Question

I’m looking at some open source projects and I’m seeing the following:

NSLog(@"%s w=%f, h=%f", #size, size.width, size.height)

What exactly is the meaning of ‘#’ right before the size symbol? Is that some kind of prefix for C strings?

Answer 1

The official name of # is the stringizing operator. It takes its argument and surrounds it in quotes to make a C string constant, escaping any embedded quotes or backslashes as necessary. It is only allowed inside the definition of a macro — it is not allowed in regular code. For example:

// This is not legal C
const char *str = #test

// This is ok
#define STRINGIZE(x) #x
const char *str1 = STRINGIZE(test);      // equivalent to str1 = "test";
const char *str2 = STRINGIZE(test2"a\"");  // equivalent to str2 = "test2\"a\\\"";

A related preprocessor operator is the token-pasting operator ##. It takes two tokens and pastes them together to get one token. Like the stringizing operator, it is only allowed in macro definitions, not in regular code.

// This is not legal C
int foobar = 3;
int x = foo ## bar;

// This is ok
#define TOKENPASTE(x, y) x ## y
int foobar = 3;
int x = TOKENPASTE(foo, bar);  // equivalent to x = foobar;