<分区>
考虑以下代码:
#define M(x) #x
#define M2(x) M(x)
M(VAR);
M2(VAR);
使用以下命令行: cpp 测试.cpp -DVAR=xxx
我希望预处理器同时改变 M(x) 和 M2(x) 变成“xxx”。
但是只有M2被替换了。这是为什么？
<分区>
考虑以下代码:
#define M(x) #x
#define M2(x) M(x)
M(VAR);
M2(VAR);
使用以下命令行: cpp 测试.cpp -DVAR=xxx
我希望预处理器同时改变 M(x) 和 M2(x) 变成“xxx”。
但是只有M2被替换了。这是为什么？
最佳答案
当我尝试这个时，我得到:
"VAR";
"abc";
(您应该将其包含在您的问题中!)
If you want to stringify the result of expansion of a macro argument, you have to use two levels of macros.
#define xstr(s) str(s)
#define str(s) #s
#define foo 4
str (foo)
==> "foo"
xstr (foo)
==> xstr (4)
==> str (4)
==> "4"
s is stringified when it is used in str, so it is not macro-expanded first. But s is an ordinary argument to xstr, so it is completely macro-expanded before xstr itself is expanded. Therefore, by the time str gets to its argument, it has already been macro-expanded.
发生这种情况是因为预处理器的预扫描宏参数规则已解释 here :
该页面的重点是:
Macros that call other macros that stringify or concatenate.
If an argument is stringified or concatenated, the prescan does not occur. If you want to expand a macro, then stringify or concatenate its expansion, you can do that by causing one macro to call another macro that does the stringification or concatenation. For instance, if you have
#define AFTERX(x) X_ ## x #define XAFTERX(x) AFTERX(x) #define TABLESIZE 1024 #define BUFSIZE TABLESIZEAFTERX(BUFSIZE) expands to X_BUFSIZE, and XAFTERX(BUFSIZE) expands to X_1024. (Not to X_TABLESIZE. Prescan always does a complete expansion.)
关于c++ - 奇怪的 C++ 预处理器行为，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/24412706/