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。

youdao

That is, a variable name should not be passed to a preprocessor directive and a symbol can only be evaluated by a preprocessor directive.

即，不应将变量名传递到预处理器指令，并且只能用预处理器指令计算符号。

youdao

That is, a variable name should not be passed to a preprocessor directive and a symbol can only be evaluated by a preprocessor directive.

即，不应将变量名传递到预处理器指令，并且只能用预处理器指令计算符号。

youdao