typically #define would be used to define a constant or a macro. However it is valid code to use #define in the following way.

#define MAX // does this do anything?
#define MAX 10 // I know how to treat this.

So, if I #define MAX 10, I know my pre-processor replaces all instances of MAX with 10. If someone uses #define MAX by itself however with no following replacement value, it's valid. Does this actually DO anything?

My reason for asking is that I am writing a compiler for c in c++ and handling preprocessor directives is required but I haven't been able to find out if there is any functionality I need to have when this occurs or if I just ignore this once my preprocess is done.

My first instinct is that this will create a symbol in my symbol table with no value named MAX, but it is equally possible it will do nothing.

As an add in question which is kind of bad form I know, but I'm really curious. Are there situations in real code where something like this would be used?

Thanks, Binx

It creates a symbol with a blank definition, which can later be used in other preprocessor operations. There are a few things it can be used for:

1) Branching.

Consider the following:

#define ARBITRARY_SYMBOL

// ...

#ifdef ARBITRARY_SYMBOL
    someCode();
#else   /* ARBITRARY_SYMBOL */
    someOtherCode();
#endif  /* ARBITRARY_SYMBOL */

The existence of a symbol can be used to branch, selectively choosing the proper code for the situation. A good use of this is handling platform-specific equivalent code:

#if defined(_WIN32) || defined(_WIN64)
    windowsCode();
#elif defined(__unix__)
    unixCode();
#endif /* platform branching */

This can also be used to dummy code out, based on the situation. For example, if you want to have a function that only exists while debugging, you might have something like this:

#ifdef DEBUG
    return_type function(parameter_list) {
        function_body;
    }
#endif  /* DEBUG */

1A) Header guards.

Building on the above, header guards are a means of dummying out an entire header if it's already included in a project that spans multiple source files.

#ifndef HEADER_GUARD
#define HEADER_GUARD

// Header...

#endif  /* HEADER_GUARD */

2) Dummying out a symbol.

You can also use defines with blank definitions to dummy out a symbol, when combined with branching. Consider the following:

#ifdef _WIN32
    #define STDCALL __stdcall
    #define CDECL __cdecl
    // etc.
#elif defined(__unix__)
    #define STDCALL
    #define CDECL
#endif  /* platform-specific */

// ...

void CDECL cdeclFunc(int, int, char, const std::string&, bool);
// Compiles as void __cdecl cdeclFunc(/* args */) on Windows.
// Compiles as void cdeclFunc(/* args */) on *nix.

Doing something like this allows you to write platform-independent code, but with the ability to specify the calling convention on Windows platforms. [Note that the header windef.h does this, defining CDECL, PASCAL, and WINAPI as blank symbols on platforms that don't support them.] This can also be used in other situations, whenever you need a preprocessor symbol to only expand to something else under certain conditions.

3) Documentation.

Blank macros can also be used to document code, since the preprocessor can strip them out. Microsoft is fond of this approach, using it in windef.h for the IN and OUT symbols often seen in Windows function prototypes.

There are likely other uses as well, but those are the only ones I can think of off the top of my head.