c++ - boost 元组+变换

Question

是否可以做到以下几点。

假设我的 boost 元组有 <std::string, T>

我想使用 std::transform + mem_fun 仅将 std::string 元素插入相应的 vector 中。是否有可能或者我们需要使用循环和 push_back(get<0>)...

即以下不喜欢编译...(未知类型...)

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

这是一个例子(尝试其中一个评论):

#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)
{

    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

Answer 1

使用成员版本的get 和Boost.Bind。我已经对此进行了测试，它确实有效，物有所值。

#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;
}