\/\/ Error: call to runtime function `g`<\/em>\n<\/code><\/pre>\n\n\n\nOrdinary invocation is fine:<\/p>\n\n\n\n
int<\/strong> n =<\/strong> f(); \/\/ Ok<\/em>\n<\/code><\/pre>\n\n\n\nSuch functions like f<\/code> are a bit deceiving: despite the constexpr<\/code> specifier, they can\u2019t be evaluated at compile time.<\/p>\n\n\n\nWith this example in mind, a constexpr<\/code> function:<\/p>\n\n\n\n\n- may<\/strong> be available for compile-time evaluation, but no guarantee that it can<\/li>\n<\/ul>\n\n\n\n
Can consteval<\/code> functions call runtime functions?<\/h3>\n\n\n\nAt first, it may seem the answer is \u201cno\u201d. consteval<\/code> functions only evaluate at compile time, so any call to runtime function must be an error, right?<\/p>\n\n\n\nBut C++ is actually quite permissive here: just like constexpr<\/code> functions, you can write code to call runtime functions, so long as you don\u2019t actually reach that code during compile-time evaluation.<\/p>\n\n\n\n\/\/ runtime function<\/em>\nint<\/strong> g<\/strong>();\n\n\/\/ Ok, even though `f` can never be called<\/em>\nconsteval<\/strong> int<\/strong> f<\/strong>() {\n return<\/strong> g();\n}\n<\/code><\/pre>\n\n\n\nSo far, the only rule to remember is:<\/p>\n\n\n\n
\n- You cannot evaluate runtime functions at compile-time<\/strong><\/li>\n<\/ul>\n\n\n\n
Can runtime functions call constexpr<\/code> functions?<\/h3>\n\n\n\nMostly, yes. In fact, most of the functions in the standard library are now constexpr<\/code>; it would be really bad if users\u2019 runtime functions can\u2019t call them!<\/p>\n\n\n\nconstexpr<\/strong> size_t<\/strong> strlen_dual<\/strong>(const<\/strong> char<\/strong>*<\/strong> s) {\n if<\/strong> consteval<\/strong> {\n return<\/strong> strlen_ct(s); \/\/ compile-time path<\/em>\n } else<\/strong> {\n return<\/strong> strlen(s); \/\/ runtime path<\/em>\n }\n}\n\n\/\/ runtime function<\/em>\nint<\/strong> main(int<\/strong> argc, char<\/strong>**<\/strong> argv) {\n int<\/strong> n1 =<\/strong> strlen_dual(argv[0]); \/\/ Ok, takes the runtime path,<\/em>\n \/\/ because argv[0] is a runtime value<\/em>\n}\n<\/code><\/pre>\n\n\n\nCan consteval<\/code> functions call constexpr<\/code> functions?<\/h3>\n\n\n\nYes. Intuitively:<\/p>\n\n\n\n
\nconstexpr<\/code> functions support (at least advertise to) compile-time evaluation<\/li>\n\n\n\nconsteval<\/code> functions must evaluate at compile-time<\/li>\n<\/ul>\n\n\n\nSo consteval<\/code> functions just take the compile-time path for the constexpr<\/code> functions they call.<\/p>\n\n\n\nconstexpr<\/strong> bool<\/strong> is_constant_evaluated<\/strong>() {\n if<\/strong> consteval<\/strong> {\n return<\/strong> true; \/\/ compile-time path<\/em>\n } else<\/strong> {\n return<\/strong> false; \/\/ runtime path<\/em>\n }\n}\n\nconsteval<\/strong> bool<\/strong> f() {\n return<\/strong> is_constant_evaluated(); \/\/ true<\/em>\n}\n<\/code><\/pre>\n\n\n\nCan runtime functions call consteval<\/code> functions?<\/h3>\n\n\n\nYes, but only if all arguments are constant expressions.<\/p>\n\n\n\n
The evaluation is immediate and done at compile-time:<\/p>\n\n\n\n
consteval<\/strong> size_t<\/strong> strlen_ct<\/strong>(const<\/strong> char<\/strong>*<\/strong> s) {\n ...\n}\n\nint<\/strong> main() {\n return<\/strong> strlen_ct(\"\"); \/\/ same as return 0<\/em>\n}\n<\/code><\/pre>\n\n\n\nCan constexpr<\/code> functions call consteval<\/code> functions?<\/h3>\n\n\n\nThis is the interesting part.<\/p>\n\n\n\n
consteval<\/code> functions can always be invoked if all the arguments are constant expressions, thus code like strlen_ct(\"\")<\/code> works in constexpr<\/code> functions too.<\/p>\n\n\n\nThere are two things that separate constexpr<\/code> functions from runtime functions, both of which have to do with passing parameters down to consteval<\/code> functions.<\/p>\n\n\n\nif consteval<\/code><\/h4>\n\n\n\nFirstly, inside the if consteval<\/code> block, constexpr<\/code> functions\u2019 formal parameters can be used as arguments for consteval<\/code> functions.<\/p>\n\n\n\nThis is why our first example works:<\/p>\n\n\n\n
consteval<\/strong> size_t<\/strong> strlen_ct<\/strong>(const<\/strong> char<\/strong>*<\/strong> s) {\n ...\n}\n\nconstexpr<\/strong> size_t<\/strong> strlen_dual(const<\/strong> char<\/strong>*<\/strong> s) {\n if<\/strong> consteval<\/strong> {\n \/\/ `s` is promoted to be usable as arguments for consteval functions<\/em>\n return<\/strong> strlen_ct(s);\n } else<\/strong> {\n return<\/strong> strlen(s);\n }\n}\n<\/code><\/pre>\n\n\n\nImportantly, if consteval<\/code> doesn\u2019t turn function parameters into a full-fledged constant expressions:<\/p>\n\n\n\nconstexpr<\/strong> int<\/strong> identity<\/strong>(int<\/strong> a) {\n if<\/strong> consteval<\/strong> {\n \/\/ Doesn't work: error: 'a' is not a constant expression<\/em>\n static_assert<\/strong>( a ==<\/strong> a );\n return<\/strong> a;\n } else<\/strong> {\n return<\/strong> a;\n }\n}\n<\/code><\/pre>\n\n\n\nClosely related is the parameters of consteval<\/code> functions:<\/p>\n\n\n\n