Enable_if check if the value type of the iterator is a pair
I would like to write a specialized template function for iterators whose value type is a pair. I expect this to match std :: map iterators.
To find a pair:
template <typename>
struct is_pair : std::false_type
{ };
template <typename T, typename U>
struct is_pair<std::pair<T, U>> : std::true_type
{ };
// also tried this, but it didn't help
template <typename T, typename U>
struct is_pair<std::pair<const T, U>> : std::true_type
{ };
Then I use enable_if in the function declaration:
template<class ITR>
decltype(auto) do_stuff(
std::enable_if<is_pair<typename ITR::value_type>::value, ITR> itr) {
//access of itr->second ok.
}
However, when I use this function with a map iterator, I get the following error messages from clang (Xcode 8.3):
Applicant template ignored: Could not match "enable_if" with "__map_iterator"
Without further explanation, why enable if not the same.
When checking the type of __map_iterator, it looks like it should match the is_pair check.
source to share
Better to move std::enable_if
to another default template argument like:
template<class ITR, typename = typename std::enable_if<is_pair<typename ITR::value_type>::value, ITR>::type>
decltype(auto) do_stuff(ITR && itr) {
//access of itr->second ok.
}
This will not block argument output as ITR && itr
it is now a generic reference.
Complete example:
#include <type_traits>
#include <utility>
#include <map>
template <typename>
struct is_pair : std::false_type
{ };
template <typename T, typename U>
struct is_pair<std::pair<T, U>> : std::true_type
{ };
template<class ITR, typename = typename std::enable_if<is_pair<typename ITR::value_type>::value, ITR>::type>
decltype(auto) do_stuff(ITR && itr) {
//access of itr->second ok.
}
int main()
{
std::map<int, int> foo{
{ 1, 2 },
{ 3, 4 },
};
do_stuff(foo.begin());
return 0;
}
source to share
I figured out basically this: SFINAE works in reverse, but not as a template parameter
So at the end I used this signature:
template<class ITR>
typename std::enable_if_t<is_pair<typename ITR::value_type>::value, ITR>::value_type::second_type& get_value_from_iterator(ITR itr)
The link was required because I need an actual reference to the value inside the map.
The configuration version of the check is not needed.
source to share