stdpopcount

stdpopcount refers to the standard library facility std::popcount, a function introduced in C++20 that counts the number of set bits (ones) in an unsigned integer value. It is defined in the <bit> header and is designed to provide a portable, efficient way to obtain the population count of a value.

The function is a template constrained to unsigned integral types. The typical prototype is a templated, constexpr function: template<class T> constexpr int popcount(T x) noexcept; where T is an unsigned integer type. The return value is the number of one-bits contained in x, as an int, ranging from 0 up to the total number of bits in the type T.

Usage is straightforward: include <bit>, then call std::popcount with an unsigned value. For example, std::popcount(13u) yields 3, since 13 in binary is 1101. The function is usable in constant expressions, so it can be evaluated at compile time when given a constant input.

Implementation notes: std::popcount is typically optimized to use a single hardware instruction (popcnt) on platforms that support it; if not available, a portable algorithm provides the result. The operation is independent of runtime context and is useful in algorithms that require bit manipulation, such as parity checks, bitmap processing, and combinatorial computations.

Compatibility: std::popcount is part of the C++20 standard and requires a compiler with <bit> support for that standard. Major compilers (GCC, Clang, MSVC) provide it in recent versions. On older compilers or non-C++20 environments, developers may rely on compiler-specific intrinsics or custom bit-count implementations as alternatives.

See also: other bit-manipulation utilities in <bit>, such as bit_width, bit_floor, and count-leading-zero functions, which complement population counting in low-level programming tasks.