a multiple definition error in c++

Question

I have 2 files, grabURL.cpp and grabURL.hpp. When I compile the both with gcc it gives me multiple definitions errors.

grabURL.cpp

#include "grabURL.hpp"
size_t buffer(void *ptr, size_t size, size_t nmemb, void *stream) { 
            int numbytes = size*nmemb; 
            char lastchar = *((char *) ptr + numbytes - 1); 
        *((char *) ptr + numbytes - 1) = '\0'; 
            ret.append((char *)ptr); 
            ret.append(1,lastchar); 
        *((char *) ptr + numbytes - 1) = lastchar;
    return size*nmemb; 
 }
void grabURL(std::string url, std::string *theRet){
    curl_easy_setopt(curl,CURLOPT_URL,url.c_str());
curl_easy_setopt(curl,CURLOPT_WRITEFUNCTION, buffer);
res = curl_easy_perform(curl);
*theRet = ret;
}

grabURL.hpp:

#ifndef CURLURLGRAB_HPP
#define CURLURLGRAB_HPP
#include <string>
#include <curl/curl.h>
CURL * curl;
CURLcode res;
std::string ret;
size_t buffer(void *ptr, size_t size, size_t nmemb, void *stream);
void grabURL(std::string url,std::string* theRet);

#endif

I have no idea what I am doing wrong, I thought I did the ifndef thing correctly. What exactly is wrong?

Answer 1

This definitions in your header file:

CURL * curl;
CURLcode res;
std::string ret;

get defined when you include the file from the cpp file, and pressumably some other translation unit (perhaps main.cpp?)

They don't look like something that should be in a header file, but if they do then you can declare them extern within the header file and move the actual definitions to the cpp file.