Why prefer constexpr or enum class over #define for constants?

A #define is preprocessor text substitution: it's untyped, ignores scope (it leaks from definition to end of translation unit), has no debug symbol, and is prone to the macro hazards from the C preprocessor lesson, precedence bugs without parentheses, and collisions because it ignores namespaces and classes. A constexpr constant (or an enum class for related integer values) is a real, typed, scoped entity: the compiler type-checks it, it respects namespaces and class membership, the debugger can show it by name, and it's guaranteed compile-time so it costs nothing at runtime and can live in flash. You get every benefit a #define constant provided, compile-time value, no runtime cost, with none of the downsides. The only place you still need the preprocessor is genuine compile-time configuration (#ifdef feature flags), not constants.