I have a callback-interface that can optionally process a "ServerError". If the callback looks at the error, there should be no default processing of the error.

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

When passing a callback function that does not look at the error, the call is ambiguous: both Boost.Functions could accept it. Is there a way to work around that problem? Can callback detect what kind of function it was passed?

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 haven't managed to realise why you get this specific error (maybe it's some weird MSVC bug), but Boost.Function expects a callable with the exact signature. Try the following: boost::function<void(int)> f1(xxx); boost::function<void(int, const ServerError&)> f2(xxx); and you'll see that the 2nd line does not compile.

On Mon, Jun 25, 2012 at 4:21 AM, Christoph Duelli <duelli@melosgmbh.de>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