🚀 go-pugleaf

In article <m0tupLs-000CajC@sqarc.sq.com> willer@carolian.com (Steve Willer)
writes:

> I saw in the brochure for Borland C++ 5.0 that it will include support for
> numerous new C++ features, including the "typename" keyword. Could anyone
> enlighten me as to what that is? I couldn't find it in the April WP.
> 
> This isn't equivalent to the "typeof" keyword that I want so much, is it?

No.  The typename keyword is a consequence of the new rules for name
resolution in templates.  Brief summary: there's a new disambiguation
rule saying that a name used in a template is assumed not to be a type
unless it has been explicitly declared to be a type.  

Disambiguation rules like this are good, since they make it possible
to check template syntax even before the template has been
instantiated.  This one, though, presents a problem.  Suppose you want
to write code like this.
    template<class Container>
    void f(Container X)
    {
       Container::value_type tmp = *(X.begin());
       // do something with tmp.
    }
This is a syntax error, since Container::value_type is assumed not to
be the name of a type.  (And I hope everyone agrees that this isn't a
contrived example.  The STL source is full of examples like this.)

The typename keyword was introduced to solve this problem; it says
that the name following it refers to a type.  The correct way to write
this code snippet, then, is as follows.
    template<class Container>
    void f(Container X)
    {
       typename Container::value_type tmp = *(X.begin());
       // do something with tmp.
    }

This is described in section 14.2 [temp.res] of the WP.
-- 
Matt Austern
SGI: MTI Compilers Group
austern@isolde.mti.sgi.com
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