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
include
/**
* 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 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]);
}