Question

I’m trying to create predicate for std::find_if by using boost::bind together with boost::contains (from boost/algoritm/string library).
Following snippet shows two ways how I’m trying to accomplish this.

#include <boost/algorithm/string.hpp>
#include <boost/bind.hpp>
#include <boost/function.hpp>

#include <iostream>
#include <string>

int main(int argc, char** argv) {
        std::string s1("hello mom");
        std::string s2("bye mom");

        boost::function<bool (std::string, std::string)> f = &boost::contains<std::string, std::string>;
        std::cout << s1 << " contains " << "hello, " << std::boolalpha << f(s1, "hello") << std::endl;
        std::cout << s2 << " contains " << "hello, " << std::boolalpha << f(s2, "hello") << std::endl;

        boost::function<bool (std::string)> contain_hello = boost::bind(boost::contains<std::string, std::string>, _1, std::string("hello"));
        std::cout << s1 << " contains " << "hello, " << std::boolalpha << contain_hello(s1) << std::endl;
        std::cout << s2 << " contains " << "hello, " << std::boolalpha << contain_hello(s2) << std::endl;
        return EXIT_SUCCESS;
}

When compiling this code with g++ 3.4.5 I’m getting following output.

error: conversion from `<unresolved overloaded function type>' to non-scalar type `boost::function<bool ()(std::string, std::string), std::allocator<void> >' requested
error: no matching function for call to `bind(<unresolved overloaded function type>, boost::arg<1>&, std::string)'

When I switch to boost::icontains which has only one overload everyting works fine.
I know how to solve similar situation when there are multiple overloads of non-template function.
Can someone help me write this correctly? Or should I write my own compare function?

Answer 1

You need to write static_cast<bool(*)(const std::string&, const std::string&)>(&boost::contains<..>) to resolve the overload.

Yes, this is a royal pain with templates and overloading. Libs written with OOP and overloading in mind are difficult to use with templates and boost::bind.

We all wait for C++0x lambda expressions, which ought to resolve things better.