Re: Friend operators don't work where class member operators do.

From:
"James Kanze" <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
8 Jan 2007 06:38:59 -0500
Message-ID:
<1168248675.446111.82650@11g2000cwr.googlegroups.com>
cesar tejeda wrote:

I have a C++ question. I'm having problems when defining operators as
friends of the class, and those problems get solved when I define the
operators as class members.


The rules concerning the two are subtly different.

The problem I have is that I have the operators defined like this:

template<class real> class vect3d{
.... skipping .....
template<class r> friend vect3d<r>
operator+(const vect3d<r>& vl,const vect3d<r>& vr);
friend real operator*(vect3d& vl,vect3d& vr){


Question: does the operator* really modify it's arguments? If
not, they should probably be declared const.

return (vl[0]*vr[0]+vl[1]*vr[1]+vl[2]*vr[2]);
};
.... skipping .....
}

and when I do like this:

vect3d a(1.,0.,0.),b(0.,1.,0.),c(0.,1.,0.);
cout<<"a*(a+b)="<<a*(a+b)<<endl;

I get the error:

test2.cpp: In function `int main()':
test2.cpp:170: error: no match for 'operator*' in 'a * operator+(const
vect3d<real>&, const vect3d<real>&) [with real = double]((&b))'
test2.cpp:55: error: candidates are: double operator*(vect3d<double>&,
vect3d<double>&)


That's because you didn't declare the parameters to operator*
const, and you cannot bind the temporary resulting from the +
operator to a non-const reference.

But doing like this :

cout<<"a*b="<<(a*b)<<"; a+b="<<(a+b)<<endl;

I get no errors.


That's because you don't try to bind a temporary to the
non-const reference.

Can someone tell me why?
I can not understand why, if I redefine the operators like this (making
them class members):

template<class real> class vect3d{
.... skipping .....
vect3d<real> operator+(vect3d<real> const & vr);
friend real operator*(vect3d& vl,vect3d& vr){


This looks exactly the same as the previous version. Are you
sure you didn't make operator* a member as well (or make its
parameters const)? Or perhaps have operator+ return a
reference?

return (vl[0]*vr[0]+vl[1]*vr[1]+vl[2]*vr[2]);
};
.... skipping .....
};

I get no error this time..., it is suppoused to be the same.


No it's not. One of the differences is that you can call a
non-const member function on a temporary, but you cannot bind a
temporary to a non-const reference. Given a member function:

     real operator*( vect3d& rhs ) ;

"a*(b+c)" should be illegal, but "(b+c)*a" no.

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