CODEWITHSUNDEEP
c++
user2826094
user2826094

Reputation: 133

Why do you use multiple header files?

Im new to learning c++ and I was curious as to what the purpose of multiple header files is. Why can't you just have everything in one header file?

Upvotes: 1

Views: 1341

Answers (6)

Escualo
Escualo

Reputation: 42182

In addition to all the answers already posted, you may want to consider that C++ has a philosophy of you pay for what you use or, conversely, you don't pay for what you don't use.

If my application needs, say, complex numbers, then I go ahead and #include this functionality. If I don't need it, then there is no reason whatsoever why my code should even be aware that such concept exists.

When I determine that I keep including the same header files over and over again for a given kind of application, I create a master header file which includes the includes, and I just #include that one.

For example:

// @file  project.hpp
// @brief provides all the functionality required by `project`
#include<library1.hpp>
#include<library2.hpp>
#include<library3.hpp>
// any type definitions go below this line

and then, in my project

// @file  project.cpp
// @brief Implements `project`
#include<project.hpp>
// go ahead and implement project

Other libraries such as boost normally offer a "master" include file such as the one described above to simplify using the library.

Upvotes: 1

Havenard
Havenard

Reputation: 27934

Headers exist to organize.

For a very simple project, one header file would do just fine. You can even not use headers at all and just write everything in a single file.

But once you grow to a bigger project full of parallel systems, classes that are used in multiple contexts, or simply using 3rd party libraries like OpenSSL or MySQL, you can't simply expect to work putting all this code in a single file with millions of lines of code.

You may also be wondering why not put all and any required header in your project in a single "common.h" and then #include it in every .cpp.

The answer is because each .cpp is compiled individually, so if you limit the headers for each .cpp to only what is required for it, you will decrease both the resulting .obj size and the compile time.

Also, you would be forced to recompile the entire project every time you make a change to any header, instead of recompiling only the .cpp involved.

Upvotes: 2

wjmolina
wjmolina

Reputation: 2675

The keyword here is modularity:

Modularity is the degree to which a system's components may be separated and recombined.

As others have already mentioned, you can write a program in a single file. Moreover, you can write it in a single line. However, you don't do this because it would then be really difficult to separate and recombine parts of this program, let alone debug it if it doesn't compile.

Upvotes: 4

SigTerm
SigTerm

Reputation: 26439

Why can't you just have everything in one header file?

If you cram everything into single header, every single change to that header will cause recompilation of every file that includes that header. In your case that'll be entire project.

It is easier to manage project, when you have many small files and include them only when they're absolutely necessary.

Upvotes: 3

Kerrek SB
Kerrek SB

Reputation: 477640

You can have everything in one header file. You can even have your entire program in one single file.

The benefit of separating things out into distinct, small files is that you compile small parts at a time, and you only need to recompile those parts whose components have changed. You can also put common code into separate files and use those files from separate projects, without the need to copy-paste code across projects. So if you find a bug, you can fix it once and all your projects benefit from the fix.

Upvotes: 1

Carl Norum
Carl Norum

Reputation: 225242

Yes, you can have everything in one header file. However, as your project grows, that header file will soon become an unmaintainable and slow-to-compile mess.

Upvotes: 0

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