Many great functions offered by the C++ standard library go criminally underused. I've listed a few of my favorites here.

C++11

• std::iota fills a data structure with incrementing values.

C++17

• std::unique iterates through a sorted data structure and puts adjacent duplicate values at the end of the structure.
• std::count counts values in a data structure that compare equal to an argument.
• std::set_intersection finds the common elements in 2 sorted data structures.

C++20

• std::clamp takes 3 arguments and "clamps" the 2nd argument into the range of the 1st and 3rd.
• std::partial_sum prefix sums a data structure.
• std::accumulate sums up the values of a data structure.
• std::adjacent_difference creates a difference array.
• <bit> has various functions for easier bitwise manipulation. Featuring std::popcount.
• std::midpoint averages 2 arguments, rounding towards the first.

C++23 (Not supported by Codeforces)

• std::unreachable marks unreachable code to enable compiler optimization and debug trapping. (Pre-C++23 alternatives: GCC/Clang: __builtin_unreachable() MSVC: __assume(false))

Miscellaneous

• std::max and std::min take an initializer list, so you don't have to nest them within themselves.

If I missed anything, comment it and I'll add it.