iterator adapter to iterate just the values in a map? - c++

Question

I'm just getting back into C++ after a couple of years of doing a lot of C#, and recently Objective C.
One thing I've done before is to roll my own iterator adapter for std::map that will deref to just the value part, rather than the key-value pair. This is quite a common and natural thing to do. C# provides this facility with its Keys and Values properties of its Dictionary class. Objective-C's NSDictionary, similarly, has allKeys and allValues.
Since I've been "away", Boost has acquired the Range and ForEach libraries, which I am now using extensively. I wondered if between the two there was some facility to do the same, but I haven't been able to find anything.
I'm thinking of knocking something up using Boost's iterator adapters, but before I go down that route I thought I'd ask here if anyone knows of such a facility in Boost, or somewhere else ready made?

Answer 1

Replacing the previous answer, in case anybody else finds this like I did. As of boost 1.43, there are some commonly used range adaptors provided. In this case, you want boost::adaptors::map_values. The relevant example:
http://www.boost.org/doc/libs/1_46_0/libs/range/doc/html/range/reference/adaptors/reference/map_values.html#range.reference.adaptors.reference.map_values.map_values_example

Answer 2

I don't think there's anything out of the box. You can use boost::make_transform.
template<typename T1, typename T2> T2& take_second(const std::pair<T1, T2> &a_pair)
{
return a_pair.second;
}
void run_map_value()
{
map<int,string> a_map;
a_map[0] = "zero";
a_map[1] = "one";
a_map[2] = "two";
copy( boost::make_transform_iterator(a_map.begin(), take_second<int, string>),
boost::make_transform_iterator(a_map.end(), take_second<int, string>),
ostream_iterator<string>(cout, "\n")
);
}

Answer 3

There is a boost range adaptor for exactly this purpose.
See http://www.boost.org/doc/libs/1_53_0/libs/range/doc/html/range/reference/adaptors/reference/map_values.html
(This example cribbed from there)
int main(int argc, const char* argv[])
{
using namespace boost::assign;
using namespace boost::adaptors;
std::map<int,int> input;
for (int i = 0; i < 10; ++i)
input.insert(std::make_pair(i, i * 10));
boost::copy(
input | map_values,
std::ostream_iterator<int>(std::cout, ","));
return 0;
}

Answer 4

Continuing David's answer, there's another possibility to put the boile by creating a derived class from boost::transform_iterator. I'm using this solution in my projects:
namespace detail
{
template<bool IsConst, bool IsVolatile, typename T>
struct add_cv_if_c
{
typedef T type;
};
template<typename T>
struct add_cv_if_c<true, false, T>
{
typedef const T type;
};
template<typename T>
struct add_cv_if_c<false, true, T>
{
typedef volatile T type;
};
template<typename T>
struct add_cv_if_c<true, true, T>
{
typedef const volatile T type;
};
template<typename TestConst, typename TestVolatile, typename T>
struct add_cv_if: public add_cv_if_c<TestConst::value, TestVolatile::value, T>
{};
} // namespace detail
/** An unary function that accesses the member of class T specified in the MemberPtr template parameter.
The cv-qualification of T is preserved for MemberType
*/
template<typename T, typename MemberType, MemberType T::*MemberPtr>
struct access_member_f
{
// preserve cv-qualification of T for T::second_type
typedef typename detail::add_cv_if<
std::tr1::is_const<T>,
std::tr1::is_volatile<T>,
MemberType
>::type& result_type;
result_type operator ()(T& t) const
{
return t.*MemberPtr;
}
};
/** #short An iterator adaptor accessing the member called 'second' of the class the
iterator is pointing to.
*/
template<typename Iterator>
class accessing_second_iterator: public
boost::transform_iterator<
access_member_f<
// note: we use the Iterator's reference because this type
// is the cv-qualified iterated type (as opposed to value_type).
// We want to preserve the cv-qualification because the iterator
// might be a const_iterator e.g. iterating a const
// std::pair<> but std::pair<>::second_type isn't automatically
// const just because the pair is const - access_member_f is
// preserving the cv-qualification, otherwise compiler errors will
// be the result
typename std::tr1::remove_reference<
typename std::iterator_traits<Iterator>::reference
>::type,
typename std::iterator_traits<Iterator>::value_type::second_type,
&std::iterator_traits<Iterator>::value_type::second
>,
Iterator
>
{
typedef boost::transform_iterator<
access_member_f<
typename std::tr1::remove_reference<
typename std::iterator_traits<Iterator>::reference
>::type,
typename std::iterator_traits<Iterator>::value_type::second_type,
&std::iterator_traits<Iterator>::value_type::second
>,
Iterator
> baseclass;
public:
accessing_second_iterator():
baseclass()
{}
// note: allow implicit conversion from Iterator
accessing_second_iterator(Iterator it):
baseclass(it)
{}
};
This leads to even cleaner code:
void run_map_value()
{
typedef map<int, string> a_map_t;
a_map_t a_map;
a_map[0] = "zero";
a_map[1] = "one";
a_map[2] = "two";
typedef accessing_second_iterator<a_map_t::const_iterator> ia_t;
// note: specify the iterator adaptor type explicitly as template type, enabling
// implicit conversion from begin()/end()
copy<ia_t>(a_map.begin(), a_map.end(),
ostream_iterator<string>(cout, "\n")
);
}