Constexpr

Constexpr is a keyword in C++ that designates expressions, variables, functions, or constructors as candidates for constant-expression evaluation, enabling compile-time computation when possible. It first appeared in C++11 and has been extended in later standards to broaden what can be evaluated at compile time. A constexpr declaration signals to the compiler that the entity should be usable in contexts that require constant expressions, such as array sizes or template parameters.

Constexpr variables must be initialized with a constant expression. They are implicitly const and have internal linkage by default at namespace scope. Example: constexpr int N = 16; This value can be used at compile time wherever a constant expression is required.

Constexpr functions are declared with the constexpr specifier. When called with constant-expression arguments, the function can be evaluated at compile time, producing a constant expression value. If the arguments are not constant expressions, the function call is performed at run time, just like a regular function. Example: constexpr int square(int x) { return x * x; } constexpr int nine = square(3);

Constexpr constructors allow creating objects in constant expressions. A class with a constexpr constructor enables objects that can participate in compile-time computations, which is useful for constant data structures and template metaprogramming. In addition, member functions and, in some cases, more complex class features can be declared constexpr to extend compile-time capabilities.

Notes: constexpr implies const for objects, but not every const variable is constexpr. The rules governing constexpr expressions have evolved across standards; early versions imposed strict limits, while newer standards broadened permissible constructs for more robust compile-time computation. Regardless, a constexpr expression is guaranteed to be a constant expression when used in a context that requires one, while calls with non-constant inputs may still execute at runtime.