Re: c++/4035: g++ actually copies a returned object when returning from a function/operator

[Gabriel Dos Reis <gdr@codesourcery.com>]

The following reply was made to PR c++/4035; it has been noted by GNATS.

From: Gabriel Dos Reis <gdr@codesourcery.com>
To: zach@cs.uni-bonn.de
Cc: gcc-gnats@gcc.gnu.org
Subject: Re: c++/4035: g++ actually copies a returned object when returning from a function/operator
Date: 16 Aug 2001 14:56:49 +0200

 zach@cs.uni-bonn.de writes:
 
 [...]
 
 | It seems to me that g++ actually copies a value when it sees a 'return X'
 | and when X is of class type.
 
 That is a conformant behaviour, assuming the return type has the same type.
 
 | If so, I think this is a major missing feature, if not really a big problem.
 | The standard does not require a copy.
 
 The standard explicitly says that that 
 	
 	return X;
 
 is semantically a copy-initialization :
 
 	ReturnType return_value = X;
 
 So, copying is a conformant behaviour.
 	
 | In fact, it explicitely allows to omit the copy completely -
 | see [class.tempoorary] and [class.copy, clause 15]:
 
 That permission isn't a requirement.  And in that respect g++ is
 violating no standard rule.
 
 And even if the actual copy is elided, the copy-contructor must be
 "callable". 
 
 [...]
 
 | In the example file I have attached, there is this piece of code:
 | 
 |     Dop DopTransform::operator * ( const Dop &dop ) const
 |     {
 |         Dop result;
 |         for ( int k = 0; k < NumOri; k ++ )
 |                 result[k] =   c[k][0] * dop[ Bb[k][0] ]
 |                             + c[k][1] * dop[ Bb[k][1] ]
 |                             + c[k][2] * dop[ Bb[k][2] ]
 |                             + o[k];
 |         return result;
 |     }
 | 
 | I get an error message saying
 | 
 | 	ColDopTree.cpp:226: no matching function for call to `col::Dop::Dop(col::Dop&)'
 | 	ColDopTree.h:74: candidates are: col::Dop::Dop(const col::Dop*)
 | 	ColDopTree.h:73:                 col::Dop::Dop(const osg::Pnt3f&)
 
 g++ is right.
 
 | (see the full output of g++ below).
 | 
 | This is because I have defined the copy constructor of class Dop 'explicit' like
 | this (of course):
 | 
 |     explicit Dop( const Dop &source );
 
 What usefulness is that?  Declaring a copy-constructor `explicit' is
 asking for trouble.
 
 | I should add, maybe, that my code compiles without any problems/warnings
 | on SGI (with SGI's C++ compiler) and under cygwin with Intel's compiler.
 
 That compiler is wrong then.
 
 Please remove the 'explicit' specifier from the copy-constructor
 declaration, it is not useful.
 
 -- Gaby