Why does this simple C++ program fail to link? I've followed as closely as possible all the articles

I have a program composed of multiple classes, many of which refer to each other. So as per recommendations I create a prototype for each one as a header file. Any module that requires another gets an appropriate #include. The files compile but fail to link and I can't figure out why. Here's an example reduced to the minimum. First a prototype, data.h:

#ifndef DATA_H#define DATA_Hclass Data {    public:        int get();        void set(int);        Data();};#endif

Next is the implementation file, data.cpp:

class Data {    private:        int content;    public:        int get() {            return content;        }        void set(int value) {            content = value;        }        Data() {}};

And finally the main program, test.cpp:

#include "data.h"int main(int argc, char* argv[]) {  Data* data = new Data();  data->set(5);  int n = data->get();  return 0;};

I compile with

g++ -c test.cpp data.cpp

No errors there so I link with

g++ test.o data.o

and the following errors result:

/usr/bin/ld: test.o: in function `main':test.cpp:(.text+0x2f): undefined reference to `Data::Data()'/usr/bin/ld: test.cpp:(.text+0x44): undefined reference to `Data::set(int)'/usr/bin/ld: test.cpp:(.text+0x50): undefined reference to `Data::get()'collect2: error: ld returned 1 exit status

Can anyone explain what I'm doing wrong?