So last week I wrote about initializing a std::map from two std::vectors. How about doing the reverse? How do I get a list of the keys or a list of the values into a vector?

So I wrote a functor, based on std::unary_function, that returns the pair.first and another that returns the pair.second:

#include <utility>
#include <functional>
 
    /**
     * A functor that returns the first in a pair
     */
    template<typename PairType>
    struct first: public std::unary_function<PairType, typename PairType::first_type>
    {
	typedef typename PairType::first_type result_type;
	typedef PairType argument_type;
 
	result_type operator()(argument_type &p) const
	{
	    return p.first;
	}
 
	result_type operator()(const argument_type &p) const
	{
	    return p.first;
	}
    };
 
    /**
     * A functor that returns the second in a pair
     */
    template<typename PairType>
    struct second: public std::unary_function<PairType, typename PairType::second_type>
    {
	typedef typename PairType::second_type result_type;
	typedef PairType argument_type;
 
	result_type operator()(argument_type &p) const
	{
	    return p.second;
	}
	result_type operator()(const argument_type &p) const
	{
	    return p.second;
	}
    };

The first() template functor (from above) is used as the functor that returns the key to the std::transform() alogrithm. Here is a little fragment code that uses first(). (The test code part of a larger CPPUNIT test suite):

void TestUtils::testFirst()
{
    typedef std::map<std::string,std::string> String2StringMap;
    String2StringMap m;
 
    m["a"] = "0";
    m["z"] = "1";
    m["x"] = "2";
 
    std::vector<std::string>	keys;
    std::transform (m.begin(), m.end(),
		    std::back_inserter (keys),
		    first<String2StringMap::value_type>());
    CPPUNIT_ASSERT_EQUAL (m.size(), keys.size());
    CPPUNIT_ASSERT_EQUAL (std::string ("x"), keys[1]);
}