在C编程的#define指令中使用NOT运算符

Question

I have defined macros as below.

#define FALSE   0
#define TRUE    (!FALSE)

What will be the data-type of TRUE and FALSE ? What literal value does TRUE take after preprocessing? Is it compiler dependent? Why?

Answer 1

#define preprocessor directive (macros) are meant to do textual replacement . It will replace all occurrence of FALSE to 0 and TRUE to !0 that essentially gets evaluated to 1 . So, the resultant data type will be same as 0 and 1 . ie, integer.

Regarding the usage of ! operator, it always produces a result of type int .

Quoting the C11 standard, chapter §6.5.3.3 ( emphasis mine )

The result of the logical negation operator ! is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0. The result has type int . [...]

Answer 2

At the time of preprocessing, macros are replaced with text. FALSE will be replace by 0, and TRUE will be replaced by 1.