C++ class member function and callback from C API

Question

I am trying to learn how to call this write_data(…) function from the funmain() function in the class as shown in the code bellow. (I know this program works if I just list these two functions without putting it inside a class).

curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data) line gives me error and wouldn't let me call the write_data(…) function. Can you please correct my code and tell me how I can achieve this. Any help would be greatly appreciated. Thanks.

error C3867: 'go_website::write_data': function call missing argument list; use '&go_website::write_data' to create a pointer to member

---

//Microsoft Visual Studio 10 in C++
#define CURL_STATICLIB
#include <stdio.h>
#include <curl/curl.h>
#include <curl/types.h>
#include <iostream>
#include <curl/easy.h>
#include <string>
using namespace std;

extern "C" typedef size_t curl_write_callback(void *ptr, size_t size, size_t nmemb, FILE *stream);
class go_website
{
public:
static curl_write_callback write_data;

void funmain()
{
    CURL *curl;
    FILE *fp;
    CURLcode res;
    char *url = "http://www.shorturl.com/";
    char outfilename[FILENAME_MAX] = "C:\\bbb.txt";
    curl = curl_easy_init();
    if (curl) {
        fp = fopen(outfilename,"wb");
        curl_easy_setopt(curl, CURLOPT_URL, url);
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, &write_data); 
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, fp);
        res = curl_easy_perform(curl);
        curl_easy_cleanup(curl);
        fclose(fp);
    }
}};

extern "C" size_t write_data(void *ptr, size_t size, size_t nmemb, FILE *stream) 
{
size_t written;
written = fwrite(ptr, size, nmemb, stream);
return written;
}

int main()
{
   go_website a;
   a.funmain();
   return 0;
}

Answer 1

It is possible http://curl.haxx.se/docs/faq.html#Using_C_non_static_functions_f

 // f is the pointer to your object.
 static YourClass::func(void *buffer, size_t sz, size_t n, void *f)
 {
   // Call non-static member function.
   static_cast<YourClass*>(f)->nonStaticFunction();
 }
 // This is how you pass pointer to the static function:
 curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass:func);
 curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);

Answer 2

These two lines won't work:

curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, fp);

The second is the easiest to fix: fp is a file pointer, not a function, you're setting the wrong attribute, I guess you want CURLOPT_WRITEDATA .

For the callback function, you need a function pointer. The name of an ordinary function automatically decays to its address, although using the address-of operator ( &functionname ) is cleaner.

Class member functions do not automatically decay. In fact, a non-static class member function is totally incompatible with a normal function pointer, since there's no way to handle this . Luckily you don't need a non-static member function, since no non-static members are used inside the callback.

Make the callback function static and extern "C" :

 
 
 
 
  
  
  extern "C" typedef size_t curl_write_callback(void *ptr, size_t size, size_t nmemb, FILE *stream); class go_website { public: static curl_write_callback write_data; // ... }; extern "C" size_t go_website::write_data(void *ptr, size_t size, size_t nmemb, FILE *stream) { size_t written; written = fwrite(ptr, size, nmemb, stream); return written; }
 
 
  

and then take its address, using the address-of operator and fully-qualified function name:

 
 
 
 
  
  
  curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, &go_website::write_data);
 
 
  

This part was even explained in the error message. But it didn't tell you that you needed static or extern "C" , this is the problem with variable argument lists, they aren't typesafe.

After reading the Standard (section 7.5 [dcl.link] , and especially paragraph 4 and its examples), this isn't allowed. The member function still has C++ language linkage, for both its name (not important) and its type (this is the deal-breaker).

You have to use a global function for the callback:

curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, &write_data);

and then pass a pointer to it:

 curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, &write_data); 

Answer 3

I know this program works if I just list these two functions without putting it inside a class

If it works outside of a class, but not inside, then you likely need to use the "this" pointer.

        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, this->write_data);

Answer 4

The problem is that you want the WRITEFUNCTION to be a member function of your class. However, curl doesn't know which instance of the class to call it on. You need to create a static function that you pass to WRITEFUNCTION and then pass your this pointer as the CURLOPT_WRITEDATA parameter. Then in your static member function you can use the user_data pointer (which is your "this" from WRITE_DATA) as the instance of the class.

Maybe this question will help: curl WRITEFUNCTION and classes