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c++c++20preprocessor-directiveif-constexpr

#if Vs if constexpr

Which one is more appropriate for compile-time configurations (such as debug/release), preprocessor directives, or if constexpr?

#define DBG

#if DBG
// some code
#endif

// --------------------------------- or

inline constexpr bool DEBUG { true };

if constexpr ( DEBUG )
{
// some code
}
Solution

  • Note that, if if constexpr is not part of a template, then the other parts of the if (such as the elses) are still compiled and checked for validity.

    From cppreference:

    If a constexpr if statement appears inside a templated entity, and if condition is not value-dependent after instantiation, the discarded statement is not instantiated when the enclosing template is instantiated .

    Outside a template, a discarded statement is fully checked. if constexpr is not a substitute for the #if preprocessing directive: