constexpr

GCC9 already implements std::is_constant_evaluated . I played a little bit with it, and I realized it is somewhat tricky. Here’s my test: constexpr int Fn1() { if constexpr (std::is_constant_evaluated()) return 0; else return 1; } constexpr int Fn2() { if (std::is_constant_evaluated()) return 0; else return 1; } int main() { constexpr int test1 = Fn1(); // Evaluates to 0 int test2 = Fn1(); // Evaluates to 0 int const test3 = Fn1(); // Evaluates to 0 constexpr int test4 = Fn2(); // Evaluates to 0 int test5 = Fn2(); // Evaluates to 1 int const test6 = Fn2(); // Evaluates to 0 } According to

Static integral data members initialized in the class definition may be declared const or constexpr , but non-integral static data members initialized in the class definition must be constexpr : class MyClass { static const int w = 5; // okay static constexpr int x = 5; // okay static const float y = 1.5; // error! static constexpr float z = 1.5; // okay }; Does anybody know why the declaration for y is not permitted? The part of the Standard making it illegal is 9.4.2/3, but why is it illegal? Prior to C++11, you could not initialize static members of non-integral/enumeration types in the

This question already has an answer here: Is it possible to use std::string in a constexpr? 4 answers Can C-Strings or std::string s be created as constexpr or must they be created at run-time? With gcc 4.9.2 I can do this: constexpr const char foo[] = "blee"; (Sadly the November 2013 Customer Technology Preview does not allow Visual Studio to support this: https://stackoverflow.com/a/29255013/2642059 ) But even with gcc 4.9.2 I cannot do this: constexpr const std::string foo = "blee"; I get the error: error: the type 'const string {aka const std::basic_string<char>}' of constexpr variable