cpp undefined reference to `class::method'

Question

When I try to compile

$ gcc -lcurl try.cpp

/tmp/ccJs0k9m.o: In function `main':
try.cpp:(.text+0x2d): undefined reference to `getURL::fetch(char*, char*)'
collect2: ld returned 1 exit status

The method appears in the header and the class body. What is wrong exactly?

try.cpp

#include <curl/curl.h>
#include <curl/easy.h>
#include "getURL.h"

int main(void) {

getURL my_getURL;

my_getURL.fetch("http://stackoverflow.com/", "file");
}

getURL.h

#ifndef _getURL
#define _getURL

class getURL {
public:
    void fetch(char *url, char *filename);
};
#endif

getURL.cpp

#include <curl/curl.h>
#include <curl/easy.h>
#include <stdio.h>


class getURL {

private CURL *curl;

public getURL() {
    //code
}

public void fetch(char *url, char *filename) {
    //code
}

private size_t write_data(void *ptr, size_t size, size_t nmemb, FILE *stream) {
    //code
}

public ~getURL() {
    //code
}

} //end class

How does getURL.cpp even compile? Doesn't look like C++. Where are the colons after the access specifiers? Semicolon after the class definition? And if it could somehow be convinced to compile, then getURL.cpp and try.cpp have completely different ideas about what a getURL object looks like. [edit: ninja'd] — Oktalist
– Oktalist, Commented Sep 1, 2012 at 17:33

6502 · Accepted Answer · 2012-09-01 17:25:42Z

3

You are not using the correct implementation syntax; it's

getURL::getURL() {
    //code
}

void getURL::fetch(char *url, char *filename) {
    //code
}

size_t getURL::write_data(void *ptr, size_t size, size_t nmemb, FILE *stream) {
    //code
}

getURL::~getURL() {
    //code
}

Note that you must not repeat the class part in the implementation (just include the header instead). Note also that you're not allowed to have private members in the implementation that are not visible in the declaration... this is unfortunate but it's how the language is defined.

answered Sep 1, 2012 at 17:25

6502

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3 Comments

juanchopanza Over a year ago

They also need to lose a lot of "public" and "private" in the function definitions.

user1640807 Over a year ago

I added getURL:: to the start of the method names, removed public and private before them. I added the private method to the header file (why?). And removed class {} from getURL.cpp. I think that sums up what you said. Exact same error.

6502 Over a year ago

Also to compile a C++ program you need to call g++, not gcc.

Drew Dormann · Accepted Answer · 2012-09-01 18:09:21Z

0

You are only compiling one of your source files.

Change:

gcc -lcurl try.cpp

to:

gcc -lcurl try.cpp getURL.cpp

answered Sep 1, 2012 at 18:09

Drew Dormann

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