Undefined symbols for constexpr function

Question

When I attempt compiling the following code I get a linker error: Undefined symbols for architecture x86_64: \"Foo()\", referenced from: _main in main.o using L

Answer 1

It's an interesting question. As Andy Prowl, constexpr makes the function inline, which means that there must be a definition of it in every translation unit which uses it; I would have expected an error from the compiler. (Actually, if I read §3.2/5 correctly, a diagnostic is required if you use the function and there is no definition.)

As to why const has different behavior: you can't mark a non-menber function const. If you write const int Foo();, it is not the function which is const, but the type it returns (except that if the return type is not a class type, cv-qualifiers are ignored, so this is really the same as int Foo();).