Comparator

In C++ tradition, the comparison condition is a predicate (a function that takes two arguments and returns true or false as a result of their comparison; satisfies the std::predicate concept), while in Python you may have seen that sort uses a key function to compare elements. I decided to mix these two approaches and create a abl::comparator struct that can be used to compare elements by both key and predicate at the same time. Anyway, this still satisfies the std::predicate concept as it is called with two arguments.

abl::comparator

The abl::comparator struct is simply an abstraction around a key function and a predicate. It can be constructed with one of them or both. The only aim is style.

Constructing

abl::comparator can be constructed with one of the following:

• a default constructor, std::identity as a key and a std::less predicate: abl::comparator<int>();
• a constructor with both a key function and a predicate, a full template should be specified:

auto square = [](const auto& a) { return a * a; };
auto leq = [](const auto& a, const auto& b) { return a <= b; };
auto comp = abl::comparator<int, decltype(square), decltype(leq)>(square, l);

• a factory member function abl::comparator<T>::make with one or both of the key function and predicate specified:

auto comp1 = abl::comparator<int>::make(square, leq);
auto comp2 = abl::comparator<int>::make(square);
auto comp3 = abl::comparator<int>::make(leq);

I personally find this syntax more readable than C++’s std::make_pair and Co.