c&c++ function pointer

Today, when reading the source code of Libcurl, we find that the method of defining function pointers is different from the usual way. Detailed analysis of a bit.

In Libcurl code, a set of function pointers that send data is defined. As shown below:

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

where curl_send is defined as follows:

Code Catalog: lib/urldata.h/* Return the count of bytes sent, or-1 On Error */typedef ssize_t (curl_send) (struct connectdata *c Onn,/* Connection data */                            int sockindex,/            * Socketindex */                            const void *BUF,/          * data to write */                            Size_ T len,/               * Max amount to write */                            curlcode *err);           /* ERROR to return */

The confusion is that we usually use TypeDef to define function pointers, which are generally written in the following form:

Defines a function pointer that receives an shaping parameter and returns an integer value. typedef int (*PFUNC) (int);

However, the curl_send definition does not have a pointer symbol on it. After reviewing some information, it is possible to find such a definition method.

So, wrote the following program to verify a bit.

#ifdef __cplusplus#include <iostream> #else # include <stdio.h> #endifint testFunc (int para) {#ifdef __ Cplusplusstd::cout << "C + + parameter is:" << para << Std::endl; #elseprintf ("C parameter is:%d\n", para ); #endifreturn 0;} int main () {typedef int (PTESTFUNC) (int);p TestFunc *pfunc = testFunc;   Mode 1:ok pfunc (1111);                   Mode 2:ok (*pfunc) (2222);                Way 3:okptestfunc *PFUNC2 = &testFunc; Way 4:okpfunc2 (3333); return 0;}

If you save the above program as a C program file (. c), compile it to get the following results:
C parameter is:1111
C parameter is:2222
C parameter is:3333

If you save as a C + + program file (. cpp), you get the result:
C + + parameter is:1111
C + + parameter is:2222
C + + parameter is:3333

From the above results you can see:
1. It is also possible to use the same function declaration method as curl_send. As the definition of ptestfunc above.
2. For the initialization of the function pointer Pfunc, it is possible to use either of these methods. Refer to the above Way 1 with mode 4.
3. The call to function pointer Pfunc can also be used in different ways, referring to mode 2 and Mode 3.

Of course, we also use the traditional method of declaring function pointers, as shown in the following procedure:

int main () {typedef int (*PTESTFUNC) (int);p TestFunc pFunc = testFunc;   Mode 5:okpfunc (1111);                  Mode 6:ok (*pfunc) (2222);               Way 7:okptestfunc PFUNC2 = &testFunc; Way 8:okpfunc2 (3333); return 0;}

The result is exactly the same as described earlier.

Copyright NOTICE: This article for Bo Master original article, without Bo Master permission not reproduced.

c&c++ function pointer