Undefined reference to functions when linking header file c++

Question

I'm trying to do the Chapter8's drill on the books of Bjarne stroustrup. I have followed all the steps but when running the programme i get two errors: undefined reference to 'print_foo' undefined reference to 'print(int)'. I use VSC.

Here are my files:

-----my.h-----

extern int foo;
void print_foo();
void print(int);

-------my.cpp--------

#include"std_lib_facilities.h"
#include"my.h"

void print_foo() 
{
    cout<<"foo = "<<foo<<'\n';
}

void print(int i)
{
    cout<<"i = "<<i<<'\n';
}

-------use.cpp--------

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

using namespace std;

inline void keep_window_open() {char cc;cin>>cc;}

int main()
{   
    int foo = 7;
    print_foo();
    print(99);

    return 0;
}

Show the command line you used for linking, and/or whatever build settings you specified in your IDE. You probably aren't including `my.cpp` in the link. — Nate Eldredge, Sep 18 '21 at 19:53
By default Visual Studio Code only builds one single source file, you need to build with both. Not knowing which OS you're using, I point you to [this VSCode with MinGW on Windows guide](https://code.visualstudio.com/docs/cpp/config-mingw), which very importantly also includes information about the configuration files used to build and which you can edit to build all source files. — Some programmer dude, Sep 18 '21 at 19:58
I don't use VSCode so I'm not sure, but take a look at https://stackoverflow.com/questions/47665886/vs-code-will-not-build-c-programs-with-multiple-ccp-source-files which may help. — Nate Eldredge, Sep 18 '21 at 19:58
However listing all the source files in the VSCode configuration file isn't going to work for larger projects, where one typically want things like dependency tracking and not rebuilding the whole project each build. For that look at something like [this VSCode and CMake guide](https://computingonplains.wordpress.com/building-c-applications-with-cmake-and-visual-studio-code/). There are also many more tools similar to CMake, it just happens to be a popular one. Do some research and find one you like. — Some programmer dude, Sep 18 '21 at 20:01
Does the `int foo = 7;` as a local variable in `main` actually work as a definition for the `extern int foo;` declared in `my.h`? — Nathan Pierson, Sep 18 '21 at 20:04

Yuno · Answer 1 · 2022-07-14T13:44:23.097

TL;DR:

compile the files use.cpp and my.cpp using g++ and declare foo outside main.

I was in this same exercise and had the same problem.

So here is my solution:

Generally the extensions in vscode use one single file, so you will need to use the terminal to specify the other files (in this case, my.cpp).

my.h

extern int foo;
void print_foo();
void print(int i);

my.cpp

#include "my.h"
#include "std_lib_facilities.h"

void print_foo()
{
    cout << foo << endl; // foo is defined in use.cpp
}

void print(int i)
{
    cout << i << endl;
}

use.cpp

#include "my.h"

int foo; // foo is declared outside main. Foo is a global var
int main()
{
    foo = 7; //definition of foo
    print_foo();

    print(99);
}

Note that foo is defined outside main because my.h calls extern int foo;.

Then, compile it using:

g++ use.cpp my.cpp

Undefined reference to functions when linking header file c++

1 Answers1