Re: [boost] How can I detect if a template static member function exists?

Subject: Re: [boost] How can I detect if a template static member function exists?
From: Edward Diener (eldiener_at_[hidden])
Date: 2016-10-16 09:10:31

On 10/16/2016 7:16 AM, Vinnie Falco wrote:
> I'd like to write a trait:
>
> template<class T>
> struct has_content_length :
>   std::integral_constant<bool, ...>{};
>
> which detects whether or not a template static member function is
> present in T. Here's a typical T:
>
> struct X
> {
>   template<bool isRequest, class Body, class Headers>
>   static
>   std::uint64_t
>   content_length(
>     message<isRequest, Body, Headers> const& m);
> };
>
> How can I detect if X::content_length(m) is callable for any m of type
> message template?

It is not possible AFAIK. The tti library can detect whether a class 
template is a member of a type but it cannot detect whether a function 
template is a member of a type. The reason for this, within tti, is that 
class templates and function templates are not treated orthogonally in 
C++; a class template can be a template parameter but a function 
template cannot be a template parameter. If a function template could be 
a template parameter then tti could detect a function template as a 
member of a type, whether that function template were a static or a 
non-static member.

I have felt that the lack of orthogonality in C++, where a class 
template can be a template parameter but a function template cannot be a 
template parameter, is a design weakness of C++. But I would have to 
convince the C++ standards committee of the practicality of allowing a 
function template to be a template parameter to possibly change this.  I 
doubt whether saying that the change would allow the Boost tti library 
to detect function templates in the same way it can detect class 
templates, would be a good enough practical reason <g>.