Re: [boost] [GSoC] [Boost.Hana] Formal review request

On 2014-08-03 15:56, pfultz2 wrote:

...

Could you post an example? Is the concept check done in the return type as with enable_if? In C++11, we can use a default template parameter, rather than the return type, which is what the `CONCEPT_REQUIRES_` uses. So for example, we can write:

template())> auto foo(T&&... xs) { ... }

This works as long as all the varidiac template types are to be deduced by the compiler.

Thanks for the explanation. Good to know!

For classes this won't work:

// ERROR template())> struct foo { ... };

Because you can't have a default template parameter after the varidiac template parameters in this context(you can when using speczializations).

Of course, you can't use `CONCEPT_REQUIRES_` for class specialization. In my library, I do provide a `TICK_CLASS_REQUIRES` which can be used for class specializations. Note that since it relies on `enable_if` it won't resolve ambiguities like Concepts Lite will. For example:

template<typename T> struct foo;

template<typename T> struct foo())> { ... }; // Here we have to add a check for `RandomAccessIterator` to avoid ambiguities template<typename T> struct foo() && !RandomAccessIterator<T>())> { ... };

Perhaps, there is a way to this using tag dispatching, I'm not sure.

Thanks and regards, Roland