When header is included for the first time in translation unit,
then preprocessor simply copy-pastes its content. If we define a
constant in a header file and this constant has internal linkage
then each and every translation unit, which includes this header
will have its own copy of this constant.
C++17 introduces inline variables, which are meant to allow creation
of variables in header files, which do not cause multiple instances.
The inline variable has a single instance when:
- constexpr is used without static (constexpr implicitly implies inline)
- inline is used without static
- inline const is used without static (const does not imply internal linkage
when used with inline)
Signed-off-by: Patryk Wrobel <patryk.wrobel@intel.com>
