Re: [Boost-users] [Boost.Function] detecting ignored arguments?

Subject: Re: [Boost-users] [Boost.Function] detecting ignored arguments?
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-06-25 13:50:48

On Mon, Jun 25, 2012 at 4:21 AM, Christoph Duelli <duelli_at_[hidden]>wrote:
[...]

> I have created a class callback that accepts two kinds of Boost.Functions:
> one with, one without an error object.
>
[...]

> Some code illustrating my question:
>
>
> #include <boost/function.hpp>
> class ServerError;
>
> template<typename T>
> struct callback
> {
>   typedef boost::function<void(T)> cb_func;
>   cb_func callback_;
>   typedef boost::function<void(T,const ServerError&)> cb_err_func;
>   cb_err_func callback_err_;
>   callback(const cb_func &cb) : callback_(cb) {}
>   callback(const cb_err_func &cb) : callback_err_(cb) {}
> };
>
>
> static void xxx(int) {}
>
> int main (int argc, char **argv)
> {
> // does not compile: ambiguous...
>   callback<int> cb(&xxx);
> // does compile: but I'd like the compiler (or my lib) to automatically
> pick
> // the right one
>   callback<int> cb(callback<int>::cb_func(&xxx));
>   return 0;
> }

[...]

I think this might due to boost::function's templated constructor being
unconstrained [1]:

template<typename F> functionN(F);

It looks to me, then, that a void (*)( int ) is, at least superficially,
convertible to boost::function< Signature > regardless of Signature (hence
the ambiguity), but the body of the converting constructor will only
compile if Signature and void ( int ) are compatible. Perhaps worth it to
add a boost::enable_if to the functionN constructor to restrict the class
of function pointers that can bind to it.

> In the meantime I have found a possible workaround:
> If I change
>   typedef boost::function<void(T,const ServerError&)> cb_err_func;
> to
>   typedef boost::function<void(const ServerError&,T)> cb_err_func;
> the code does compile.

Okay, never mind, my theory above seem to run counter to this fact :(

- Jeff

[1] http://www.boost.org/doc/libs/1_49_0/doc/html/boost/functionN.html