Constexpr

constexpr is a specifier keyword in the C and C++ programming languages which, roughly speaking, specifies that something may be evaluated at compile time. Unlike the usual const, which specifies read-only access, constexpr is a stronger form that implies const (in most contexts). While in C this is limited only to objects, constexpr may be used on objects, functions, or structured bindings in C++.

Additional keywords in the C++ family of "const" keywords include consteval, used for guaranteeing a function executes at compile time and compile time only, and constinit, used for guaranteeing static/thread storage duration, ensuring compile-time initialization (but not immutability).

Background

The word constexpr is a portmanteau of constant expression, first introduced in C++11. Prior to the introduction of constexpr, options for specifying compile-time constants and functions in C++ were limited to preprocessor macros (such as #define PI 3.1415926535 or procedural macros like #define SQUARE(x) ((x) * (x))), which performed textual substitution and thus lacked the type safety of a core language feature, or declaring an enumerated constant (such as enum { WINDOW_SIZE = 600 };), which was limited only to integral types. A particular motivation for the development of compile-time expressions was that std::numeric_limits<T>::max(), although being equivalent to the macro INT_MAX, produced a non-constant expression.

Initial proposals for generalised constant expressions and compile-time expressions were first proposed for inclusion into C++ by Gabriel Dos Reis in 2003, proposing generalizing constant expressions to include constant-valued functions, functions which could be evaluated at compile time, with a particular goal being to improve type-safety and portability. This proposed was further revised with the co-authorship of Bjarne Stroustrup and Jens Maurer in 2006 to introduce the constexpr keyword, along with including user-defined literals (objects of user-defined types with constexpr constructors and destructors) into the definition of constant expressions., and adopted into the draft for C++11 in July 2007. Herb Sutter, chairman of the C++ committee, in 2026 described the adoption of constexpr as one of the most 'momentous' polls of C++ history.