铸造NULL在C一个struct指针? [英] Casting NULL to a struct pointer in C?
问题描述
有铸造的NULL在C一个struct指针任何好处?
Is there any benefit of casting NULL to a struct pointer in C ?
例如:
typedef struct List
{
....
} List;
List *listPtr = ((List *) NULL) ;
这是PostgreSQL源代码示例:
Example from PostgreSQL source:
#define NIL ((List *) NULL)
http://doxygen.postgresql.org/pg__list_8h_source.html
推荐答案
在分配例子明确投不作任何有用的意义。然而,似乎你的问题实际上是关于的#define NIL((名单*)NULL)
宏,它的实用性超出分配。
In assignment example the explicit cast make no useful sense. However, it seems that you the question is really about #define NIL ((List *) NULL)
macro, whose usability extends beyond assignment.
一个地方,它可能是有意义的是,当你把它传递给一个可变参数函数,或者没有原型声明的函数。该标准 NULL
可以被定义为 0
或 0L
或((无效*)0)
或以其它方式,这意味着它可以是除$ p $在这种类型的无上下文PTED不同。显式类型转换将确保它是正确PTED为指针间$ P $。
One place where it might make sense is when you pass it to a variadic function or to a function declared without a prototype. The standard NULL
can be defined as0
or 0L
or ((void *) 0)
or in some other way, meaning that it might be interpreted differently in such type-less contexts. An explicit cast will make sure that it is interpreted correctly as a pointer.
例如,这通常是无效的(行为是未定义)
For example, this is generally invalid (behavior is undefined)
void foo();
int main() {
foo(NULL);
}
void foo(List *p) {
/* whatever */
}
在更换与呼叫
foo(NIL);
使得它有效。
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