What is the difference between "functions" and "function-like entities" in C++ algorithms?

Question

In C++20 the new namespace std::ranges was added to the standard, with many algorithms there duplicating already-existing ones, e.g. std::find_if and std::ranges::find_if. On the cppreference page the "old" algorithms are called "functions", and the "new" - "function-like entities". Yet, I can't use it in a "functional" way, e.g.

auto result = myMapObj
                | std::ranges::views::keys
                | std::ranges::find_if([](const auto& key) { return key == 42; });

I need to use it the same way as the good old std::find_if. So what is the difference, and which one should I choose in which case?

Answer 1

They're global objects with overloaded operator().

There are some marginal benefits in designing them this way (compared to function templates), e.g. being able to pass them as arguments to other functions (foo(std::ranges::find_if)) using a sane syntax (to pass std::find_if in this manner you'd need to wrap it in a lambda).

which one should I choose in which case?

You should always prefer the std::ranges::... versions, assuming your standard library implementation supports them.