Expand macros inside quoted string [duplicate]

Question

I have a function which accepts one argument of type char*, like f("string");
If the string argument is defined by-the-fly in the function call, how can macros be expanded within the string body?

For example:

#define COLOR #00ff00
f("abc COLOR");

would be equivalent to

f("abc #00ff00");

but instead the expansion is not performed, and the function receives literally abc COLOR.

In particular, I need to expand the macro to exactly \"#00ff00\", so that this quoted token is concatenated with the rest of the string argument passed to f(), quotes included; that is, the preprocessor has to finish his job and welcome the compiler transforming the code from f("abc COLOR"); to f("abc \"#00ff00\"");

The preprocessor does not touch string literals.

alk
– alk

2012-10-18 16:11:36 +00:00
Commented Oct 18, 2012 at 16:11 — alk
– alk, Commented Oct 18, 2012 at 16:11

András Kovács · Accepted Answer · 2016-05-08 15:23:53Z

58

You can't expand macros in strings, but you can write

#define COLOR "#00ff00"

f("abc "COLOR);

Remember that this concatenation is done by the C preprocessor, and is only a feature to concatenate plain strings, not variables or so.

edited May 8, 2016 at 15:23

András Kovács

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answered Oct 18, 2012 at 16:06

tomahh

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4 Comments

davide Over a year ago

Is this equivalent to passing 2 arguments to f() ?

singpolyma Over a year ago

@davide no, putting two string literals next to each other makes the C compiler concatenate them.

tomahh Over a year ago

No, the strings will be concat by the compiler. At compile time, the string will result in "abc #00ff00"

Barun Over a year ago

I believe the concatenation "abc ""#00ff00" to "abc #00ff00"is performed by compiler instead of preprocessor. gcc.gnu.org/onlinedocs/cpp/Stringification.html Notice in the second paragraph:

The preprocessor will replace the stringified arguments with string constants. The C compiler will then combine all the adjacent string constants into one long string.

singpolyma · Accepted Answer · 2012-10-18 16:07:35Z

4

#define COLOR "#00ff00"
f("abc "COLOR);

answered Oct 18, 2012 at 16:07

singpolyma

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1 Comment

phimuemue Over a year ago

I think it's worth noting that this works because C "automatically concatenates" two strings if they appear without anything in between.

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