c23 typeof

本文最后更新于：2024年9月28日

C23 标准新增了typeof 支持，其实就是gcc等编译器的扩展“转正”

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typeof(int) a;//等价于 int a
typeof(int*) p;//等价于 int* p
typeof(int[3]) arr;//等价于 int arr[3]

下面给出一个可能比较有用的使用方式

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#define pointer(T)  typeof(T *)
#define array(T, N) typeof(T [N])
array(pointer(char), 4) a;

可读性远远优于int(*a)[4]。

typeof 在 linux 内核开发中被大量使用，比如

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/*
 * Check at compile time that something is of a particular type.
 * Always evaluates to 1 so you may use it easily in comparisons.
 */
#define typecheck(type,x) \
({  type __dummy; \
    typeof(x) __dummy2; \
    (void)(&__dummy == &__dummy2); \
    1; \
})

/*
 * Check at compile time that 'function' is a certain type, or is a pointer
 * to that type (needs to use typedef for the function type.)
 */
#define typecheck_fn(type,function) \
({  typeof(type) __tmp = function; \
    (void)__tmp; \
})

/*
 * swap - swap value of @a and @b
 */
#define swap(a, b) \
    do { typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; } while (0)

/**
 * container_of_const - cast a member of a structure out to the containing
 *			structure and preserve the const-ness of the pointer
 * @ptr:		the pointer to the member
 * @type:		the type of the container struct this is embedded in.
 * @member:		the name of the member within the struct.
 */
#define container_of_const(ptr, type, member)				\
	_Generic(ptr,							\
		const typeof(*(ptr)) *: ((const type *)container_of(ptr, type, member)),\
		default: ((type *)container_of(ptr, type, member))	\
	)

此外也可以参考笔者之前有关宏的文章。