C语言 预处理器相等测试，这是标准吗？
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Preprocessor equality test, is this standard?
提问by Mawg says reinstate Monica
I had envisaged one of these in the project preferences
我曾在项目偏好中设想过其中之一
TESTING=HOSTTESTING=TARGETTESTINGnot defined at all
TESTING=HOSTTESTING=TARGETTESTING根本没有定义
My problem is with the latter.
我的问题是后者。
It seems that instead of
似乎，而不是
#if TESTING==HOST
#error "HOST defined" // add temporarilly for testing porpoises
#endif
I need to code
我需要编码
#ifdef TESTING
#if TESTING==HOST
#error "HOST defined" // add temporarilly for testing porpoises
#endif
#endif
I am convinced that this is not-standard behaviour, since if TESTINGis not defined then it certainly doesn't equal HOST, and I do not need that extra #ifdef TESTINGwith the GCC compiler.
我确信这不是标准行为，因为如果TESTING没有定义，那么它肯定不等于HOST，并且我不需要#ifdef TESTINGGCC 编译器的额外行为。
However, when I use the Atmel AVR Studio (which I thinkis based on MS Visual Studio), it is necessary to add that initial #ifdef TESTINGin a few dozen places :-(
但是，当我使用 Atmel AVR Studio（我认为它基于 MS Visual Studio）时，有必要#ifdef TESTING在几十个地方添加该首字母:-(
It looks like I have no choice, but I just wondered if any C standard acually requires this.
看起来我别无选择，但我只是想知道是否有任何 C 标准需要这样做。
回答by ouah
#if TESTING==HOST
If TESTINGis not defined, then
如果TESTING没有定义，那么
it is equivalent to:
它相当于：
#if 0==HOST
From the C Standard:
来自 C 标准：
(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""
(C99, 6.10.1p4)“在执行了由于宏扩展和定义的一元运算符导致的所有替换之后，所有剩余的标识符（包括与关键字词法相同的标识符）都被替换为 pp-number 0”
回答by tomlogic
And note that you can do this:
请注意，您可以这样做：
#ifndef TESTING
...
#elif TESTING == HOST
...
#elif TESTING == TARGET
...
#else
#error "Unexpected value of TESTING."
#endif
Also:
还：
#if defined(TESTING) && TESTING == HOST
...
#endif
If you want to collapse the tests. The parenthesis are optional (#if defined TESTINGis valid) but I think it's clearer to include them, especially when you start adding additional logic.
如果你想折叠测试。括号是可选的（#if defined TESTING有效），但我认为包含它们更清晰，尤其是当您开始添加其他逻辑时。
回答by wallyk
The original C preprocessors required explicit #ifdefvalidation before using a symbol. It is a relatively recent innovation (perhaps driven by scripting languages like Javascript) to assume that undefined symbols have a default value.
原始 C 预处理器#ifdef在使用符号之前需要显式验证。假设未定义的符号具有默认值是一项相对较新的创新（可能由 Javascript 等脚本语言驱动）。
Why don't you always insure the symbol is defined?:
为什么不总是确保定义了符号？：
#ifndef TESTING
#define TESTING (default value)
#endif
#if TESTING==HOST
...
#elif TESTING==TARGET
...
#else
...
#endif
Alternative, maybe force a selection?:
替代方案，也许强制选择？：
#ifndef TESTING
#error You must define a value for TESTING
#endif
回答by jdavis
I had the same problem. It used to be the compiler would error if the #if parameter was not defined which is what I wanted. I learned the hard way this is no longer the case. The solution I came up with is as follows:
我有同样的问题。如果没有定义我想要的 #if 参数，它曾经是编译器会出错。我学会了艰难的方式，这不再是这种情况。我想出的解决方案如下：
#define BUILDOPT 3
#if 3/BUILDOPT == 1
<compile>
#endif
If BUILDOPT is not defined you get a compile error for divide by 0. If BUILDOPT != 3 && > 0 the #if fails.
如果未定义 BUILDOPT，则除以 0 时会出现编译错误。如果 BUILDOPT != 3 && > 0，则 #if 失败。
回答by jdavis
My final implementation is to set BUILDOPT to 1 to enable the code, > 1 to disable. Then the #if becomes #if 1/BUILDOPT==1.
我的最终实现是将 BUILDOPT 设置为 1 以启用代码，> 1 以禁用。然后#if 变成#if 1/BUILDOPT==1。
