Creating a C++ class to read data from a website to a string using libcurl - c++

Question

I am using http://ubuntuforums.org/showthread.php?t=781021 as a guide for how to write a c++ program that use libcurl to check a website and downloads the text to a c++ app. When I copy the code to my visual studio project, it works with only slight modification. However, I hit a snag trying to convert it to class based files.
I created a .h and .cpp files to run the query. What I didn't realize before was that handle_data was not a typical function, since it is not given any arguments or even () in the midway down Viewer.cpp. This is how the above site did it, and it worked, but I'm at a loss as to why or how to convert it now.
Does anyone know whats going on here or how to fix it?
Thanks!
Viewer.h:
#pragma once
#include <string>
#include "curl.h"
class Viewer
{
public:
Viewer(void);
~Viewer(void);
std::string view(std::string q);
private:
size_t handle_data(void *ptr, size_t size, size_t nmemb, void *stream);
std::string contents;
};
Viewer.cpp:
#include "stdafx.h"
#include "Viewer.h"
#include <iostream>
Viewer::Viewer(void)
{
std::cout << "ViewerCreated!\n";
}
Viewer::~Viewer(void)
{
}
size_t Viewer::handle_data(void *ptr, size_t size, size_t nmemb, void *stream)
{
int numbytes = size*nmemb;
// The data is not null-terminated, so get the last character, and replace
// it with '\0'.
char lastchar = *((char *) ptr + numbytes - 1);
*((char *) ptr + numbytes - 1) = '\0';
contents.append((char *)ptr);
contents.append(1,lastchar);
*((char *) ptr + numbytes - 1) = lastchar; // Might not be necessary.
return size*nmemb;
}
std::string Viewer::view(std::string q)
{
char* url = "www.google.com";
CURL* curl = curl_easy_init();
if(curl)
{
// Tell libcurl the URL
curl_easy_setopt(curl,CURLOPT_URL, url);
// Tell libcurl what function to call when it has data
curl_easy_setopt(curl,CURLOPT_WRITEFUNCTION,handle_data);
// Do it!
CURLcode res = curl_easy_perform(curl);
curl_easy_cleanup(curl);
if (res == 0)
std::cout << contents << std::endl;
else
std::cerr << "Error: " << res << std::endl;
}
return contents;
}

Answer 1

You should not replace the last character of the chunk that you have received. Instead you should force that the buffer containing the appended blocks has a '\0' just after the end of the data. This way it can be printed, but if you are going to store that in a file, you should just write the bytes that you have received.
This has always worked for me:
struct MemoryStruct chunk;
chunk.memory=NULL;
chunk.size = 0;
...
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteMemoryCallback);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, (void *)&chunk);
And the callback:
struct MemoryStruct {
char *memory;
size_t size;
};
static void *myrealloc(void *ptr, size_t size)
{
if(ptr)
return realloc(ptr, size);
else
return malloc(size);
}
static size_t WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
{
size_t realsize = size * nmemb;
struct MemoryStruct *mem = (struct MemoryStruct *)data;
mem->memory = (char *)myrealloc(mem->memory, mem->size + realsize + 1);
if (mem->memory) {
memcpy(&(mem->memory[mem->size]), ptr, realsize);
mem->size += realsize;
mem->memory[mem->size] = 0;
}
return realsize;
}

Answer 2

handle_data is used as a parameter for curl_easy_setopt. What that means is that you have to program what handle_data does when it receives the next data chunk (providing the data chunk is what curl will do for you).

Answer 3

handle_data is being passed to curl_easy_setopt as a callback function, which it will call when there's data available. Because of that, handle_data must be called as a static function (that is, one not receiving an object pointer, because cURL doesn't have one). However, you're not declaring it as static. Even if you do, you are accessing contents inside it, which is invalid.