can't get forward declaration to work in c++

Asked May 01 '19 at 20:15

Active May 01 '19 at 20:15

Viewed 26 times

I'm learning C++, using g++ (Ubuntu 7.4.0-1ubuntu1~18.04) 7.4.0

I have the following code :

main.cpp :

#include <iostream>

void log(std::string message);

int main() {
    log("HI");
}

log.cpp :

#include <iostream>

void log(std::string message) 
{
    std::cout << message << std::endl;
}

When i compile and run, i get the following error :

/tmp/ccYckYXk.o: In function `main':
main.cpp:(.text+0x43): undefined reference to `log(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)'
collect2: error: ld returned 1 exit status

Can i please know what's wrong with my code ?

asked May 01 '19 at 20:15

Mohamed Benkedadra

1,964
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48

What's your compiler command line? – Etienne de Martel May 01 '19 at 20:17
Do you compile `log.cpp` then link against it? – Vivick May 01 '19 at 20:17
3

@Etienne ^ More specifically what's your linker command – πάντα ῥεῖ May 01 '19 at 20:17
1

@πάνταῥεῖ I'm fairly sure he doesn't compile and link separately, but yes, that's what I meant ;) – Etienne de Martel May 01 '19 at 20:18
You need `#include "log.h"` in each of the files. You need a log.h file and you need to compile with `g++ main.cpp log.cpp` – Adam May 01 '19 at 20:19
1

This article talks about what you are asking https://www.cs.fsu.edu/~myers/c++/notes/compilation.html – Adam May 01 '19 at 20:23

can't get forward declaration to work in c++

0 Answers0