预处理程序相等性测试，这是标准吗？ [英] Preprocessor equality test, is this standard?

问题描述

我已经在项目首选项中设想了其中之一

I had envisaged one of these in the project preferences

• 测试 = 主机


• 测试 = 目标


• 测试完全没有定义

• TESTING = HOST
• TESTING = TARGET
• TESTING not defined at all

我的问题是后者。

似乎不是

#if TESTING==HOST
#error "HOST defined"  // add temporarilly for testing porpoises
#endif

我需要编码

#ifdef TESTING   
#if TESTING==HOST
#error "HOST defined"  // add temporarilly for testing porpoises
#endif
#endif

我坚信这不是标准行为，因为如果未定义 TESTING ，那么它肯定不会等于 HOST ，而我不需要使用GCC编译器进行额外的 #ifdef TESTING 。

I am convinced that this is not-standard behaviour, since if TESTING is not defined then it certainly doesn't equal HOST, and I do not need that extra #ifdef TESTING with the GCC compiler.

但是，当我使用Atmel AVR St udio（我认为是基于MS Visual Studio的），有必要在几十个地方添加初始的 #ifdef TESTING ： （

However, when I use the Atmel AVR Studio (which I think is based on MS Visual Studio), it is necessary to add that initial #ifdef TESTING in a few dozen places :-(

看起来我别无选择，但我只是想知道是否有任何C标准都要求这样做。

It looks like I have no choice, but I just wondered if any C standard acually requires this.

推荐答案

#if TESTING==HOST

如果未定义 TESTING ，则

等效于：

#if 0==HOST

根据C标准：

（C99，6.10.1p4）由于宏扩展和定义的一元变量而进行的所有替换之后操作符已执行，所有剩余的标识符（包括与关键字在词法上相同的标识符）都替换为pp-number 0

(C99, 6.10.1p4) "After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers (including those lexically identical to keywords) are replaced with the pp-number 0""

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