Objective-C中的宏调用isEqualToString：产生有关无效令牌的错误 [英] Macro in Objective-C calling isEqualToString: produces error about invalid token

问题描述

我正在尝试定义这样的宏：

I'm trying to define a macro like this:

#define SOME_DEF [[TTys getString] isEqualToString:ANOTHER_STRING]

然后执行以下操作：

#if SOME_DEF
...
#endif

• [TTys getString] 返回NSString


• ANOTHER_STRING 之前定义为 #define ANOTHER_STRINGhello


• [TTys getString] returns an NSString
• ANOTHER_STRING is defined earlier as #define ANOTHER_STRING "hello"

我在 #if SOME_DEF 行收到以下错误：




无效预处理器表达式开始时的标记

Invalid token at start of a preprocessor expression

基于此 SO问题这可能是由于在编译时无法解决的问题引起的，但我已经定义了所有内容。我怀疑是
isEqualToString 方法，但我不知道另一种方法。

Based on this SO question this might be caused by something that can't be resolved at compile time, but I have everything defined. My suspect is the isEqualToString method, but I don't know of another way to do this.

推荐答案

当您编写 #if SOME_DEF 时，预处理器将其解析为：

When you write #if SOME_DEF the preprocessor resolves it into:

#if [[TTys getString] isEqualToString:ANOTHER_STRING]

这不是 #if 预处理程序指令的有效条件：

This is not a valid condition for the #if preprocessor directive:




'＃if'指令允许您测试算术
表达式的值，而不仅仅是存在一个宏。它的语法是

The ‘#if’ directive allows you to test the value of an arithmetic expression, rather than the mere existence of one macro. Its syntax is

 #if expression

 controlled text

 #endif /* expression */ 

表达式是整数类型的C表达式，受到严格的限制。它可能包含

expression is a C expression of integer type, subject to stringent restrictions. It may contain



• 整数常量。


• 字符常量，解释为
  将采用普通代码。


• 用于加法，
  减法，乘法，除法，按位运算，移位，
  比较和逻辑运算（&&和||）的算术运算符。后两者遵守标准C的通常短路规则
  。


• 宏。在
  表达式值的实际计算开始之前，表达式中的所有宏都扩展了
  中的所有宏。



• 使用已定义的运算符，让
  检查是否在'#if'中间定义了宏。

• Integer constants.
• Character constants, which are interpreted as they would be in normal code.
• Arithmetic operators for addition, subtraction, multiplication, division, bitwise operations, shifts, comparisons, and logical operations (&& and ||). The latter two obey the usual short-circuiting rules of standard C.
• Macros. All macros in the expression are expanded before actual computation of the expression's value begins.

• Uses of the defined operator, which lets you check whether macros are defined in the middle of an ‘#if’.

不是宏的标识符，它们都被认为是
数字零。这允许你编写#if MACRO而不是#ifdef
MACRO，如果你知道定义的MACRO将始终具有
非零值。没有函数调用
括号的函数式宏也被视为零。

Identifiers that are not macros, which are all considered to be the number zero. This allows you to write #if MACRO instead of #ifdef MACRO, if you know that MACRO, when defined, will always have a nonzero value. Function-like macros used without their function call parentheses are also treated as zero.

来自 GCC文档。

你可以做的是使用运行时评估的常规if语句：

What you can do instead is using a runtime-evaluated regular if-statement:

if(SOME_DEF) {
    ...
}

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