How can I make std::find_if and std::map work together using some boost library?

问题

This question is inspired from another topic which poses this question:

Find the first value greater than user specified value from a map container

which can be solved in several ways. A typical C++03 solution defines a dedicated function (or functor) and pass it to std::find_if as third argument.

In C++11, one can avoid defining a dedicated function (or functor), and can instead make use of lambda as:

auto it = std:: find_if(m.begin(), mp.end(), 
                    [n](const std::pair<std::string, int> & x) -> bool
                       { return x.second > n; }
                   );

which is the accepted answer.

I'm still looking for a short and cool solution. If it were a vector, then I just learnt a cool solution which makes use of Boost.Phoenix and the solution becomes very concise (ideone demo):

std::vector<int> v = ...;
auto it = std::find_if(v.begin(), v.end(), arg1 > 4);

Here arg1 is a functor object defined in boost::phoenix::arg_names namespace, and the expression arg1>4 evaluates to another functor which then gets passed to std::find_if.

A quick test is (ideone),

std::cout<< (arg1 > 9)(v) << std::endl; //prints 0 if as v > 9 is false, else 1

//or store the functor first and then use it
const auto & f = arg1 > 9;
std::cout<<  f(v) << std::endl; //prints 0 if as v > 9 is false, else 1

---

My question is, I want to solve the map problem, in a similar way. Is there any such solution? Something like:

auto it = std::find_if(m.begin(),mp.end(), (???).second > n); //m is std::map

Or,

auto it = std::find_if(m.begin(),mp.end(), at<1>(arg1) > n);  //m is std::map

For it to work, the expression at<1>(arg1) > 2 has to evaluate to a functor which takes const std::pair & as argument. My gut feelings tells me that boost has this solution. :-)

回答1:

Indeed, Boost.Fusion and Boost.Phoenix have exactly what you want built-in.

If one includes the necessary header to adapt std::pair<> as a conforming Fusion sequence, then one can use Phoenix's lazy version of boost::fusion::at_c<> to access std::pair<>::first or std::pair<>::second (be sure to #include <boost/phoenix/fusion.hpp>).

namespace phx = boost::phoenix;
using phx::arg_names::arg1;

auto it = std::find_if(m.begin(), m.end(), phx::at_c<1>(arg1) > n);

---

EDIT: Full sample, tested with VC++ 2010 SP1 + Boost 1.47.0:

#include <algorithm>
#include <map>
#include <string>
#include <iostream>
#include <boost/fusion/include/std_pair.hpp>
#include <boost/phoenix/core.hpp>
#include <boost/phoenix/operator.hpp>
#include <boost/phoenix/fusion.hpp>

int main()
{
    namespace phx = boost::phoenix;
    using phx::arg_names::arg1;

    std::map<std::string, int> m;
    m["foo"]    = 1;
    m["bar"]    = 2;
    m["baz"]    = 3;
    m["qux"]    = 4;
    m["quux"]   = 5;
    m["corge"]  = 6;
    m["grault"] = 7;
    m["garply"] = 8;
    m["waldo"]  = 9;
    m["fred"]   = 10;
    m["plugh"]  = 11;
    m["xyzzy"]  = 12;
    m["thud"]   = 13;

    int const n = 6;
    auto it = std::find_if(m.cbegin(), m.cend(), phx::at_c<1>(arg1) > n);
    if (it != m.cend())
        std::cout << it->first << '\n'; // prints "fred"
}

来源：https://stackoverflow.com/questions/7337414/how-can-i-make-stdfind-if-and-stdmap-work-together-using-some-boost-library