expressing _1.second->pair().first == r in boost::lambda

Question

I've an expression that I need to put in std::transform as callback, but I don't want to write another method for it. I want to express the expression _1.second->pair().first == r to boost::lambda assuming _1 is the only argument passed to it of type std::pair

I've a generic shorthand functors util::shorthand::pair_second and util::shorthand::pair_first which returns second and first item of a pair.

I can do boost::bind(util::shorthand::pair_second, _1) but then do what ? how to implement rest of the expression ?

-1.second is templated of type Connection<T>.I cannot use C++11

Answer 1

Take a look at Boost.Lambda bind expressions. You can use them to bind member functions and member data.

I don't have enough information as to the types that you are using, but something like this should work:

bind(&std::pair<S, R>::first,
     bind(&Connection<T>::pair,
          bind(&std::pair<U, Connection<T> >::second, _1))) == r

Note: The bind in this sample is the one in the boost::lambda namespace, not Boost.Bind.

EDIT: Complete, compilable example:

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

#include <boost/lambda/lambda.hpp>
#include <boost/lambda/bind.hpp>

// http://stackoverflow.com/questions/12026884/expressing-1-second-pair-first-r-in-boostlambda/

typedef int S;
typedef int R;

template <typename T>
struct Connection
{
    std::pair<S, R> pair() const {
        return std::make_pair(0, 0);
    }
};

int main()
{
    using namespace boost::lambda;

    typedef int T;
    typedef int U;

    std::vector<std::pair<U, Connection<T> > > vec;
    vec.push_back(std::make_pair(3, Connection<T>()));
    std::vector<bool> res(vec.size());

    int r = 0;
    std::transform(vec.begin(), vec.end(), res.begin(),
        bind(&std::pair<S, R>::first,
            bind(&Connection<T>::pair,
                bind(&std::pair<U, Connection<T> >::second, _1))) == r);

    std::vector<bool>::iterator it, end = res.end();
    for (it = res.begin(); it != end; ++it) {
        std::cout << ' ' << *it;
    }
    std::cout << std::endl;
}