How to pass a pointer to object method? - c++

Question

I'm trying to make a CurlResponse object encapsulating libcurl response. My implementation of curl options WRITEFUNCTION and HEADERFUNCTION is mostly the same, the only difference being that in first case I'm calling response->appendBody and in the second - response->appendHeader. I would like to have one function and pass a pointer to appropriate method as a parameter, e.g. WRITEDATA would be response->appendBody, and I could call writer(data). However when I execute the below code, I get an error:
error: cannot pass objects of non-trivially-copyable type ‘struct std::_Bind<std::_Mem_fn<void (CurlResponse::*)(std::basic_string<char>)>(CurlResponse*, std::_Placeholder<1>)>’ through ‘...’
...
#include <iostream>
#include <string>
#include <functional>
#include <curl/curl.h>
using namespace std;
class CurlResponse {
public:
void appendBody(string data) {
cout << "Append body " << data << endl;
}
void appendHeader(string data) {
cout << "Append header " << data << endl;
}
};
//size_t WriteMemoryCallback(char * contents, size_t size, size_t nmemb, CurlResponse* response)
size_t WriteMemoryCallback(char * contents, size_t size, size_t nmemb, function<void(string)> writer)
{
size_t realsize = size * nmemb;
string data(contents, realsize);
// response->appendBody(data);
writer(data);
return realsize;
}
size_t WriteHeaderCallback(char * contents, size_t size, size_t nmemb, CurlResponse* response)
{
size_t realsize = size * nmemb;
string data(contents, realsize);
response->appendHeader(data);
return realsize;
}
int main() {
CURL *curl;
CURLcode res;
curl = curl_easy_init();
if (! curl) return 1;
curl_easy_setopt(curl, CURLOPT_URL, "http://localhost");
CurlResponse* response = new CurlResponse();
auto writeBody = std::bind(&CurlResponse::appendBody, response, placeholders::_1);
writeBody("Test writing to body");
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteMemoryCallback);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, writeBody);
// curl_easy_setopt(curl, CURLOPT_WRITEDATA, response);
curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, WriteHeaderCallback);
curl_easy_setopt(curl, CURLOPT_HEADERDATA, response);
res = curl_easy_perform(curl);
if(res != CURLE_OK) {
fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
}
curl_easy_cleanup(curl);
return 0;
}
How do I fix this? Does this mean I can use std::binded functions in the same function, but can't pass them anywhere?

Answer 1

The problem is that you are trying to pass a complex object through ..., as the compiler already tells you. The probably best solution is to first wrap the std::bind in a std::function object, to avoid having to repeat the complete type:
function<void(string)> writeBodyPass(writeBody);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, &writeBodyPass);
You then have to fix the signature (and body) of your callback, as you are now receiving a pointer to the std::function object:
size_t WriteMemoryCallback(char * contents, size_t size, size_t nmemb, function<void(string)> *writer)
{
size_t realsize = size * nmemb;
string data(contents, realsize);
(*writer)(data);
return realsize;
}