还可以创建没有参数的宏，在这种情况下不指定任何数字。
Macros can also be created without arguments, in which case they don't specify any number.
类型安全意味着宏在进行类型转换之前，会验证指定操作可以在指定类型上安全地执行。
Type-safe means that the macro verifies that the specified operation can be performed on the given type before attempting to cast.
如果宏不能执行指定操作，就会提供警告。
If the macro can't perform the operation, it provides a warning.
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