Re: Using assignment operator when rhs is temporary object.

Victor Bazarov <>
Mon, 05 Oct 2009 09:50:07 -0400
Vladimir Grigoriev wrote:

I have found a difference in behavior of a Borland C++ compiler and Visual
C++ 2005 EE compiler.

class A
    A( int i = 0 ): a( i ) {}
    A( A &rhs ): a( rhs.a ) {}
    virtual ~A(){}
    A & operator=( A &rhs ) // without const!
        if ( this != &rhs )
            a = rhs.a;

        return ( *this );

    virtual std::ostream & out( std::ostream &os ) const
        os << a;
        return ( os );

    int a;

inline std::ostream & operator<<( std::ostream &os, const A &rhs )
    return ( rhs.out( os ) );

A f()
    return ( A( 5 ) );

int _tmain(int argc, _TCHAR* argv[])
    A a1( 10 );
    A a2;

    a2 = f();

    return 0;

For the Borland C++ compliker the compilation is failed as the compiler does
not find a required function for the assignment statement a2 = f();
because the assignment operator has no attribute const for rhs parameter
while Visual C++ compiles the code without any error.

Which compiler is wrong according to the C++ standard?

Visual C++ is not wrong because you're most likely have C++ extensions
enabled. See in the settings: C/C++ | Language | Disable Language
Extensions. If extensions are not disabled, VC++ allows you to bind a
reference to non-const to a temporary. The Standard does not allow
that. As soon as you disable the extensions, you will get an error from
VC++ as well, IIRC. The problem, of course, is that with extensions
disabled, no Windows header will compile. :-)

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