C++ issue with class declared in a separate object file

Question

I've got a problem when declaring a class in a separate object file and then using it in the main function of another file:

main.cpp:

#include <helloclass.hpp>


using namespace std;

int main() {
    Hello hi;
    hi.hello();

    return 0;
}

helloclass.cpp:

#include <iostream>

using namespace std;

class Hello {
    public:

    void hello() {
        cout << "Hello world\n";
    }

    Hello() {}
};

helloclass.hpp:

class Hello {
    public:

    void hello();


    Hello();
};

Then I ran the following commands:

g++ -I. -c main.cpp
g++ -c helloclass.cpp
g++ -o main main.o helloclass.o

However, the last command gives the following output:

main.o: In function `main':
main.cpp:(.text+0x1f): undefined reference to `Hello::Hello()'
main.cpp:(.text+0x2b): undefined reference to `Hello::hello()'
collect2: error: ld returned 1 exit status

To me, it seems like I'm missing something pretty obvious. Does anyone know how to fix this?

You should not try to program in C++ by trial and error. You should go get a [good book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — Rakete1111, Jun 14 '17 at 17:08

score 1 · Accepted Answer · answered Jun 14 '17 at 17:10

1

You should not redefine the class in the separate source file. Instead include the header-file and implement the Hello::hello function (and the Hello::Hello constructor).

answered Jun 14 '17 at 17:10

Some programmer dude

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I already included the header file in the first line of `main.cpp` and didn't _re_define the class in the source file. – asc11 Jun 14 '17 at 17:25
@asc11 The problematic source file is `helloclass.cpp`. – Some programmer dude Jun 14 '17 at 17:44

C++ issue with class declared in a separate object file

1 Answers1