Question

I’d like to use std::make_pair usable with e.g. std::bind2nd so that I get an unary function object which I could use with e.g. std::transform.

Right now I’m using

template <typename T, typename U>
struct pair_creator : std::binary_function<T, U, std::pair<T, U> >
{
    std::pair<T, U> operator()( T arg1, U arg2 ) const {
        return std::make_pair( arg1, arg2 );
    }
};

// ...

std::transform( start1, end2, start2, std::bind2nd( pair_creator<int, bool>(), true ) );

but I wonder – is there an easier way to make std::make_pair (or potentially any other binary function) usable with the binders except by writing little wrapper classes like pair_creator by hand?

I’d need a C++03 solution (for some obscure reason, stackoverflow always rewrites my c++0x tag to c++11 when saving the post…).

Answer 1

You need std::ptr_fun, which turns a plain function pointer into an adaptable binary function object (or a unary function object, if you pass it a one-arg function):

#include <functional>
#include <utility>
#include <vector>
#include <algorithm>
#include <iostream>

int main() {
    std::vector<int> intvec;
    intvec.push_back(0);
    intvec.push_back(1);
    std::vector<std::pair<int,bool> > pairvec(intvec.size());
    std::transform(
        intvec.begin(),
        intvec.end(),
        pairvec.begin(),
        // this is the significant line
        std::bind2nd(std::ptr_fun(std::make_pair<int, bool>), true)
    );
    std::cout << pairvec[1].first << " " << pairvec[1].second << "\n";
}

ptr_fun is declared:

template <class Arg1, class Arg2, class Result>
pointer_to_binary_function<Arg1,Arg2,Result>
ptr_fun(Result (*)(Arg1,Arg2));

And for the unary version:

template <class Arg, class Result>
pointer_to_unary_function<Arg,Result>
ptr_fun(Result (*)(Arg));