Boost tuple + transform

Is it possible to do the following.

Say my boost tuple has <std::string, T>

I would like to use std::transform + mem_fun to insert only the std::string element in a corresponding vector. Is it possible or are we required to use a loop and push_back(get<0>)...

Ie the following doesn't like to compile... (unknown types...)

result.resize(storage.size())
std::transform(storage.begin(), storage.end(), result.begin(), std::mem_fun(&boost::get<0>));

Here is an example (trying one of the comments):

#include <boost/tuple/tuple.hpp>
#include <vector>
#include <string>
#include <algorithm>
#include <boost/bind.hpp>

template <typename T>
class TestClass
{
private:
    typedef boost::tuple<std::string,T> PairType;
    std::vector<PairType> storage;
public:
    void extract(std::vector<std::string> &result)
    {
        result.resize(storage.size());
        std::transform(storage.begin(), storage.end(), result.begin(), boost::bind(&PairType::get<0>, _1));
    }
};

int main(int argc, char**argv)
{

开发者_Python百科    TestClass<int> bb;
    std::vector< std::string> result;
    bb.extract(result);
    return 0;
}

g++ test.cpp 
test.cpp: In member function `void TestClass<T>::extract(std::vector<std::string, std::allocator<std::string> >&)':
test.cpp:17: error: expected primary-expression before ',' token
test.cpp: In member function `void TestClass<T>::extract(std::vector<std::string, std::allocator<std::string> >&) [with T = int]':
test.cpp:26:   instantiated from here
test.cpp:17: error: address of overloaded function with no contextual type information

---

Use the member version of get and Boost.Bind. I've tested this and it works, for what it's worth.

#include <algorithm>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>

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

int main()
{
    typedef boost::tuple<std::string,int> T;
    std::vector<T> v1;
    v1.push_back(T("Blah", 23));
    v1.push_back(T("Wibble", 9));

    std::vector<std::string> v2;
    std::transform(v1.begin(), v1.end(), std::back_inserter(v2), boost::bind(&T::get<0>, _1));

    std::copy(v2.begin(), v2.end(), std::ostream_iterator<std::string>(std::cout, "\n"));

    return 0;
}

---

The type of the get<0> overload that you want is:

const std::string& (*)(const boost::tuples::cons<std::string, boost::tuples::cons<int, boost::tuples::null_type> >&)

If you typedef that to get0_fn_t, then you can declare a pointer to this get<0> overload with:

get0_fn_t getter_fn = &boost::tuples::get<0, std::string, boost::tuples::cons<int, boost::tuples::null_type> >;

EDIT: This program is a complete working example:

#include <algorithm>
#include <cstdlib>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>
#include <boost/bind.hpp>
#include <boost/tuple/tuple.hpp>

int main()
{
    typedef boost::tuple<std::string, int> tuple_type;
    std::vector<tuple_type> tuples;
    tuples.push_back(boost::make_tuple(std::string("test3"), 3));
    tuples.push_back(boost::make_tuple(std::string("test0"), 0));

    std::vector<std::string> strings;
    typedef const std::string& (*get0_fn_t)(const boost::tuples::cons<std::string, boost::tuples::cons<int, boost::tuples::null_type> >&);
    get0_fn_t getter_fn = &boost::tuples::get<0, std::string, boost::tuples::cons<int, boost::tuples::null_type> >;
    std::transform(tuples.begin(), tuples.end(), std::back_insert_iterator<std::vector<std::string> >(strings), getter_fn);

    std::vector<std::string>::const_iterator it, end = strings.end();
    for (it = strings.begin(); it != end; ++it)
        std::cout << *it << std::endl;

    return EXIT_SUCCESS;
}

EDIT2: This shows how to integrate it into the TestClass template:

template <typename T>
class TestClass
{
private:
    typedef boost::tuple<std::string, T> PairType;
    std::vector<PairType> storage;

public:
    void extract(std::vector<std::string>& result) const
    {
        result.clear();
        typedef const std::string& (*get0_fn_t)(const boost::tuples::cons<std::string, boost::tuples::cons<T, boost::tuples::null_type> >&);
        get0_fn_t getter_fn = &boost::tuples::get<0, std::string, boost::tuples::cons<T, boost::tuples::null_type> >;
        std::transform(storage.begin(), storage.end(), result.begin(), getter_fn);
    }
};