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Constant Expressions Specifier

  • Book Excerpt from "Generative AI in C++"
  • by David Spuler, Ph.D.

Constant Expressions Specifier

The constexpr keyword is an optimization hint for the compiler that's more powerful than “const." Whereas const only guarantees that something won't change, constexpr is a guarantee by the human that something can be evaluated at compile-time.

The compiler should use the constexpr hint to try to propagate constant values throughout the evaluation of expressions and function calls, producing an overall speedup. However, if the compiler doesn't have the capability to do the level of compile-time optimization required, or if the human has told the machine a bald-faced lie, there's no penalty and the code just runs like it never had a constexpr specifier.

There's not a whole lot of difference between const and constexpr if you use it only for named constants: 

    const float PI = 3.14f;
    constexpr float PI = 3.14f;  // Same same

 

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