Declaring unordered_set with free functions for user-defined hash and comparison

c++, c++11

Solution

You can use:

std::unordered_set<int, size_t(*)(int), bool(*)(int, int)> my_set;
my_set s( 10, &IntHash, &IntComp );

And in order to use `std::bind`, you can use `decltype` or `std::function`

Problem

This code compiles fine with g++ 4.4 and '-std=c++0x'. ``` #include <unordered_set> namespace { size_t IntHash ( int i ) { return i; } bool IntComp ( int i, int j ) { return i == j; } } int main ( int argc, char* argv[] ) { typedef std::pointer_to_unary_function<int, size_t> CustomHash; typedef std::pointer_to_binary_function<int, int, bool> CustomComp; typedef std::unordered_set<int, CustomHash, CustomComp> DeprecatedSet; DeprecatedSet deprecatedSet ( 10, std::ptr_fun ( IntHash ), std::ptr_fun ( IntComp ) ); deprecatedSet.insert ( 5 ); deprecatedSet.insert ( 10 ); } ``` Say, however, I didn't want to use the deprecated std::pointer_to_unary_function and std::ptr_fun, but still use the free functions: ``` #include <unordered_set> #include <functional> namespace { size_t IntHash ( int i ) { return i; } bool IntComp ( int i, int j ) { return i == j; } } int main ( int argc, char* argv[] ) { typedef std::unordered_set<int /*, UserDefinedHash?, UserDefinedComparison? */> NewSet; NewSet newSet ( 10, std::bind ( IntHash, std::placeholders::_1 ), std::bind ( IntComp, std::placeholders::_1, std::placeholders::_2 ) ); newSet.insert ( 5 ); newSet.insert ( 10 ); } ``` This doesn't compile, I assume because I'm not sure what to put in for UserDefinedHash and UserDefinedComparison. It doesn't look like std::bind has a member type defining the type of the bind object itself. I'm aware that there's other ways to define custom hash functions and comparisons, just curious if free/class functions can be used without deprecated standard library types and functions.

Original source