[boost] [bind] Function call operator is a template?!

Subject: [boost] [bind] Function call operator is a template?!
From: Andy Venikov (avenikov_at_[hidden])
Date: 2009-12-16 17:38:06

All this time I've been using bind quite happily without really going 
into the details of its implementation.
Recently, however, I had to discover the hard way that the function call 
operator that is produced as part of the bind() return is a template!

I guess most of the time one would not care if it's a template or not as 
long as you can call the resulting functor with the right signature.

It does, however, make a difference. The difference can be seen when 
calling the resulting functor with wrong arguments. The error detection 
happens earlier in the case when functor's operator() is a concrete 
function as opposed to a template. And that plays bad tricks with SFINAE.

You see, recently I was faced with a challenge where I had to check at 
compile time whether calling a template argument with a specific 
signature is a well-formed expression. Something similar to Eric 
Niebler's "can_be_called<>":
http://www.boost.org/doc/libs/1_41_0/doc/html/proto/appendices.html#boost_proto.appendices.implementation.function_arity

Well, this technique won't work with bind(), because the template member 
operator() will happily accept ANY argument during SFINAE and the error 
detection will happen much later, when you actually try to call the functor.

I'm not going to debate whether it's right or wrong, but I'm still 
curious - why did it have to be a template? When bind() is called it 
already has all the information about the function's signature so it 
could construct a concrete member operator() without resorting to 
templates. For example bind(main) could easily determine that the 
signature is <int (int, char * [])> and could create a functor with a 
single "int operator()(int, char *[])", instead of a template. It's not 
like it's lambdas, where when you say _1 + _2 you literally mean 
parameters of any type.

Thanks,
     Andy.