[c voice] explain the advanced pointer and points for attention (2)

Preface : This section is next to 《 Explain the initial and advanced levels of the pointer and points for attention （1）》,
     At last, I got to know the function pointer , Next, we will continue to start around the function pointer .

One 、 Let's talk about function pointers

As we know before, if you want to get the address of the function , There are two ways , One is to use the function name directly ( Such as test), One is to use address symbols ( Such as &test).
So how to store addresses with pointers ？
Let's take a look at the code

int test(const char* str)
{
    
	printf("test()\n");
	return 0;
}

int main()
{
    
	printf("%p\n", test);
	printf("%p\n", &test);

	// With a pointer pf Deposit test Address 
	int (*pf)(const char*) = test;
	// perhaps 
	int (*pf)(const char*) = &test;
	// perhaps 
	int pf(const char*) = &test;
	// perhaps 
	int (*****pf)(const char*) = &test;
	
	// It can be called in the following ways .
	(*pf)("abc");
	test("abc");
	pf("abc");

	return 0;
}

We found that ：

1. Pointer stores function address , There are two ways to express function addresses , The pointer does not need to add * Or add more *（ Add * You need to put parentheses around it ）.
2. image int (* )(const char *) This , We call it function pointer type .
3. Because pointers and function names can represent function addresses , In the call , It can be directly through such as pf(“abc”),test(“abc”) Call directly .

So what's the use of function pointers ？
Now look at a piece of code

void menu()
{
    
	printf("*****************************\n");
	printf("***** 1.add 2.sub *****\n");
	printf("***** 3.mul 2.div *****\n");
	printf("***** 0.exit *****\n");
	printf("*****************************\n");
}

int Add(int x, int y)
{
    
	return x + y;
}

int Sub(int x, int y)
{
    
	return x - y;
}

int Mul(int x, int y)
{
    
	return x * y;
}

int Div(int x, int y)
{
    
	return x / y;
}

void calc(int (*pf)(int, int))
{
    
	int x = 0;
	int y = 0;
	int ret = 0;

	printf(" Please enter 2 Operands :>");
	scanf("%d %d", &x, &y);
	ret = pf(x, y);
	printf("%d\n", ret);

}

int main()
{
    
	int input = 0;

	do
	{
    
		menu();
		printf(" Please select :>");
		scanf("%d", &input);

		switch (input)
		{
    
		case 1:
			calc(Add);
			// If it is not established by function pointer calc function , Then the code will be written as follows and become very redundant .
			//int x = 0;
			//int y = 0;
			//int ret = 0;

			//printf(" Please enter 2 Operands :>");
			//scanf("%d %d", &x, &y);
			//ret = Add(x, y);
			//printf("%d\n", ret);
			break;
		case 2:
			calc(Sub);
			//int x = 0;
			//int y = 0;
			//int ret = 0;

			//printf(" Please enter 2 Operands :>");
			//scanf("%d %d", &x, &y);
			//ret = Sub(x, y);
			//printf("%d\n", ret);
			break;
		case 3:
			calc(Mul);
			//..
			break;
		case 4:
			calc(Div);
			//..
			break;
		case 0:
			printf(" Exit calculator \n");
			break;
		default:
			printf(" Wrong choice \n");
			break;
		}
	} while (input);

	return 0;
}

Using function pointers , In some special cases , Simplify the code .

 

After understanding the function pointer , Let's look at two interesting pieces of code , See what these two pieces of code are for .

// Code 1
(*(void (*)())0)();
// Code 2
void (*signal(int , void(*)(int)))(int);

1. In the first piece of code , First of all ( void (*)() ),（ type ） Type in parentheses means strong conversion , take 0 Strong conversion to function pointer ,* Dereference address , So the essence is a function call , Call address is 0 Function at .
2. From the outside ,void (* )(int) Is a function pointer type , Inside signal(int, void()(int)), function signal, Parameter is (int, void()(int)), The type is void(* )(int), There are return types , There are parameter types , Is a function declaration .

 
 

Two 、 Function pointer array

seeing the name of a thing one thinks of its function , It's an array with function pointers .
So how to express ?
The function pointer is like this

int (*pf)();

We learned pointer array before , Know that means

int* arr[];

So together , The array of function pointers is represented in this way ,pf The first and [] The combination description is an array , The type is int (*)() A function pointer .

int (*pf[])();

 
So what's the use of function pointer arrays ？
Look at this code

void menu()
{
    
	printf("*****************************\n");
	printf("***** 1.add 2.sub *****\n");
	printf("***** 3.mul 2.div *****\n");
	printf("***** 0.exit *****\n");
	printf("*****************************\n");
}

int Add(int x, int y)
{
    
	return x + y;
}

int Sub(int x, int y)
{
    
	return x - y;
}

int Mul(int x, int y)
{
    
	return x * y;
}

int Div(int x, int y)
{
    
	return x / y;
}

int main()
{
    
	int input = 0;
	int x = 0;
	int y = 0;
	int ret = 0;

	int (*pfArr[])(int, int) = {
     0,Add,Sub,Mul,Div };

	do
	{
    
		menu();
		printf(" Please select :>");
		scanf("%d", &input);

		if (input == 0)
		{
    
			printf(" Exit calculator \n");
		}
		else if (input >= 1 && input <= 4)
		{
    
			printf(" Please enter 2 Operands :>");
			scanf("%d%d", &x, &y);
			ret = (*pfArr[input])(x, y);// Subscript access function name , Function name ()  Call the function in this form 
			printf("%d\n", ret);
		}
		else
		{
    
			printf(" Wrong choice \n");
		}
	} while (input);

	return 0;
}

Access functions by accessing array subscripts , More beautification , Convenient for our code design .

 
Function pointer array pointer
Of course, the function pointer array can also be put into the pointer through the address of the array , Here is just to meet , You can also go down with a wireless doll , But that's it .
How to say ？

// Function pointer array 
int (*pfun[5])();

// Function pointer array pointer 
//(*ppfun) First explain ppfun It's a pointer , After with [] The combination description points to a 5 Array of elements ,
// The element type in the array is int (*)()
int (*(*ppfun)[5]))()  

 

3、 ... and 、 Callback function

What is a callback function ?

A callback function is a function called through a function pointer . If you put a pointer to a function （ Address ） Pass as a parameter to another
function , When this pointer is used to call the function it points to , Let's just say this is a callback function . The callback function is not controlled by this function
The implementer of directly calls , It's called by another party when a particular event or condition occurs , Used to enter the event or condition
Row response .

Suppose there is a function F1 I called the function F2 When , function F1 Given by parameters F2 Passed another function F3 The pointer to , In function F2 During execution , function F2 I called the function F3, This action is called callback , It is first passed as a pointer 、 The function called back later F3 It's a callback function .

Understand as follows ：
F3 The address is passed as a parameter to F2, Store F3 The pointer of the address is used to call the original F3 function , Just say F3 It's a callback function .
But it's worth it Be careful Yes. , It's not through F3 Function calls directly （ Not directly called by the implementer ）, But through a storage F3 Function pointer to call .

int F3()
{
    
	return 0;
}

//F2 I called the function F3
int F2(int (*F3)())
{
    
	F3();// And through the F3 Visited F3 function 
	return 0;
}

int F1()
{
    
	// function F1 call F2 Pass the F3 The pointer 
	F2(F3)
	return 0;
}

 
 
Next, let's look at the callback function qsort The implementation of the .
adopt MSDN, We see some qsort Information about ,qsort It is realized by referring to the fast scheduling , We won't discuss how to realize it here , We learn how to use it .
Let's look at the function declaration .

By simplifying the function declaration, we get ：

void qsort(void* base, // You need to sort the starting position of the data 
	size_t num, // The number of data elements to be sorted 
	size_t width, // The size of the data elements to be sorted ( Unit is byte )
	int (*cmp)(const void* e1, const void* e2) // A function pointer - Comparison function 
);

Because we don't know what kind of data to sort , So when defining the starting position , I chose void* type , How to understand it ？
Look at the code

int main()
{
    
	int a = 10;
	//char* pa = &a;//int*
	void* pv = &a;//void* Is a pointer without a specific type , Can accept any type of address 
	//void* Is a pointer without a specific type , Therefore, you cannot dereference the operation , Also can not +- Integers 
	//
	return 0;
}

Now let's try sorting
Look at the code

// Integer sort 
// Callback function 
int cmp_int(const void* e1, const void* e2)
{
    
	// What type of data is excluded , First, try to turn around 
	return (*(int*)e1 - *(int*)e2);
}

void test1()
{
    
	int arr[] = {
     9,8,7,6,5,4,3,2,1,0 };

	int sz = sizeof(arr) / sizeof(arr[0]);

	qsort(arr, sz, sizeof(arr[0]), cmp_int);

	int i = 0;
	for (i = 0; i < sz; i++)
	{
    
		printf("%d ", arr[i]);// result {0,1,2,3,4,5,6,7,8,9}
	}
}


// Structure ordering 
int cmp_stu_name(const void* e1, const void* e2)
{
    
	return strcmp(((struct stu*)e2)->name, ((struct stu*)e1)->name);
}

int cmp_stu_age(const void* e1, const void* e2)
{
    
	return (((struct stu*)e1)->age -((struct stu*)e2)->age);
}

void test2()
{
    
	struct stu s[] = {
     {
    "ade",20 }, {
    "qwe", 30}, {
    "wer", 25} };
	int sz2 = sizeof(s) / sizeof(s[0]);
	qsort(s, sz2, sizeof(s[0]), cmp_stu_name);
	printf("\n");
	for (i = 0; i < sz; i++)
	{
    
		printf("%d ", (s + i)->name);
	}
}

void test3()
{
    
	struct stu s[] = {
     {
    "ade",20 }, {
    "qwe", 30}, {
    "wer", 25} };
	int sz2 = sizeof(s) / sizeof(s[0]);
	qsort(s, sz2, sizeof(s[0]), cmp_stu_age);
	printf("\n");
	for (i = 0; i < sz; i++)
	{
    
		printf("%d ", (s + i)->age);
	}
}

int main()
{
    
	test1();
	test2()
	test3()
	return 0;
}

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