Mac OSX/iPhone 上的 C 预处理器，使用 '# 钥匙？

发布于 2024-07-18 02:43:55 字数 171 浏览 10 评论 0原文

我正在查看一些开源项目，我看到以下内容：

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

大小符号之前的“#”到底是什么意思？这是 C 字符串的某种前缀吗？

原文

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?

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追星践月 2024-07-25 02:43:55

为了详细说明 dirkgently 的答案，这看起来像是一个宏的实现，它接受 NSSize （或类似的）参数，并打印变量的名称（这就是 # 所做的；将变量的名称转换为包含的字符串）变量的名称），然后是它的值。因此，在：中，

NSSize fooSize = NSMakeSize(2, 3);
MACRO_NAME_HERE(fooSize);

宏将扩展为：

NSLog(@"%s w=%f h=%f", "fooSize", fooSize.width, fooSize.height);

并打印：（

fooSize w=2.0 h=3.0

类似于 NSStringFromSize，但带有变量名称）

To elaborate on dirkgently's answer, this looks like the implementation of a macro that takes an NSSize (or similar) argument, and prints the name of the variable (which is what the # is doing; converting the name of the variable to a string containing the name of the variable) and then its values. So in:

NSSize fooSize = NSMakeSize(2, 3);
MACRO_NAME_HERE(fooSize);

the macro would expand to:

NSLog(@"%s w=%f h=%f", "fooSize", fooSize.width, fooSize.height);

and print:

fooSize w=2.0 h=3.0

(similar to NSStringFromSize, but with the variable name)

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苹果你个爱泡泡 2024-07-25 02:43:55

# 的正式名称是字符串化运算符。它采用其参数并将其用引号括起来以形成 C 字符串常量，并根据需要转义任何嵌入的引号或反斜杠。它只允许在宏定义中使用——在常规代码中不允许使用。例如：

// 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\\\"";

相关的预处理器运算符是标记粘贴运算符 ##。它需要两个令牌并将它们粘贴在一起以获得一个令牌。与字符串化运算符一样，它只允许在宏定义中使用，而不能在常规代码中使用。

// 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;

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;

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