const in a header implicitely means
Recently on SO:
constexprimplies const and const on global/namespace scope implies static (internal linkage), which means that every translation unit including this header gets its own copy of PI. The memory for that static is only going to be allocated if an address or reference to it is taken, and the address is going to be different in each translation unit.
That implied static for const variables was introduced specifically to use const instead of #define in header files in C++ to define constants. Without static there would be multiple symbol definitions linker error if that header file is included in more than one translation unit which were linked together.
In C++17 you can also make it
inline, so that there is only ever a single copy of PI if an address or reference to it is taken (i.e. not
inlinevariables were introduced in C++17 to allow for header-only libraries with non-const variable definitions in the header files.
In other words, you should use
constexprfor your constants in header files, if possible, otherwise
const. And if you require the address of that constant to be the same everywhere mark it as