C++ ##（双哈希）在预处理器指令中做什么？
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What does ## (double hash) do in a preprocessor directive?
提问by DavidColson
#define DEFINE_STAT(Stat) \
struct FThreadSafeStaticStat<FStat_##Stat> StatPtr_##Stat;
The above line is take from Unreal 4, and I know I could ask it over on the unreal forums, but I think this is a general C++ question that warrants being asked here.
上面的内容取自 Unreal 4，我知道我可以在虚幻论坛上提问，但我认为这是一个通用的 C++ 问题，值得在这里提问。
I understand the first line defines a macro, however I am not well versed in preprocessor shenanigans in C++ and so I'm lost over there. Logic tells me the backslash means the declaration continues onto the next line.
我知道第一行定义了一个宏，但是我不熟悉 C++ 中的预处理器恶作剧，所以我在那里迷路了。逻辑告诉我反斜杠意味着声明继续到下一行。
FThreadSafeStaticStat looks a bit like a template, but there's #'s going on in there and a syntax I've never seen before in C++
FThreadSafeStaticStat 看起来有点像模板，但里面有 # 和我以前从未在 C++ 中见过的语法
Could someone tell me what this means? I understand that you may not have access to Unreal 4, but it's just the syntax I don't understand.
有人能告诉我这是什么意思吗？我知道您可能无法访问 Unreal 4，但这只是我不理解的语法。
回答by Susam Pal
##is the preprocessor operator for concatenation.
##是用于连接的预处理器运算符。
So if you use
所以如果你使用
DEFINE_STAT(foo)
DEFINE_STAT(foo)
anywhere in the code, it gets replaced with
在代码中的任何地方，它被替换为
struct FThreadSafeStaticStat<FStat_foo> StatPtr_foo;
struct FThreadSafeStaticStat<FStat_foo> StatPtr_foo;
before your code is compiled.
在你的代码编译之前。
Here is another example from a blog postof mine to explain this further.
#include <stdio.h>
#define decode(s,t,u,m,p,e,d) m ## s ## u ## t
#define begin decode(a,n,i,m,a,t,e)
int begin()
{
printf("Stumped?\n");
}
This program would compile and execute successfully, and produce the following output:
该程序将成功编译并执行，并产生以下输出：
Stumped?
When the preprocessor is invoked on this code,
当预处理器在这段代码上被调用时，
beginis replaced withdecode(a,n,i,m,a,t,e)decode(a,n,i,m,a,t,e)is replaced withm ## a ## i ## nm ## a ## i ## nis replaced withmain
begin被替换为decode(a,n,i,m,a,t,e)decode(a,n,i,m,a,t,e)被替换为m ## a ## i ## nm ## a ## i ## n被替换为main
Thus effectively, begin()is replaced with main().
因此，有效地，begin()被替换为main()。
