Question

#include <algorithm>
#include <functional>
#include <vector>

struct blah {
    int member;
};

int main (int argc, const char* argv[])
{
    blah a = { 1 };
    auto are_same = std::bind(
            std::equal_to<blah>(),
            a,
            std::mem_fn(&blah::member)   // Obviously not a function, but I tried.
        );

    std::vector<blah> blahs = { {0}, {1}, {2} };
    return static_cast<int>(std::any_of(blahs.begin(), blahs.end(), are_same));
}

This program fails to compile (GCC 4.4):

error: no match for call to ‘(std::equal_to<blah>) (blah&, std::_Mem_fn<int blah::*>&)’

Aside from coding the loop myself, what’s the correct way to check for equivalence based on a data member?

Answer 1

If you’re already using C++11, why not use a lambda?

int main (int argc, const char* argv[])
{
    blah a = { 1 };
    auto are_same = [=](blah const& b) -> bool {
        return a.member == b.member;
    };

    std::vector<blah> blahs = { {0}, {1}, {2} };
    return static_cast<int>(std::any_of(blahs.begin(), blahs.end(), are_same));
}

If you can’t use lambdas, you can use bind to do function composition:

using namespace std::placeholders;
auto are_same = std::bind(
  std::equal_to<int>(),
  std::bind(&blah::member, _1),
  std::bind(&blah::member, a)
);