Argument Preceded by a # Token in a Macro

Question

#define LINK_ENTITY_TO_CLASS(mapClassName,DLLClassName) \
    static CEntityFactory<DLLClassName> mapClassName( #mapClassName );

This is a macro from the Alien Swarm mod for Half-Life 2, meant to be compiled with MSVC.

I've never seen an argument preceded by a # in a macro before, and I'm not sure if this is a MSVC specific thing or just uncommon. What does it mean?

Answer 1

This is part of both standard C and C++ and is not implementation-specific. The # preprocessing operator stringizes its argument. It takes whatever tokens were passed into the macro for the parameter designated by its operand (in this case, the parameter mapClassName) and makes a string literal out of them. So, for a simple example,

#define STRINGIZE(x) # x

STRINGIZE(Hello World)
// gets replaced with
"Hello World"

Note that the argument tokens are not macro replaced before they are stringized, so if Hello or World were defined as a macro, the result would still be the same. You need to use an extra level of indirection to get the arguments macro replaced (that linked answer discusses the concatenation operator, ##, but applies equally to the stringization operator.