Pound sign in #define function parameters

Question

What does the pound sign indicate in this line of code?

#define CONDITION(x)    if(!(x)){ HandleError(#x,__FUNCTION__,__LINE__);return false;}

This is how it is being called:

CONDITION(foo != false);

AFAIK It's stringifying the condition. so you'll get "foo != false" in the handleerror call. — Borgleader, Feb 18 '15 at 14:17
@Anonymouse: Except in American English, where # is a pound sign (sometimes used to represent a pound weight), hash is banned in most states, and £ is a symbol of colonial oppression. If you want to be unambiguous, it's an "octothorpe". Or just #, since we're typing not speaking. — Mike Seymour, Feb 18 '15 at 14:34
@MikeSeymour Same in Canada. It's probably a NA vs Europe distinction. — Carcigenicate, Feb 18 '15 at 14:54

score 14 · Accepted Answer · answered Feb 18 '15 at 14:19

14

A single # before a macro parameter converts it to a string literal.

#define STRINGIFY(x) #x
STRINGIFY(hello)   // expands to "hello"

In your example, the string would be "foo != false", so that the error message shows the code that was being tested.

A double ## between two tokens within a macro combines them into a single token

#define GLOM(x,y) x ## y
GLOM(hello, World) // expands to helloWorld

answered Feb 18 '15 at 14:19

Mike Seymour

can you elaborate on the double pound example. – Mohit May 11 '15 at 06:15
@ahujamoh: As I said, it combines two tokens to make a single token (in the example, `hello` and `World` are combined to make `helloWorld`). I'm not sure how I can elaborate further, but your introductory book should describe it in detail. – Mike Seymour May 11 '15 at 10:10

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