Re: [Boost-users] Determine at compile time whether type has subtraction semantics?

Subject: Re: [Boost-users] Determine at compile time whether type has subtraction semantics?
From: John Reid (j.reid_at_[hidden])
Date: 2011-02-21 11:41:06

On 19/02/11 06:51, Frédéric Bron wrote:
>> Is it possible to determine at compile-time if a type has subtraction
>> semantics? E.g. in struct A<T>  I would like to decide at compile-time not to
>> execute&A::subtract in the register_fns method. Then the subtract method is
>> not instantiated and there is no error.
>
> You can look at the code that will be reviewed soon for an extension
> of type traits.
>
> #include<boost/type_traits/has_operator_minus.hpp>
>
> boost::has_operator_minus<T, T, T>::value is what you want:
> is true if x-y returns something convertible to T
> is false if operator-(T, T) does not exist.
>
> http://www.boostpro.com/vault/index.php?action=downloadfile&filename=type_traits.tar.bz2&directory=Extension&

Is there any way to do use the type traits to conditionally compile some 
code using some brief syntax? I.e. it would be nice if something like 
the following worked:

template< typename T >
void g( T x, T y ) {
        if( boost::has_operator_minus<T, T, T>::value ) {
                return x - y;
        } else {
                return T();
        }
}

but of course gcc 4.4.3 tries to compile the x-y regardless of whether 
it can ever be executed.

Or do you always need to specialise some struct or function elsewhere 
and then call a method on that struct or execute the function?

Thanks,
John.