Re: This fails in g++ but compiles in VC++ 2008. Need Help.

From:

"Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sun, 11 Jul 2010 07:30:38 +0200

Message-ID:

<i1bku7$tqi$1@news.eternal-september.org>

* Jag, on 11.07.2010 06:56:

template<typename T>
vertex<T>* sgraph<T>::find_a_vertex(const vertex<T>& v) {
        typename set<vertex<T> >::iterator itt = _vertices.find(v) ;
        if (itt == _vertices.end()) {
                return NULL ;
        }
        vertex<T>& sv = *itt ; //LINE X
        vertex<T>* asv =&(sv) ;
        return asv ;
}

LINE X
error: invalid initialization of reference of type 'vertex<int>&' from
expression of type 'const vertex<int>'

What exactly is the problem here? Thanks in advance

A std::set consists of immutable objects. If you could modify an object O in a
set directly, then the set object (that contains the object O) would know
nothing about the change of O, and could wind up with two or more elements that
compared as equal. Worse, in the internal structure that's hosting the elements,
the changed element could now be in an unsorted position.

So the std::set elements are 'const'.

And hence std::set<T>::iterator is really a const_iterator.

Cheers & hth.,

- Alf

--
blog at <url: http://alfps.wordpress.com>