Why should I not include cpp files and instead use

Question

So I finished my first C++ programming assignment and received my grade. But according to the grading, I lost marks for including cpp files instead of compiling and linking them. I'm not too clear on what that means.

Taking a look back at my code, I chose not to create header files for my classes, but did everything in the cpp files (it seemed to work fine without header files...). I'm guessing that the grader meant that I wrote '#include "mycppfile.cpp";' in some of my files.

My reasoning for #include'ing the cpp files was: - Everything that was supposed to go into the header file was in my cpp file, so I pretended it was like a header file - In monkey-see-monkey do fashion, I saw that other header files were #include'd in the files, so I did the same for my cpp file.

So what exactly did I do wrong, and why is it bad?

Answer 1

While it is certainly possible to do as you did, the standard practice is to put shared declarations into header files (.h), and definitions of functions and variables - implementation - into source files (.cpp).

As a convention, this helps make it clear where everything is, and makes a clear distinction between interface and implementation of your modules. It also means that you never have to check to see if a .cpp file is included in another, before adding something to it that could break if it was defined in several different units.

Answer 2

Think of cpp files as a black box and the .h files as the guides on how to use those black boxes.

The cpp files can be compiled ahead of time. This doesn't work in you #include them, as it needs to actual "include" the code into your program each time it compiles it. If you just include the header, it can just use the header file to determine how to use the precompiled cpp file.

Although this won't make much of a difference for your first project, if you start writing large cpp programs, people are going to hate you because compile times are going to explode.

Also have a read of this: Header File Include Patterns

Answer 3

It's like writing a book, you want to print out finished chapters only once

Say you are writing a book. If you put the chapters in separate files then you only need to print out a chapter if you have changed it. Working on one chapter doesn't change any of the others.

But including the cpp files is, from the compiler's point of view, like editing all of the chapters of the book in one file. Then if you change it you have to print all the pages of the entire book in order to get your revised chapter printed. There is no "print selected pages" option in object code generation.

Back to software: I have Linux and Ruby src lying around. A rough measure of lines of code...

     Linux       Ruby
   100,000    100,000   core functionality (just kernel/*, ruby top level dir)
10,000,000    200,000   everything 

Any one of those four categories has a lot of code, hence the need for modularity. This kind of code base is surprisingly typical of real-world systems.

Answer 4

If you #include a cpp file in several other files in your program, the compiler will try to compile the cpp file multiple times, and will generate an error as there will be multiple implementations of the same methods.

Compilation will take longer (which becomes a problem on large projects), if you make edits in #included cpp files, which then force recompilation of any files #including them.

Just put your declarations into header files and include those (as they don't actually generate code per se), and the linker will hook up the declarations with the corresponding cpp code (which then only gets compiled once).

Answer 5

I will suggest you to go through Large Scale C++ Software Design by John Lakos. In the college, we usually write small projects where we do not come across such problems. The book highlights the importance of separating interfaces and the implementations.

Header files usually have interfaces which are supposed not to be changed so frequently. Similarly a look into patterns like Virtual Constructor idiom will help you grasp the concept further.

I am still learning like you :)

Answer 6

The typical solution is to use .h files for declarations only and .cpp files for implementation. If you need to reuse the implementation you include the corresponding .h file into the .cpp file where the necessary class/function/whatever is used and link against an already compiled .cpp file (either an .obj file - usually used within one project - or .lib file - usually used for reusing from multiple projects). This way you don't need to recompile everything if only the implementation changes.