What are the applications of the ## preprocessor operator and gotchas to consider?

ghz 1years ago ⋅ 2908 views

Question

As mentioned in many of my previous questions, I'm working through K&R, and am currently into the preprocessor. One of the more interesting things — something I never knew before from any of my prior attempts to learn C — is the ## preprocessor operator. According to K&R:

The preprocessor operator ## provides a way to concatenate actual arguments during macro expansion. If a parameter in the replacement text is adjacent to a ##, the parameter is replaced by the actual argument, the ## and surrounding white space are removed, and the result is re-scanned. For example, the macro paste concatenates its two arguments:

#define paste(front, back) front ## back

so paste(name, 1) creates the token name1.

How and why would someone use this in the real world? What are practical examples of its use, and are there gotchas to consider?

Answer

CrashRpt: Using ## to convert macro multi-byte strings to Unicode

An interesting usage in CrashRpt (crash reporting library) is the following:

#define WIDEN2(x) L ## x
#define WIDEN(x) WIDEN2(x)
//Note you need a WIDEN2 so that __DATE__ will evaluate first.

Here they want to use a two-byte string instead of a one-byte-per-char string. This probably looks like it is really pointless, but they do it for a good reason.

 std::wstring BuildDate = std::wstring(WIDEN(__DATE__)) + L" " + WIDEN(__TIME__);

They use it with another macro that returns a string with the date and time.

Putting L next to a __ DATE __ would give you a compiling error.

Windows: Using ## for generic Unicode or multi-byte strings

Windows uses something like the following:

#ifdef  _UNICODE
    #define _T(x)      L ## x
#else
    #define _T(x) x
#endif

And _T is used everywhere in code

Various libraries, using for clean accessor and modifier names:

I've also seen it used in code to define accessors and modifiers:

#define MYLIB_ACCESSOR(name) (Get##name)
#define MYLIB_MODIFIER(name) (Set##name)

Likewise you can use this same method for any other types of clever name creation.

Various libraries, using it to make several variable declarations at once:

#define CREATE_3_VARS(name) name##1, name##2, name##3
int CREATE_3_VARS(myInts);
myInts1 = 13;
myInts2 = 19;
myInts3 = 77;
#c