在 C/C++ 中实现 UNUSED 宏的通用编译器独立方式
在实现存根等时，您希望避免未使用的变量"警告.多年来，我遇到了一些 UNUSED() 宏的替代方案，但从来没有一种被证明适用于所有"编译器，也没有一种按照标准是密封的.
When implementing stubs etc. you want to avoid "unused variable" warnings. I've come across a few alternatives of UNUSED() macros over the years, but never one which either is proven to work for "all" compilers, or one which by standard is air tight.
或者我们是否坚持使用每个构建平台的 #ifdef 块?
Or are we stuck with #ifdef blocks for each build platform?
由于有许多不符合 c 的替代方案的答案，我想澄清一下，我正在寻找一个对 C 和 C++ 都有效的定义，所有风格等等
Due to a number of answers with non c-compliant alternatives, I'd like to clarify that I'm looking for a definition which is valid for both C and C++, all flavours etc.
推荐答案
根据this answer by 用户 GMan 典型方式是强制转换为void:
According to this answer by user GMan the typical way is to cast to void:
#define UNUSED(x) (void)(x)
但是如果 x 被标记为 volatile 这将强制从变量中读取并因此产生副作用，因此几乎可以保证无操作和抑制编译器警告如下:
but if x is marked as volatile that would enforce reading from the variable and thus have a side effect and so the actual way to almost guarantee a no-op and suppress the compiler warning is the following:
// use expression as sub-expression,
// then make type of full expression int, discard result
#define UNUSED(x) (void)(sizeof((x), 0))
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