CODEWITHSUNDEEP
cgccmacros
ph1ash
ph1ash

Reputation: 371

How does the C pre-proccesor know what to expand and what not to?

Let's say we have a macro defined as such

#define VALUE_ENTRY(a, b, c, d, e, f, g) \
    case E_##a##e: temp.a##e = d; break;

How does the pre-processor know that the 'e' in 'temp' shouldn't be expanded? Is it due to not having ## in front of the 'e'?

Also, should temp.a##e be temp.##a##e?

Upvotes: 4

Views: 63

Answers (1)

Qaz
Qaz

Reputation: 61950

The preprocessor works on tokens. e by itself is a token, whereas the e in temp is just a character that's part of the larger temp token. ## applies only to tokens.

Furthermore, pasting must produce a single token. Therefore, pasting . and a is not valid; temp.a##e is correct, and pastes a and e to form temp.<a><e>, where <a> and <e> are the replacement texts of a and e.

Upvotes: 6

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