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C++ - a complete example with polymorphism, and how to split it into project files

Archive - Originally posted on "The Horse's Mouth" - 2010-11-16 12:16:43 - Graham Ellis

One of the things that newcomers to Object Oriented projects find most difficult is to know how all the various parts of the OO paradigm are related to each other - and that's especially the case on larger C++ projects, where the number of files of source involved is likely to be substantial.

Yesterday and today, I have written a brand new object oriented example, showing a base class of object (an Animal), two subclasses (Pet and Human) and a test program. Initially, we wrote the code all in a single file and it's [here]. From the top, you'll find the header definitions for animal, pet and human, then the methods for animal, pet and human, and finally a test program.

But that's not how you'll want the project in the longer term. You'll want the test harness separated out, and you'll want the other classes each in their own files. You'll also need the header sections in separate files so that you can successfully compile the inherited classes and / or the test program without having to duplicate definitions. All these extras are done using the C preprocessor directive #include, and we also use #ifndef directives and defined preprocessor variables to avoid base class definitions being loaded multiple times.

The net result of this is that a single file demonstration has split into no fewer that 8 individual files to show you how even this - quite simple - project would sprout. All eight files are available on our server, as follows:

second_inheritance.cpp - the main test program
pet.cpp - one of the subclasses
human.cpp - the other subclass
animal.cpp - the base class on which the subclasses are based
pet.h - the headers for the pet subclass
human.h - the headers for the human subclass
animal.h - the headers for the base class (#ifndef is vital in this one!)
Makefile - the directives for make defining how the build is to work

I've added a "make test" to the Makefile - so that the user can check that the build has worked; this is also a nice running demonstration of make, and how it will only do those jobs that it needs to, based on file timestamps:

wizard:cpp graham$ make
g++ -c -o second_inheritance.o second_inheritance.cpp
g++ -c -o pet.o pet.cpp
g++ -c -o animal.o animal.cpp
g++ -c -o human.o human.cpp
g++ -o second_i second_inheritance.o pet.o animal.o human.o
wizard:cpp graham$ make
make: `second_i' is up to date.
wizard:cpp graham$ make test
Living there ... Mrs Smith aged 21
Living there ... Mrs Smith aged 52
Living there ... Robin aged 6
Living there ... Tigger aged 10
Test Completed - should have give 4 line report
wizard:cpp graham$

Subject covered on all three of our C++ courses ... we offer a five day course for delegates who have done very little programming before (perhaps none), a four day course for experienced programmers who have not written much C, and a two day conversion course for experienced C programmers. The next dates for public runnings of these course can be found [here], and I can also run any of the courses for your private group - at Melksham, or elsewhere in the UK, or further afield.