Detecting if an expression is constant in C

Detecting if an expression is constant in C 21 Apr 2025 Here's a fun little language puzzle: implement a macro that takes an expression as an argument and: Verifies that the expression is a constant expression (i.e, known at compile time), otherwise aborts the compilation. "Returns" back the same value. Optionally, the returned value has the same type as the original expression. There's a number of ways to solve this, depending on which C standard you're using and whether compiler extensions are allowed or not. Here are a few I've come across along with some pros and cons. static compound literals If you're using C23 or later then you can specify a storage duration for compound literals combined with typeof (also standardized in C23). #define C(x) ( (static typeof(x)){x} ) This keeps the same type, and since initializers of static storage duration need to be constant expression, the compiler will ensure that x is a constant expression. Cons: Requires C23, which isn't widely ... Read full article.