C & amp; C ++ function pointer

C & C ++ function pointer

Today, when I read the libcurl source code, I found that the method for defining the function pointer inside is different from the method I used at ordinary times. I analyzed it in detail.

The libcurl Code defines a set of function pointers for sending data. As follows:

 

// Code Directory: lib/urldata. hstruct connectdata {... Curl_send * send [2]; ......};

The definition of Curl_send is as follows:

// Code Directory: lib/urldata. h/* return the count of bytes sent, or-1 on error */typedef ssize_t (Curl_send) (struct connectdata * conn,/* connection data */int sockindex, /* socketindex */const void * buf,/* data to write */size_t len,/* max amount to write */CURLcode * err);/* error to return */

I am confused that if we usually use typedef to define function pointers, they are generally written in the following format:

// Define a function pointer. the pointer to the function receives an integer parameter and returns an integer value. Typedef int (* pFunc) (int );

However, the curl_send definition does not contain a pointer. After reading some documents, we found that such a definition method is also possible.

 

So I wrote the following program to verify it.

 

# Ifdef _ cplusplus # include
 
  
# Else # include
  
   
# Endifint testFunc (int para) {# ifdef _ cplusplusstd: cout <C ++ parameter is: <para <std: endl; # elseprintf (C parameter is: % d, para); # endifreturn 0;} int main () {typedef int (pTestFunc) (int); pTestFunc * pFunc = testFunc; // Method 1: OK · pFunc (1111); // Method 2: OK (* pFunc) (2222); // method 3: okpTestFunc * pFunc2 = & testFunc; // Method 4: okpFunc2 (3333); return 0 ;}
  
 

If you save the above program as a C program file (. c) and compile it, the following running result is obtained:
C parameter is: 1111
C parameter is: 2222
C parameter is: 3333

If the file is saved as a C ++ program file (. cpp), run the following command:
C ++ parameter is: 1111
C ++ parameter is: 2222
C ++ parameter is: 3333

The above results show that:
1. It is also possible to use the same function declaration method as curl_send. The above pTestFunc definition.
2. For the initialization of the function pointer pFunc, either method is acceptable. Refer to the methods 1 and 4 above.
3. For function pointer pFunc calls, you can also use different methods. For more information, see Methods 2 and 3.
4. For C and C ++, these definitions and function pointers are supported.

Of course, we can also use the traditional function pointer declaration method, as shown in the following program:

 

Int main () {typedef int (* pTestFunc) (int); pTestFunc pFunc = testFunc; // Method 5: okpFunc (1111); // Method 6: OK (* pFunc) (2222); // Method 7: okpTestFunc pFunc2 = & testFunc; // Method 8: okpFunc2 (3333); return 0 ;}

The running result is exactly the same as described above.