#if 0作为定义 [英] #if 0 as a define

问题描述

我需要一种方法来定义FLAGS_IF宏(或等效宏)，以使

FLAGS_IF(expression)
<block_of_code>
FLAGS_ENDIF

在调试中进行编译时(例如使用特定的编译器开关)会编译为

if (MyFunction(expression))
{
    <block_of_code>
}

而在发布时不会产生任何指令，就像这样

#if 0
    <block_of_code>
#endif

由于对 C /C ++预处理器一无所知，我想不出任何幼稚的方式(因为#define FLAGS_IF(x) #if 0甚至没有编译)，您能帮上忙吗?

我需要一个解决方案:

• 如果<block_of_code>内部存在*/
，不会弄乱
• 即使在任何深度的内联函数中，也可以确保在释放中生成0条指令(我想这不包括if (false){<block_of_code>}吧?)
• 是否符合标准

解决方案

以下应做您想做的事情:

#ifdef DEBUG
# define FLAGS_IF(x) if (MyFunction((x))) {
# define FLAGS_ENDIF }
#else
# define FLAGS_IF(x) if(0) {
# define FLAGS_ENDIF }
#endif

if(0)应该不变成任何指令，或者至少在大多数编译器中都这样.

编辑:Hasturkun指出您确实不需要FLAGS_ENDIF，因此可以这样编写代码:

FLAGS_IF(expression) {
   <block_of_code>
}

具有以下宏:

#ifdef DEBUG
# define FLAGS_IF(x) if (MyFunction((x)))
#else
# define FLAGS_IF(x) if(0)
#endif

I need a way to define a FLAGS_IF macro (or equivalent) such that

FLAGS_IF(expression)
<block_of_code>
FLAGS_ENDIF

when compiling in debug (e.g. with a specific compiler switch) compiles to

if (MyFunction(expression))
{
    <block_of_code>
}

whereas in release does not result in any instruction, just as it was like this

#if 0
    <block_of_code>
#endif

In my ignorance on the matter of C/C++ preprocessors I can't think of any naive way (since #define FLAGS_IF(x) #if 0 does not even compile) of doing this, can you help?

I need a solution that:

• Does not get messed up if */ is present inside <block_of_code>
• Is sure to generate 0 instructions in release even inside inline functions at any depth (I guess this excludes if (false){<block_of_code>} right?)
• Is standard compliant if possible

解决方案

The following should do what you want:

#ifdef DEBUG
# define FLAGS_IF(x) if (MyFunction((x))) {
# define FLAGS_ENDIF }
#else
# define FLAGS_IF(x) if(0) {
# define FLAGS_ENDIF }
#endif

The if(0) should turn into no instructions, or at least it does so on most compilers.

Edit: Hasturkun commented that you don't really need the FLAGS_ENDIF, so you would instead write your code like this:

FLAGS_IF(expression) {
   <block_of_code>
}

with the follow macros:

#ifdef DEBUG
# define FLAGS_IF(x) if (MyFunction((x)))
#else
# define FLAGS_IF(x) if(0)
#endif

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