A more elegant way for a template function grouping values? - c++

Question

The following code groups values for any container with a generic grouping lambda:
template<class Iterator, class GroupingFunc,
class T = remove_reference_t<decltype(*declval<Iterator>())>,
class GroupingType = decltype(declval<GroupingFunc>()(declval<T&>()))>
auto groupValues(Iterator begin, Iterator end, GroupingFunc groupingFunc) {
map<GroupingType, list<T>> groups;
for_each(begin, end,
[&groups, groupingFunc](const auto& val){
groups[groupingFunc(val)].push_back(val);
} );
return groups;
}
With the following usage:
int main() {
list<string> strs = {"hello", "world", "Hello", "World"};
auto groupOfStrings =
groupValues(strs.begin(), strs.end(),
[](auto& val) {
return (char)toupper(val.at(0));
});
print(groupOfStrings); // assume a print method
list<int> numbers = {1, 5, 10, 24, 13};
auto groupOfNumbers =
groupValues(numbers.begin(), numbers.end(),
[](int val) {
int decile = int(val / 10) * 10;
return to_string(decile) + '-' + to_string(decile + 9);
});
print(groupOfNumbers); // assume a print method
}
I am a bit reluctant regarding the (over?)-use of declval and decltype in groupValues function.
Do you see a better way for writing it?
(Question is mainly for better style and clarity unless of course you see any other issue).
Code: http://coliru.stacked-crooked.com/a/f65d4939b402a750

Answer 1

I would probably move the last two template parameters inside the function, and use std::result_of to give a slightly more tidy function:
template <typename T>
using deref_iter_t = std::remove_reference_t<decltype(*std::declval<T>())>;
template<class Iterator, class GroupingFunc>
auto groupValues(Iterator begin, Iterator end, GroupingFunc groupingFunc) {
using T = deref_iter_t<Iterator>;
using GroupingType = std::result_of_t<GroupingFunc(T&)>;
std::map<GroupingType, std::list<T>> groups;
std::for_each(begin, end, [&groups, groupingFunc](const auto& val){
groups[groupingFunc(val)].push_back(val);
});
return groups;
}
live demo