How to build a binary from multiple c++ files? - c++

Question

I have started learning C++, and have gotten stuck when working with multiple files. To practice basic classes, I wrote three different files,
working.cpp
word.cpp
word.h
word.cpp:
#include <iostream>
#include "word.h"
using namespace std;
class word{
public:
char *word;
void createWord(char *str)
{
word = str;
}
void print_word(void)
{
cout<<word<<endl;
}
char * getWord()
{
return word;
}
}
working.cpp
#include <iostream>
#include "word.h"
void printWord(word);
using namespace std;
int main()
{
word one;
one.createWord("one");
printWord(one);
}
void printWord(word a)
{
cout<<a.getWord()<<endl;
}
word.h
class word;
These are three different files, so I am not sure how to compile them. What I have tried is
g++ working.cpp word.cpp
However, the compiler doesn't recognize word as a class, and gives me the following errors
working.cpp: In function 'int main()':
working.cpp:7:7: error: aggregate 'word one' has incomplete type and cannot be defined
working.cpp:7:12: error: aggregate 'word two' has incomplete type and cannot be defined
working.cpp:7:17: error: aggregate 'word three' has incomplete type and cannot be defined
working.cpp: In function 'void printWord(word)':
working.cpp:19:6: error: 'aha' has incomplete type
In file included from working.cpp:2:0:
word.h:2:7: error: forward declaration of 'class word'
word.cpp:25:1: error: expected ';' after class definition
What am I doing wrong while compiling?

Answer 1

You need to include more of the definition of word in the header file. Something like this:
class word
{
public:
char *word;
void createWord(char *str);
void print_word(void);
char * getWord();
};
Then, change word.cpp to just have the implementations:
void word::createWord(char *str)
{
word = str;
}
void word::print_word(void)
{
cout<<word<<endl;
}
char * word::getWord()
{
return word;
}
compile and link!
You need to have more of the word class in the header so that your other translation unit can know how big the class is (to reserve enough space for the instance you're creating) as well as to know the names of the methods you want to call.

Answer 2

Just mentioning the class name in the header file (a so-called forward declaration) is not enough; you need a complete class declaration (which declares all the fields and functions of the class):
class word {
public:
char *word;
void createWord(char *str);
void print_word(void);
char * getWord();
};

Answer 3

There is no actual declaration of class word in word.h
word.h:2:7: error: forward declaration of 'class word'
I would advise you to read Bjarne Stroustrup's brilliant book "The C++ Programming Language" to get started.