[英]What does #ifndef _Python_CALL mean in a C++ code?
I am debugging a C++ code which contains something related to Python.我正在调试包含与 Python 相关的内容的 C++ 代码。 In a function:
在 function 中:
void normalize(string &text) {
...
#ifdef _Python_CALL
newContentStr = contentStr;
#endif
#ifndef _Python_CALL
...
...
#endif
return 0;
}
I am using GDB to keep track of the code logic, and I found that after it reaches the line:我正在使用 GDB 来跟踪代码逻辑,我发现到达该行后:
newContentStr = contentStr;
It then directly jumps to the last line in the function:然后直接跳转到function的最后一行:
return 0;
Why is the code between the following is skipped?为什么下面之间的代码被跳过了?
#ifndef _Python_CALL
...
...
#endif
Also note that the first is "#ifdef" and the 2nd is "#ifndef".另请注意,第一个是“#ifdef”,第二个是“#ifndef”。 Does that make the skip?
这会跳过吗?
#ifndef
is the opposite of #ifdef
. #ifndef
与#ifdef
相反。
In your case, #ifdef
is true, so it jump to return 0
directly and omit #ifndef
block.在你的情况下,
#ifdef
是真的,所以它跳转到直接return 0
并省略#ifndef
块。
see this official doc看这个官方文档
Judging from the code fragment you've shown, _Python_CALL
is a macro name, possibly defined somewhere via #define _Python_CALL
(or maybe by some other means, such as using a command line argument to the C++ compiler during compilation).从您显示的代码片段来看,
_Python_CALL
是一个宏名称,可能通过#define _Python_CALL
在某处定义(或者可能通过其他方式,例如在编译期间使用 C++ 编译器的命令行参数)。
Then, line #ifdef _Python_CALL
means that everything that follows it until the line #endif
will be compiled (and thus executed in the compiled program) if and only if the macro name _Python_CALL
is defined (the #ifdef
means "if defined").然后,
#ifdef _Python_CALL
行意味着当且仅当宏名称_Python_CALL
被定义( #ifdef
表示“如果定义”)时,在它之后的所有内容直到#endif
行将被编译(并因此在编译的程序中执行)。 Since you claim that the line newContentStr = contentStr;
由于您声称该行
newContentStr = contentStr;
was executed, we can assume that the macro name _Python_CALL
was indeed defined during compilation.执行后,我们可以假设宏名
_Python_CALL
确实是在编译期间定义的。
Now, the line #ifndef _Python_CALL
means that everything that follows it until the line #endif
will be compiled (and executed) if and only if the macro name _Python_CALL
is NOT defined.现在,
#ifndef _Python_CALL
行意味着当且仅当宏名称_Python_CALL
未定义时,直到#endif
行之前的所有内容都将被编译(并执行)。 (Note the n in #ifndef
, it means "if not defined"). (注意
#ifndef
中的n ,它的意思是“如果未定义”)。 But, as we already know (from the conclusion we made in the previous paragraph), this is not the case, because _Python_CALL
is indeed defined.但是,正如我们已经知道的(从我们在上一段中得出的结论),情况并非如此,因为确实定义了
_Python_CALL
。 Thus, this block will not be compiled/executed.因此,该块将不会被编译/执行。
On Cppreference, you can read more about C++ preprocessor , especially about #define
and #ifdef
/ #ifndef
directives, to gain deeper understanding.在 Cppreference 上,您可以阅读有关 C++预处理器的更多信息,尤其是有关
#define
和#ifdef
/ #ifndef
指令的信息,以获得更深入的了解。
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