Why would a .hpp file be included in a .h file?

Question

I'm tasked with implementing a class. I have been provided with a .h file entitled "pro.h" that I cannot change in any way. The .h file only contains function prototypes, no definitions. After the class block, it has a

#include "pro.hpp"

I'm supposed to create a file.hpp to add the definitions although, I'm confused because definitions are usually included in a .cpp file not another header file. Why do you think this would be done? And would I format the definitions the same why I'd do it in a .cpp file, or rewrite the header file to include the definitions (which would seem inefficient).

There's nothing magical about a `.h` file, a `.hpp` file, or any other file. The compiler simply processes all `#include` files, as directed. Your file extensions can be `.h`, `.hpp`, `.xyz`, or `.hjgiurejkcxkj`. It'll still be valid C++, as long as the files in question, after being logically-included, contain valid C++ code. — Sam Varshavchik, Feb 17 '17 at 00:33
Thank you, I just wanted to make sure compilers would not compile it differently because of the extension. — Alessandro Lorusso, Feb 17 '17 at 00:35
@AlessandroLorusso You (usually) don't compile header files on their own to begin with, so there is no issue. — Baum mit Augen, Feb 17 '17 at 00:44
At a guess, I'd say the reason you must do things this way is that you have an incompetent teacher. — Beta, Feb 17 '17 at 00:45
@BaummitAugen You're right, It's been working just fine, now I know, thank you! — Alessandro Lorusso, Feb 17 '17 at 00:58
Any chance that you are supposed to use something called templates? — Kupto, Feb 17 '17 at 01:06

score 1 · Accepted Answer · edited May 23 '17 at 10:29

Your teacher wants you to implement a template class. The thing about templates is that they need to get instantiated with the correct template type, which means that you can't create the binary before you know what type its gonna be. That is why the implementation is usually written inside the header file. Have a look f.e. here...

If you are new to templates, just ask uncle google or ant wiki ;)

ADD:

To put it simply... when you have something in foo.cpp, it gets translated to binary and the corresponding header foo.hpp serves as a reference to what functions are there in the binary that I could use from my other code. The important thing is that this binary does not (really) change anymore.

On the other hand templates can't be transformed into the binary, until you know what type it is going to be operating on... If you use the template class once with int and another time with vector<double> for template parameter, the resulting binary could be very very different... Therefore you can only compile the code into the binary once you know the type, and therefore you need to pass along the code (inside the header) instead of just function prototypes...

Hope this is clear. It is after all almost 3 o'clock in the morning here.

If not, this should be very comprehensive.

This did help a lot, thank you – Alessandro Lorusso Feb 17 '17 at 16:50 — Alessandro Lorusso, Feb 17 '17 at 16:50

Why would a .hpp file be included in a .h file?

1 Answers1