Re: [Boost-users] [bind, tuple] Custom comparison of tuple elements

Mark Ruzon wrote:

Assume we have a range of objects of type tuple and a function object that compares two objects of type T1. How would a person sort this range using this function object and bind? I want something along the lines of:

std::sort(first, last, boost::bind<bool>(comp, boost::bind(boost::get<0>, _1), boost::bind(boost::get<0>, _2)));

but this isn't remotely close to compiling. I could create a custom function object, but I'd rather learn how to use bind properly.

A very inconvenient question. :-) In principle, when you want to use bind with a function template: template <typename T> int f( T const & x); you supply its template parameters when binding it: bind( f<int>, _1 ); When you have several overloads: template <typename T> int f(const type1<T>& x); template <typename T> int f(const type2<T>& x); you cast it to the correct type or use a function pointer: int (*pf)( const type2<int>& ) = &f; bind( pf, _1 ); Going by this logic: T1 const & (*get0)( tuple const & ) = &boost::get<0>; std::sort(first, last, boost::bind<bool>(comp, boost::bind(get0, _1), boost::bind(get0, _2))); This doesn't work, though. get<0> doesn't take a tuple, it actually takes a cons, an implementation detail of tuple: T1 const & (*get0)( tuple::inherited const & ) = &boost::get<0>; This works under VC 8, at least. It doesn't link under VC 7.1; the compiler doesn't instantiate the correct get<0>. In conclusion... boost::get<> and boost::bind don't mix well. It would be easier to just write a get0 function for your tuple type or even a function object for the comparison.