xcode 用于静音未使用变量警告的跨平台宏
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cross platform macro for silencing unused variables warning
提问by Jean-Denis Muys
In porting a large piece of C++ code from Visual Studio (2008) to Xcode (4.4+), I encounter lines such as:
在将一大段 C++ 代码从 Visual Studio (2008) 移植到 Xcode (4.4+) 时，我遇到了如下几行：
UNUSED_ALWAYS(someVar);
the UNUSED_ALWAYS(x)(through UNUSED(x)) macro expands to xwhich seems to silence Visual C++ just fine. It's not enough for Clang however.
在UNUSED_ALWAYS(x)（通过UNUSED(x)）宏扩展到x这似乎沉默的Visual C ++就好了。然而，这对 Clang 来说还不够。
With Clang, I usually use the #pragma unused xdirective.
对于 Clang，我通常使用#pragma unused x指令。
The UNUSED_ALWAYSand UNUSEDmacros are defined in an artificial windows.hheader which I control that contains a number of utilities to help Xcode compile Windows stuff.
在UNUSED_ALWAYS和UNUSED宏在人工定义的windows.h头，我控制包含了一些工具来帮助Xcode的编译的Windows的东西。
Is there a way to define UNUSED(x)to expand to #pragma unused x? I tried this, which Clang fails to accept:
有没有办法定义UNUSED(x)扩展到#pragma unused x？我试过这个，但 Clang 无法接受：
#define UNUSED(x) #pragma unused(x)
I also tried:
我也试过：
#define UNUSED(x) (void)(x)
Which seems to work. Did I miss anything?
这似乎有效。我错过了什么吗？
采纳答案by justin
Yup - you can use this approach for GCC and Clang:
是的 - 您可以将这种方法用于 GCC 和 Clang：
#define MON_Internal_UnusedStringify(macro_arg_string_literal) #macro_arg_string_literal
#define MONUnusedParameter(macro_arg_parameter) _Pragma(MON_Internal_UnusedStringify(unused(macro_arg_parameter)))
although mine did have the (void)approach defined for clang, it appears that Clang now supports the stringify and _Pragmaapproach above. _Pragmais C99.
尽管我的确实(void)为 clang 定义了方法，但似乎 Clang 现在支持_Pragma上面的 stringify 和方法。_Pragma是C99。
回答by Christian Stieber
(void)x;
is fine; has always worked for me. You can't usually expand a macro to a #pragma, although there is usually a slightly different pragma syntax that can be generated from a macro (_Pragma on gcc and clang, __pragma on VisualC++).
很好；一直对我来说有效。您通常不能将宏扩展为 #pragma，尽管通常可以从宏（gcc 和 clang 上的 _Pragma，VisualC++ 上的 __pragma）生成略有不同的编译指示语法。
Still, I don't actually need the (void)x anymore in C++, since you can simply not give a name to a function parameter to indicate that you don't use it:
尽管如此，我实际上不再需要 C++ 中的 (void)x，因为您不能简单地为函数参数命名以表明您不使用它：
int Example(int, int b, int)
{
... /* only uses b */
}
works perfectly fine.
工作得很好。
回答by Ajay
#defineand #pragmaboth are preprocessor directives. You cannot define one macro to expand as preprocessor directive. Following would be incorrect:
#define并且#pragma两者都是预处理器指令。您不能定义一个宏来扩展为预处理器指令。以下将是不正确的：
#define MY_MACRO #if _WIN32
MY_MACRO cannot expand to #if _WIN32for the compiler.
MY_MACRO 不能扩展#if _WIN32为编译器。
Your best bet is to define your own macro:
最好的办法是定义自己的宏：
#define UNUSED(_var) _var
