Why should I not include cpp files and instead use a header?

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?

To the best of my knowledge, the C++ standard knows no difference between header files and source files. As far as the language is concerned, any text file with legal code is the same as any other. However, although not illegal, including source files into your program will pretty much eliminate any advantages you would've got from separating your source files in the first place.

Essentially, what #include does is tell the preprocessor to take the entire file you've specified, and copy it into your active file before the compiler gets its hands on it. So when you include all the source files in your project together, there is fundamentally no difference between what you've done, and just making one huge source file without any separation at all.

"Oh, that's no big deal. If it runs, it's fine," I hear you cry. And in a sense, you'd be correct. But right now you're dealing with a tiny tiny little program, and a nice and relatively unencumbered CPU to compile it for you. You won't always be so lucky.

If you ever delve into the realms of serious computer programming, you'll be seeing projects with line counts that can reach millions, rather than dozens. That's a lot of lines. And if you try to compile one of these on a modern desktop computer, it can take a matter of hours instead of seconds.

"Oh no! That sounds horrible! However can I prevent this dire fate?!" Unfortunately, there's not much you can do about that. If it takes hours to compile, it takes hours to compile. But that only really matters the first time -- once you've compiled it once, there's no reason to compile it again.

Unless you change something.

Now, if you had two million lines of code merged together into one giant behemoth, and need to do a simple bug fix such as, say, x = y + 1, that means you have to compile all two million lines again in order to test this. And if you find out that you meant to do a x = y - 1 instead, then again, two million lines of compile are waiting for you. That's many hours of time wasted that could be better spent doing anything else.

"But I hate being unproductive! If only there was some way to compile distinct parts of my codebase individually, and somehow link them together afterwards!" An excellent idea, in theory. But what if your program needs to know what's going on in a different file? It's impossible to completely separate your codebase unless you want to run a bunch of tiny tiny .exe files instead.

"But surely it must be possible! Programming sounds like pure torture otherwise! What if I found some way to separate interface from implementation? Say by taking just enough information from these distinct code segments to identify them to the rest of the program, and putting them in some sort of header file instead? And that way, I can use the #include preprocessor directive to bring in only the information necessary to compile!"

Hmm. You might be on to something there. Let me know how that works out for you.

This is probably a more detailed answer than you wanted, but I think a decent explanation is justified.

In C and C++, one source file is defined as one translation unit. By convention, header files hold function declarations, type definitions and class definitions. The actual function implementations reside in translation units, i.e .cpp files.

The idea behind this is that functions and class/struct member functions are compiled and assembled once, then other functions can call that code from one place without making duplicates. Your functions are declared as "extern" implicitly.

/* Function declaration, usually found in headers. */
/* Implicitly 'extern', i.e the symbol is visible everywhere, not just locally.*/
int add(int, int);

/* function body, or function definition. */
int add(int a, int b) 
{
   return a + b;
}

If you want a function to be local for a translation unit, you define it as 'static'. What does this mean? It means that if you include source files with extern functions, you will get redefinition errors, because the compiler comes across the same implementation more than once. So, you want all your translation units to see the function declaration but not the function body.

So how does it all get mashed together at the end? That is the linker's job. A linker reads all the object files which is generated by the assembler stage and resolves symbols. As I said earlier, a symbol is just a name. For example, the name of a variable or a function. When translation units which call functions or declare types do not know the implementation for those functions or types, those symbols are said to be unresolved. The linker resolves the unresolved symbol by connecting the translation unit which holds the undefined symbol together with the one which contains the implementation. Phew. This is true for all externally visible symbols, whether they are implemented in your code, or provided by an additional library. A library is really just an archive with reusable code.

There are two notable exceptions. First, if you have a small function, you can make it inline. This means that the generated machine code does not generate an extern function call, but is literally concatenated in-place. Since they usually are small, the size overhead does not matter. You can imagine them to be static in the way they work. So it is safe to implement inline functions in headers. Function implementations inside a class or struct definition are also often inlined automatically by the compiler.

The other exception is templates. Since the compiler needs to see the whole template type definition when instantiating them, it is not possible to decouple the implementation from the definition as with standalone functions or normal classes. Well, perhaps this is possible now, but getting widespread compiler support for the "export" keyword took a long, long time. So without support for 'export', translation units get their own local copies of instantiated templated types and functions, similar to how inline functions work. With support for 'export', this is not the case.

For the two exceptions, some people find it "nicer" to put the implementations of inline functions, templated functions and templated types in .cpp files, and then #include the .cpp file. Whether this is a header or a source file doesn't really matter; the preprocessor does not care and is just a convention.

A quick summary of the whole process from C++ code (several files) and to a final executable:

• The preprocessor is run, which parses all the directives which starts with a '#'. The #include directive concatenates the included file with inferior, for example. It also does macro-replacement and token-pasting.
• The actual compiler runs on the intermediate text file after the preprocessor stage, and emits assembler code.
• The assembler runs on the assembly file and emits machine code, this is usually called an object file and follows the binary executable format of the operative system in question. For example, Windows uses the PE (portable executable format), while Linux uses the Unix System V ELF format, with GNU extensions. At this stage, symbols are still marked as undefined.
• Finally, the linker is run. All the previous stages were run on each translation unit in order. However, the linker stage works on all the generated object files which were generated by the assembler. The linker resolves symbols and does a lot of magic like creating sections and segments, which is dependent on the target platform and binary format. Programmers aren't required to know this in general, but it surely helps in some cases.

Again, this was definetely more than you asked for, but I hope the nitty-gritty details helps you to see the bigger picture.

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.

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

Header files usually contain declarations of functions / classes, while .cpp files contain the actual implementations. At compile time, each .cpp file gets compiled into an object file (usually extension .o), and the linker combines the various object files into the final executable. The linking process is generally much faster than the compilation.

Benefits of this separation: If you are recompiling one of the .cpp files in your project, you don't have to recompile all the others. You just create the new object file for that particular .cpp file. The compiler doesn't have to look at the other .cpp files. However, if you want to call functions in your current .cpp file that were implemented in the other .cpp files, you have to tell the compiler what arguments they take; that is the purpose of including the header files.

Disadvantages: When compiling a given .cpp file, the compiler cannot 'see' what is inside the other .cpp files. So it doesn't know how the functions there are implemented, and as a result cannot optimize as aggressively. But I think you don't need to concern yourself with that just yet (:

The basic idea that headers are only included and cpp files are only compiled. This will become more useful once you have many cpp files, and recompiling the whole application when you modify only one of them will be too slow. Or when the functions in the files will start depending on each other. So, you should separate class declarations into your header files, leave implementation in cpp files and write a Makefile (or something else, depending on what tools are you using) to compile the cpp files and link the resulting object files into a program.

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).

re-usability, architecture and data encapsulation

here's an example:

say you create a cpp file which contains a simple form of string routines all in a class mystring, you place the class decl for this in a mystring.h compiling mystring.cpp to a .obj file

now in your main program (e.g. main.cpp) you include header and link with the mystring.obj. to use mystring in your program you don't care about the details how mystring is implemented since the header says what it can do

now if a buddy wants to use your mystring class you give him mystring.h and the mystring.obj, he also doesn't necessarily need to know how it works as long as it works.

later if you have more such .obj files you can combine them into a .lib file and link to that instead.

you can also decide to change the mystring.cpp file and implement it more effectively, this will not affect your main.cpp or your buddies program.

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.

If it works for you then there is nothing wrong with it -- except that it will ruffle the feathers of people who think that there is only one way to do things.

Many of the answers given here address optimizations for large-scale software projects. These are good things to know about, but there is no point in optimizing a small project as if it were a large project -- that is what is known as "premature optimization". Depending on your development environment, there may be significant extra complexity involved in setting up a build configuration to support multiple source files per program.

If, over time, your project evolves and you find that the build process is taking too long, then you can refactor your code to use multiple source files for faster incremental builds.

Several of the answers discuss separating interface from implementation. However, this is not an inherent feature of include files, and it is quite common to #include "header" files that directly incorporate their implementation (even the C++ Standard Library does this to a significant degree).

The only thing truly "unconventional" about what you have done was naming your included files ".cpp" instead of ".h" or ".hpp".

When you compile and link a program the compiler first compiles the individual cpp files and then they link (connect) them. The headers will never get compiled, unless included in a cpp file first.

Typically headers are declarations and cpp are implementation files. In the headers you define an interface for a class or function but you leave out how you actually implement the details. This way you don't have to recompile every cpp file if you make a change in one.

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 :)

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.

There are times when non conventional programming techniques are actually quite useful and solve otherwise difficult (if not impossible problems).

If C source is generated by third party applications such as lexx and yacc they can obviously be compiled and linked separately and this is the usual approach.

However there are times when these sources can cause linkage problems with other unrelated sources. You have some options if this occurs. Rewrite the conflicting components to accommodate the lexx and yacc sources. Modify the lexx & yacc componets to accommodate your sources. '#Include' the lexx and yacc sources where they are required.

Re-writing the the components is fine if the changes are small and the components are understood to begin with (i.e: you not porting someone else's code).

Modifying the lexx and yacc source is fine as long as the build process doesn't keep regenerating the source from the lexx and yacc scripts. You can always revert to one of the other two methods if you feel it is required.

Adding a single #include and modifying the makefile to remove the build of the lexx/yacc components to overcome all your problems is attractive fast and provides you the opportunity to prove the code works at all without spending time rewriting code and questing whether the code would have ever worked in the first place when it isn't working now.

When two C files are included together they are basically one file and there are no external references required to be resolved at link time!