Re: Using SFINAE to check for function existance

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Mon, 22 Oct 2007 22:36:02 -0700

Message-ID:

<ffk18q$c5c$1@murdoch.acc.Virginia.EDU>

dascandy@gmail.com wrote:

I think it would be possible to use SFINAE or something to detect
whether a certain function is present or not. I would like to use one
approach if it has the function (calling it for a result) and another
when it doesn't exist (calling a substitute function).

Here is an example:

  template < typename T >
  class has_clone {
    /*
      stolen from Rani Sharoni, who attributes this to
      Richard Smith and also Artem Livshits

      With a fix to deal with const and non-const versions.
    */

    template < typename S, S* ( S::* ) ( void ) >
    struct dummy {};

    template < typename S, S* ( S::* ) ( void ) const >
    struct dummy_const {};

    template < typename S >
    static
    yes_type check ( dummy< S, &S::clone > * );

    template < typename S >
    static
    yes_type check ( dummy_const< S, &S::clone > * );

    template < typename S >
    static
    no_type check ( ... );

  public:

    static bool const value = sizeof( check<T>(0) ) == sizeof( yes_type );

  }; // has_clone

I've thought about using an interface instead, but that requires all
classes using it to implement that interface;

and that would be bad?

moreover it won't ever be called virtually.

huh? I lost the reference of "it". If "it" still refers to "interface", then
this sentence seems to be false.

I would like to have it work mostly "automatically".

Again, the reference of "it" seems to be dangling.

My thoughts were along the lines of "pass the size of a function
pointer of that function in the respective class, if it doesn't exist
pass 0 instead".

How do you pass an unsigned int "in the respective class"? I am not
following.

I can then select based on the sizeof whether it
supports the function. The problem is, if it has the function, both
templates match equally well. If it doesn't have it, the first is
eliminated based on sfinae but the second still matches, so it works
only if it doesn't exist.

You lost me completely. Could you illustrate your idea in code?

Am I missing something?

I cannot tell.

How can I make this work other than using an interface for matching?

If only I knew what "this" refers to ...

Best

Kai-Uwe Bux