What is the diffrence between std::greater{} and std::greater<int>()?

Question

Some people write

std::nth_element(v.begin(), v.begin()+1, v.end(), std::greater{});

and some write like this

std::nth_element(v.begin(), v.begin()+1, v.end(), std::greater<int>());

What is thre diffrence between std::greater{} and std::greater<int>()?

Answer 1

This is new as of C++14, and this results in two completely different classes.

In C++14, std::greater acquires a default value for its template parameter: void.

You end up with either std::greater<void> or std::greater<int>.

std::greater<void> is a specialization for a so-called "transparent" comparator that deduces its parameters, see its reference for more information.

Answer 2

std::greater<int> has an bool operator()(int const&,int const&) const that calls >.

std::greater<>, aka std::greater<void>, has a template references auto operator()(T const&, U const&) const that calls > and deduces its return value.

In c++11 and earlier, greater<void> and <> won't work.

In practice there will be no difference here. Under any non zero optimization, both versions of the comparison will be inlined.

In other contexts, the void version will let you compare dissimilar types (with suitable < or conversion overloads), and lets you neglect the type you are comparing.

The other big detail is that <void> advertises a is_transparent tag. When passed to a container like std::map, it changes some of the methods to have template arguments. This permits using a std::string key without forcing the lookup strings to become the same object type, which can save copies and allocations.