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。

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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。

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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。

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