Its only purpose is to check at runtime whether or not the supplied preprocessor symbol is defined.
它唯一的目的是在运行时检查给定的预定义符号是否被定义。
This symbol should be defined in the Debug version and not defined in the Release version.
该符号应在调试版本中定义而不应在发布版本中定义。
A symbol that was not defined was used with a preprocessor directive.
未定义的符号与预处理器指令一起使用。
On the other hand, if a defined global symbol exists, the appearance of a weak symbol with the same name will not cause an error.
另一方面，如果一个已定义的全局符号存在，则即便另一个具有相同名字的弱符号存在也不会引起错误。
On the other hand, if a defined global symbol exists, the appearance of a weak symbol with the same name will not cause an error.
另一方面，如果一个已定义的全局符号存在，则即便另一个具有相同名字的弱符号存在也不会引起错误。
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