Why do we need to write a header file?

Question

Before you whip out your snarky comments, I know -- this is a nooby question. This is my first time using a C based language.

I'm an undergrad student learning Objective C for a computer science course on mobile development. I know that, in an academic setting, a lot of real-world considerations aren't necessary since you're building smaller projects, work in smaller teams, etc.

But our professor demands -- and XCode supports -- .h header files for every .m implementation file. To me, it kind of seems like busy work. I have to make sure I copy every method signature and instance variable over to the other file. If I change one file, I have to make sure it's consistent with the other file. It seems like just a bunch of small annoyances like that.

But I know there has to be some real-world use for header files. A great answer would address both:

1. What is a header file useful for that an implementation file isn't suited for? What is its purpose?
2. Why do we as programmers have to manually write our header files? It seems like they could easily be generated automatically.

Thanks in advance!

Answer 1

1. In short;

   • The header file defines the API for a module. It's a contract listing which methods a third party can call. The module can be considered a black box to third parties.

   • The implementation implements the module. It is the inside of the black box. As a developer of a module you have to write this, but as a user of a third party module you shouldn't need to know anything about the implementation. The header should contain all the information you need.

2. Some parts of a header file could be auto generated - the method declarations. This would require you to annotate the implementation as there are likely to be private methods in the implementation which don't form part of the API and don't belong in the header.

Header files sometimes have other information in them; type definitions, constant definitions etc. These belong in the header file, and not in the implementation.

Answer 2

The main reason for a header is to be able to #include it in some other file, so you can use the functions in one file from that other file. The header includes (only) enough to be able to use the functions, not the functions themselves, so (we hope) compiling it is considerably faster.

Maintaining the two separately most results from nobody ever having written an editor that automates the process very well. There's not really a lot of reason they couldn't do so, and a few have even tried to -- but the editors that have done so have never done very well in the market, and the more mainstream editors haven't adopted it.

Answer 3

Including a header allows your code to call functions or methods which are written elsewhere and which may or may not be a part of your software project/build, but which can be found by the 'linker' when you are building the software.

For instance, when you call a function in the standard C library then you don't want to have to put all the innards of that function inside your project - you simply expect your program to be able to call it.

But your compiler needs to be told that the function exists somewhere even if it cannot see the code and the header does that. When the compiler is finished, the linker is called and it looks to 'link' those bits of your code that are left dangling, such as library calls.

Answer 4

Header files are mainly used to declare and include the signatures (i.e. function name, return value and arguments) of your functions in other files. The compiler needs to know these signatures when compiling and linking your files together.

Read this article for more information.