Constexpr Functions recap

This section explored how to leverage compile-time evaluation to create more efficient and type-safe programs. Let's review the key concepts.

Constexpr Functions

A constexpr function is a function allowed to be called in a constant expression. To make a function a constexpr function, we simply use the constexpr keyword in front of the return type. Constexpr functions are only guaranteed to be evaluated at compile-time when used in a context requiring a constant expression. Otherwise they may be evaluated at compile-time (if eligible) or runtime. Constexpr functions are implicitly inline, and the compiler must see the full definition of the constexpr function to call it at compile-time.

Consteval Functions

A consteval function is a function that must evaluate at compile-time. Consteval functions otherwise follow the same rules as constexpr functions.

Key Terminology

  • Constexpr function: A function allowed to be called in a constant expression
  • Consteval function: A function that must be evaluated at compile-time
  • Constant expression: An expression that can be evaluated at compile-time
  • Compile-time evaluation: Evaluation performed during compilation rather than at runtime

Looking Forward

These concepts enable you to write more efficient code by moving computations from runtime to compile-time, resulting in faster program execution and earlier error detection. Constexpr and consteval functions are powerful tools that become increasingly important as you work with templates and compile-time programming in C++.