In C, with #define, we can use the token pasting operator ## to concatenate an identifier with some other text.

Is it possible to do the reverse as seen in the example below?

#define millimeter
//truncate to milli
//convert to millisecond

I am specifically trying to use this for function name generation, so a call to a non-existent function millimeter() can generate code containing millisecond.

The previous paragraph is an example of how I would use this. I have included it for context, but I am not sure if that is possible even if the identifier can be truncated. If you can link related questions for this part, that would be helpful.

CodePudding user response：

Terrible code adapted from this answer: https://stackoverflow.com/a/37355243/1046249

TRUNCATE(x) will do the reverse of paste by truncating a predefined set of tokens. Then CAT(x,y) pastes y onto the end of x so CAT(TRUNCATE(millimeter),second)() becomes millisecond().

I have no idea why you would ever want to do this.

#define TRUNCATE_millimeter  milli
#define TRUNCATE_milligram   milli

#define TRUNCATE(s) TRUNCATE_ ## s

#define _CAT(x,y) x ## y
#define CAT(x,y) _CAT(x,y)

int millisecond() {
    return 0;
}

int main()
{
    CAT(TRUNCATE(millimeter), second)();
    CAT(TRUNCATE(milligram), second)();

    return 0;
}

CodePudding user response：

The units of input to the C preprocessor are (preprocessing) tokens such as identifiers, operators, and string literals. Tokens are atomic as far as the preprocessor is concerned -- it has no mechanism for dividing them into pieces or for operating in any other way on partial tokens.

Under some circumstances, you can use macro substitution to emulate truncation of a pre-selected (by you) set of specific tokens that have the form of identifiers or keywords, but this is not generalizable.

I am specifically trying to use this for function name generation

Where this sort of thing is done, it is generally done by pasting tokens together, not truncating them.