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C / C + + knowledge sharing: function pointer and pointer function, can you understand after reading this article?

2020-11-08 16:58:00 【Muge love programming】

One 、 What is a pointer ？

Definition ： A pointer is the address of program data in memory , And pointer variables are variables that hold these addresses ;

The above a 4GB The memory of can store 2^32 Bytes of data . The consecutive hexadecimal number on the left is the memory address , Each memory address corresponds to a byte of memory space . The pointer variable holds this number , Memory address .

Pointer declaration ：

A pointer is actually a variable , The way to declare a pointer is similar to a general variable declaration , as follows ：

int *p; // Make a statement int Pointer to type p, The pointer points to a int Object of type

char *p // Make a statement char Pointer to type p, The pointer points to a int Object of type

int *arr[10] // Declare an array of pointers , The array has 10 Elements , Each of these elements is a point to int Pointer to type object

int (*arr)[10] // Declare an array pointer , The pointer points to a int One dimensional array of type

int **p; // Declare a pointer p , The pointer points to a int Pointer to type

Declaring a pointer variable does not automatically allocate any memory . Before making indirect access to pointers , The pointer must be initialized ： Or point it to existing memory , Or dynamically allocate memory to him , Otherwise, we don't know where the pointer is , This problem needs special attention .

Two 、 What is a function pointer ？

Function pointer definition ： A function pointer is a pointer variable to a function . therefore “ A function pointer ” First of all, it should be a pointer variable , It's just that the pointer variable points to the function . It's like pointing to an integer variable with a pointer variable 、 Character 、 Array is the same , Here is the pointing function .

Its general expression is ： Type specifier (* Function name ) ( Parameters )

int (*fun)(int x) // Definition of function pointer

int (*fun)(int x,int y) // Definition of function pointer

Function pointer in PC Less used in software development , It is used more in the embedded industry , But whether it's PC Software or embedded software , Understand the definition and use of function pointers , It's good for understanding programming .

Assignment of function pointer

Function pointers, like other pointers, need to be initialized after they are defined and used .

Function pointers have two uses ： Call function and do function parameters

int (*fun)(int x,int y) // Definition of function pointer

fun = &Function // How function pointers are assigned 1

fun = Function // How function pointers are assigned 2

x = (*fun)() // Call method of function pointer 1

x = fun() // Call method of function pointer 2

The address operator is taken when the function is assigned & Not required , Because a function identifier represents its address , In addition, the function does not need to be enclosed in parentheses ;

If it's a function call , It must also contain a parameter list enclosed in parentheses .

Function pointer usage

When we use the pointer , Need to go through the key * To get the value in the memory it points to , The same is true for function pointers . By using (*pf) Take out the function that exists in this address , Then call it. .

char* fun(char* p1,char* p2)

{

int i = 0;

i = strcmp(p1,p2);

if(0 == i)

{

return p1;

}

else

{

return p2;

}

}

int main()

{

char * (*pf)(char* p1,char* p2);

pf = &fun;

(*pf)("aa","bb");

return 0;

}

It's important to note here that yes , stay Visual C++6.0 in , When assigning a function pointer , It can be used &fun Or use the function name directly fun. This is because the function name is actually an address after being compiled , So there is no essential difference between the two usages .

The usage is extended to

When we are not satisfied with the simple use of function pointers , In this case, we need an advanced usage to extend our cognitive boundary of function pointer .

Interested students can take a look at the following usage , And try to understand how the expression is used by the function pointer .

(* (void(*)()) 0)(); // come from 《C Trap and Pitfalls》 This classic book

The answer is as follows ： ``

First step ： adopt void(*) (), You can see that this is a function pointer type . This function has no arguments , no return value .

The second step ： adopt (void(*) ())0, You can see that this is going to 0 Cast to function pointer type ,0 It's an address , In other words, a function exists with the first address of 0 In a region of .

The third step ： adopt (*(void(*) ())0), It can be understood that this is taking 0 Address the beginning of a section of memory content .

Step four ： Finally understand (*(void(*) ())0)(), This is a function call .

Let the program jump to the absolute address of 0x0113F90C

Method 1 ：

take 0x0113F90C Address cast to function pointer type , namely : (void (*)())0x0113F90C

And then call ：((void (*)())0x0113F90C)()

Method 2 ：

typedef (void (*)()) VoidFuncPtr;

((VoidFuncPtr)0x0113F90C)();

Interview questions ： Point out program errors

#include<stdio.h>

void main(void)

{

int max(x,y);

int *p=max;

int a,b,c,d;

scanf("%d %d %d",a,b,c);

d=p(p(a,b),c);

printf("d:%d\n",d);

return;

}

int max(int a,int y)

{

return(x > y ？x:y);

}

answer ：

int max(x ,y); Function declaration error , Change it to ：int max(int x,int y);

analysis ：max The function declaration just writes out the name of the formal parameter of the function , This is meaningless for the type of parameter , The compiler will put x and y As a data type , There will be errors when compiling ,max The call of must also be wrong .

int *p=max; Pointer definition error , Change it to ：int (*p)(int ,int)=max;

analysis ： The pointer to a function cannot be assigned directly to int Type pointer .

scanf("%d %d %d",a,b,c); Wrong use of library function , Change it to scanf("%d %d%d",&a,&b,&c);

analysis ： Wrong use of library function , The second part should be the address of receiving data , It's written here as a variable .

d=p(p(a,b),c); Error calling function by function pointer , Change it to d=(*p)((*p)(a,b),c);`

analysis ： The format of calling a function with a function pointer is as follows ：（【*】【 Function pointer name 】）（【 parameter list 】）; Can't call directly with function pointer and parameter .

3、 ... and 、 What is a pointer function ？

Pointer function definition ： The foothold of pointer function is a function , The return value of this function is a pointer , And ordinary functions int function(int,int) similar , It's just that the returned data types are different .

_type_ *function(int, int) // The return is the pointer address

int function(int,int) // The return is int Type data .

int * fun(int x,int y) // Definition of pointer function

int* fun(int x,int y) // Definition of pointer function

All three of the above are correct , however * Closer to the return value is easier to understand .int *fun(int x) // Definition of pointer function

Pointer function call

When calling a pointer function , A pointer of the same type is required to receive the return value of its function .

typedef struct _Data{

int a;

int b;

}Data;

// Pointer function

Data* f(int a,int b){

Data * data = new Data;

data->a = a;

data->b = b;

return data;

}

int main(int argc, char *argv[])

{

QApplication a(argc, argv);

// Call pointer function

Data * myData = f(4,5);

qDebug() << "f(4,5) = " << myData->a << myData->b;

return a.exec();

}

However, you can also define its return value as void* type , When calling, cast the return value to the type you want .

The output is the same , However, this is not recommended , Because coercion can bring risks . The return type can be any basic type or compound type . A function that returns a pointer has a wide range of uses .

in fact , Every function , Even if it doesn't have a pointer that returns a certain type , It itself has an entry address , The address is equivalent to a pointer .

For example, a function returns an integer value , In fact, it is equivalent to returning the value of a pointer variable , But the variable is the function itself , And the whole function is equivalent to one “ Variable ”.

Four 、 Function pointer is different from pointer function

Through the above introduction , Partners should be able to understand the definition of both . So simply sum up the difference between the two ：

1. Different definitions

A pointer function is essentially a function , Its return value is a pointer .

A function pointer is essentially a pointer , It points to a function .

2. The writing is different

Pointer function ：int* fun(int x,int y);

A function pointer ：int (*fun)(int x,int y);

It can be simply understood as , Pointer function * It's data type , The asterisk of the function pointer belongs to the function name .

More simply , You can distinguish the two in this way ： The function name with parentheses is the function pointer , Otherwise, it's a pointer function .

3. Use different

The above function pointer and pointer function usage have , But function pointers will be used more , Relatively speaking, it is more difficult , For example, function pointers and callback functions , If it is C++ Non static member function pointer , There will be some differences in its usage , Interested students can follow up tweets or consult relevant books by themselves .

To make a long story short , It's easy to confuse these two things , We must have a deep understanding of the two definitions and differences , Avoid making mistakes .

See here , Are you right “C/C++” I have a little new understanding ~ If you like this article , Move your little finger , Pay attention to it ~