Re: [boost] Adding helpful requires macros to the enable_if utility

On Fri, 27 Jun 2014 09:35:14 -0700, Andrey Semashev wrote:

On Friday 27 June 2014 09:10:33 paul Fultz wrote:

...

These following set of macros, I have found very useful in C++:

#define BOOST_ERROR_PARENTHESIS_MUST_BE_PLACED_AROUND_THE_RETURN_TYPE(...) __VA_ARGS__>::type #define BOOST_FUNCTION_REQUIRES(...) typename boost::enable_if_c<(__VA_ARGS__), BOOST_ERROR_PARENTHESIS_MUST_BE_PLACED_AROUND_THE_RETURN_TYPE

#define BOOST_CLASS_REQUIRES(...) typename boost::enable_if_c<(__VA_ARGS__)>::type

#define BOOST_REQUIRES(...) typename boost::enable_if_c<(__VA_ARGS__), int>::type = 0

Which I use like the following

For functions:

template <class T> BOOST_FUNCTION_REQUIRES(boost::is_arithmetic<T>::value) (T) foo(T t) { return t; }

And C++11 functions:

template T foo(T t) { return t; }

Class specializations:

template class A { ... };

template <class T> class A { ... };

template <class T> class A { ... };

It would be nice if these were the added with the enable_if utility with boost.

Frankly, I'd prefer using enable_if directly instead of macros.

-1. I find the use of macros to be far more readable than the direct use of templates. In fact, I was earlier toying with the idea of writing such macros myself. Needless to say that I would find them to be very helpful. Paul, can the concept be extended to a DSEL-like utility? Something along the lines of: template <class T> BOOST_FUNCTION_REQUIRES(is_foo(T) and is_bar(T)) (T) foo(T t) { return t; } where "is_foo", "and", and "is_bar" are PPMP keywords. "Non-standard" conditions can be introduced via the "explicit" keyword, like: template <class T> BOOST_FUNCTION_REQUIRES(is_foo(T) and is_bar(T) or explicit(is_baz<T>::value)) (T) foo(T t) { return t; } , or clients can be given a mechanism to hook in their own keywords. (I have a mental sketch of how the latter could work, nothing implemented or tested.) Just thinking out loud ...