[英]Difference between function with returned type pointer and function pointer
I have a code with struct defination with member function pointers like this我有一个结构定义的代码,其中成员 function 指针像这样
struct file_system_type {
struct dentry *(*mount) (struct file_system_type *, int,
const char *, void *);
void (*kill_sb) (struct super_block *);
};
and object of file_system_type
like this和
file_system_type
的 object 像这样
static struct file_system_type minix_fs_type = {
.mount = minix_mount,
.kill_sb = kill_block_super,
};
and .mount
like this和
.mount
像这样
static struct dentry *minix_mount(struct file_system_type *fs_type,
int flags, const char *dev_name, void *data)
I like to know what is the difference of above from function with return type some pointer like if I had something like this我想知道上面与 function 有什么区别,返回类型是一些指针,就像我有这样的东西
static struct dentry* minix_mount(...)
struct dentry *(*mount) (struct file_system_type *, int,
const char *, void *);
void (*kill_sb) (struct super_block *);
are pointers a function that have return type struct dentry *
resp.是指针 function 具有返回类型
struct dentry *
resp。 void
. void
。 First you have to assign an actual function to these pointers to call these functions through these pointer.首先,您必须为这些指针分配一个实际的 function 以通过这些指针调用这些函数。 The pointers in your code are assigned with
代码中的指针分配有
.mount = minix_mount,
.kill_sb = kill_block_super,
static struct dentry *minix_mount(struct file_system_type *fs_type,
int flags, const char *dev_name, void *data)
is a function that returns a pointer.是一个返回指针的 function。 It already has a static function body and can instantly be called.
它已经有一个 static function 主体,可以立即调用。
Both calls return a value with the same type struct dentry *
.两个调用都返回一个具有相同类型
struct dentry *
的值。
A big advantage of function pointers is that you can write generic code and assign a different functions to this pointer at run time. function 指针的一大优势是您可以编写通用代码并在运行时为该指针分配不同的函数。 Common use-cases are algorithms like sort or find algorithms where you can pass a predicate or compare function to a function pointer.
常见的用例是排序或查找算法等算法,您可以在其中传递谓词或将 function 与 function 指针进行比较。
Another advantage is that structs in C can contain function pointers but can't contain functions.另一个优点是 C 中的结构可以包含 function 指针但不能包含函数。 That's a way to simulate OOP in C.
这是在 C 中模拟 OOP 的一种方法。
Here is an example of a function pointer to a function returning a pointer:下面是一个指向 function 的 function 指针返回一个指针的示例:
#include <stdio.h>
struct S {
int x;
int y;
// pointers to functions returning a pointer
int *(*compare1)(int *, int *);
int *(*compare2)(int *, int *);
};
// functions returning a pointer
int *min(int *a, int *b) {
return *a < *b ? a : b;
}
int *max(int *a, int *b) {
return *a > *b ? a : b;
}
int main() {
struct S s = {
.x = 5,
.y = 7,
.compare1 = &max,
// .compare2 = &min;
// name of a function can be used as function pointer
.compare2 = min
};
int *result1 = s.compare1(&s.x, &s.y);
int *result2 = s.compare2(&s.x, &s.y);
++*result1;
--*result2;
printf("%d %d", s.x, s.y);
}
Output: Output:
4 8
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