Question

I have a question about the relationship between header and implementation files in C++.

I have received two empty header files and one empty implementation file. When I say “empty” I mean that the files only contain method declarations; I have to implement the code inside each method. (I thought that posting the code for the three files would be redundant, since they only contain declarations..)

The three files are:

• interface_complexmutex.h
• complexmutex.h
• complexmutex.cpp

interface_complexmutex.h has virtual methods for complexmutex. Also, the complexmutex class inherits from interface_complexmutex.

These files have basic mutex functions that I have to implement. I have 2 questions:

1. Why do I need interface_complexmutex.h? It does not really have a .cpp file, and the entire implementation will be in complexmutex files, right?

2. How should I include these files and use complexmutex class in a program? (if I have the init() method in complexmutex.cpp, for example)

   Should main.cpp look like this?

   #include <stdlib.h>
   #include <stdio.h>
   #include "interface_complexmutex.h"
   #include "complexmutex.h"
   using namespace std;
   int main(){
   
       complexmutex a;
   
       /* do something with mutex to test */
   
       return 0;
   }
   

   I made my own Makefile, and the code above seems to run fine so far.

Answer 1

1) You need an interface only if it is likely that the implementation will change later on. That will leave you free to make all the changes you want to the internals of your code without impacting the people using it to create software. That means that the users should only see the interface class and never the implementation.

To do that you’ll need to implement what is called a factory pattern. This is a function that will create an instance of the implementation class and return it as a pointer (or reference) to the interface class. You probably can write that function as a static method of your interface class.

2) In a program you should image you are the user of your code. The user doesn’t care how you implement your feature, so it should not use the implementation at all. If possible it should only include the interface header and call the factory method like this:

 #include "interface_complexmutex.h"
 using namespace std;

 int main(...) {
     interface_complemutex* mutex = interface_complexmutex::create_new(...);
     // do something interesting with the mutex
 }

In this example, create_new is your factory method. You can have parameters for it (usually an enumeration) if you have different implementations.

When using interfaces (actually abstract classes in C++), you need to use pointers or references. You can’t create values for abstract classes. Of course, if you write test code, it’s a bit different because test code is not meant to be seen/used by the users of your library.

Notes:

Let me point out that the naming is quite bad. The interface should be named complexmutex and the implementation complexmutex_impl since users should use the interface more, its name should be shorter (and you can replace the “impl” by something relevant such as “pthread” if you use pthreads to implement your mutex class.

As a PS note, in C++0x there is a thread library including mutex available. It is supported by some compilers already.