C++ #include not working

Question

I'm trying to include a function like this in C++ and I can't understand why it's not working. I have 3 files.

test.cc

int test() {
  std::cout << "This is a test" << std::endl;
}

test.h

int test();

main.cc

#include "test.h"

int main() {

    test();
    return 0;
}

This is the error I got and the command I used.

c++ main.cc -o main
Undefined symbols for architecture x86_64:
  "test()", referenced from:
      _main in main-12ba52.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

What is the error that you're getting? Post it please. I suspect this is a linker problem, not a compiler problem. — Steve, Jul 15 '14 at 20:15
Nothing wrong with the include, as long as `test.cc` is listed for compilation, howover it wont compile. `std::cout` is not declared. You must `#include ` in `test.cc`. — Havenard, Jul 15 '14 at 20:17
`c++ main.cc -o main` only compiles `main.cc`. You also need to compile `test.cc` into `test.o`, then link `test.o` and `main.o` together into an executable. — Steve, Jul 15 '14 at 20:20
This error is telling you that even though you declared `test()`, it was not defined. It happened because `test.cc` was not included in your project for compilation. — Havenard, Jul 15 '14 at 20:21
Can anyone give me an example makefile for compiling two files together? I tried to make one myself but I kept getting the same linker error. — Francis, Jul 15 '14 at 20:21
possible duplicate of [What is an undefined reference/unresolved external symbol error and how do I fix it?](http://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix) — πάντα ῥεῖ, Jul 15 '14 at 20:38

score 6 · Accepted Answer · answered Jul 15 '14 at 20:26

6

Here's the culprit.

c++ main.cc -o main

You need to link test.o.

c++ main.cc test.cc -o main

answered Jul 15 '14 at 20:26

user3813353

174
4

score 3 · Answer 2 · edited Jul 15 '14 at 20:28

3

Assuming you are using your compiler properly, try to include "test.h" on the top of your test.cc file:

#include "test.h"

int test() {
   std::cout << "This is a test" << std::endl;
}

Compile with:

g++ main.cc test.cc -o main

edited Jul 15 '14 at 20:28

haccks

104,019
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answered Jul 15 '14 at 20:23

Marco

131
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8

score 2 · Answer 3 · answered Jul 15 '14 at 20:21

2

You should add the following to test.cc:

#include "test.h"
#include <iostream>

And make sure you're building/linking with test.cc

answered Jul 15 '14 at 20:21

Ranic

486
4
12

I tried that and it still gave me the linker error. – Francis Jul 15 '14 at 20:22
Just edited to say that you need to make sure you're linking against it. – Ranic Jul 15 '14 at 20:22

Vlad from Moscow · Answer 4 · 2014-07-15T20:23:53.560

1

The definition of function test has undefined behaviour because it does not return a value though it has return type int

int test() {
  std::cout << "This is a test" << std::endl;
}

As you did not report what is the error I think that the problem is that you did not include in the project file test.cc. You have to link it along with file main.cc that the linker would be able to find the definition of test.

edited Jul 15 '14 at 20:23

answered Jul 15 '14 at 20:17

Vlad from Moscow

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Thanks for your answer, I hadn't actually realised that my function was wrong. The problem is that I'm getting a linker error. – Francis Jul 15 '14 at 20:19
@Francis You have to link the both modules together that the linker would be able to find the definition of test. – Vlad from Moscow Jul 15 '14 at 20:21

C++ #include not working

4 Answers4