()
{
MyClass mc;
IFormatter f;
f.func( MyClass( ) );
return 0;
}


surprisingly, now even comeau complains:

Comeau C/C++ 4.3.3 (Aug 6 2003 15:13:37) for ONLINE_EVALUATION_BETA1
Copyright 1988-2003 Comeau Computing. All rights reserved.
MODE:strict errors C++

"ComeauTest.c", line 35: error: "MyClass::MyClass(const MyClass &)",
required for
copy that was eliminated, is inaccessible
f.func( MyClass( ) );
^


Could it be possible that comeau is wrong for one or the other example?
Or did I change something significantly in my simplification?

Binding an rvalue to a const reference invokes implementation-defined
behaviour. An implementation can bind directly to the object or copy
construct a temporary object. Comeau obviously does one or the other
depending on the context.

In other words for your code to be portable the copy constructor must
be accessible.

Could someone try this with a gcc 4.1.x or 3.x.x ?

gcc 4.1.1 complains as well in both cases.

 

[Markus Schoder]

Binding an rvalue to a const reference invokes implementation-defined
behaviour. An implementation can bind directly to the object or copy
construct a temporary object. Comeau obviously does one or the other
depending on the context.

In other words for your code to be portable the copy constructor must
be accessible.

Actually from the wording in the standard it looks like the copy
constructor must be accessible in all cases even if the copy is not
done which would mean that Comeau gets this wrong.

 

[nutty]

Markus Schoder schrieb:

Actually from the wording in the standard it looks like the copy
constructor must be accessible in all cases even if the copy is not
done which would mean that Comeau gets this wrong.

Thank you,

your answers, both, finally helped.

So that means for me, either I need to avoid this syntax or I make the
copy constructor available, knowing that at least in release mode no
compiler would ever use it.

Ingo

 

[Tom Widmer]

Actually from the wording in the standard it looks like the copy
constructor must be accessible in all cases even if the copy is not
done which would mean that Comeau gets this wrong.

Indeed, Daveed Vandevoorde admitted the bug in this post:

http://groups.google.co.uk/group/comp.lang.c++.moderated/msg/0bb0ffb131e94bea

Tom