Re: [boost] ConceptGCC 4.3.0 Alpha 6 is now available

From: Douglas Gregor (doug.gregor_at_[hidden])
Date: 2007-04-12 10:48:58

On Apr 12, 2007, at 12:16 AM, Mathias Gaunard wrote:
> Thank you.
> This is awesome.
>
> A version of GCC, and even precompiled binaries, with all those C++0x
> features is a good thing as it will allow early usage and spreading of
> those new features.

The "OSL5" regression tester for Boost CVS HEAD tests a version of  
GCC with all of those C++0x features *except* concepts, so we can  
start (conditionally) using these features in Boost libraries.

> Could you give more info about the incompability between variadic
> templates and concepts though?

Don't write a constrained template (i.e., one with any concept  
requirements placed on it) that uses variadic templates, e.g., the  
following will probably crash:

        auto concept OutputStreamable<typename T> {
                std::ostream operator<<(std::ostream&, const T&);
        }

        void print() { }

        template<typename T, typename... Rest>
        requires OutputStreamable<T>, OutputStreamable<Rest>...
        void print(const T& x, const Rest&... rest) {
                print(x);
                print(rest...);
        }

Take away the "requires" clause and it should be fine, because you  
would be using variadic templates separately from concepts.

        Cheers,
        Doug