*.h or *.hpp for your class definitions

I've always used a *.h file for my class definitions, but after reading some boost library code, I realised they all use *.hpp. I've always had an aversion to that file extension, I think mainly because I'm not used to it.

What are the advantages and disadvantages of using *.hpp over *.h?

Here are a couple of reasons for having different naming of C vs C++ headers:

• Automatic code formatting, you might have different guidelines for formatting C and C++ code. If the headers are separated by extension you can set your editor to apply the appropriate formatting automatically
• Naming, I've been on projects where there were libraries written in C and then wrappers had been implemented in C++. Since the headers usually had similar names, i.e. Feature.h vs Feature.hpp, they were easy to tell apart.
• Inclusion, maybe your project has more appropriate versions available written in C++ but you are using the C version (see above point). If headers are named after the language they are implemented in you can easily spot all the C-headers and check for C++ versions.

Remember, C is not C++ and it can be very dangerous to mix and match unless you know what you are doing. Naming your sources appropriately helps you tell the languages apart.

I use .hpp because I want the user to differentiate what headers are C++ headers, and what headers are C headers.

This can be important when your project is using both C and C++ modules: Like someone else explained before me, you should do it very carefully, and its starts by the "contract" you offer through the extension

.hpp : C++ Headers

(Or .hxx, or .hh, or whatever)

This header is for C++ only.

If you're in a C module, don't even try to include it. You won't like it, because no effort is done to make it C-friendly (too much would be lost, like function overloading, namespaces, etc. etc.).

.h : C/C++ compatible or pure C Headers

This header can be included by both a C source, and a C++ source, directly or indirectly.

It can included directly, being protected by the __cplusplus macro:

• Which mean that, from a C++ viewpoint, the C-compatible code will be defined as extern "C".
• From a C viewpoint, all the C code will be plainly visible, but the C++ code will be hidden (because it won't compile in a C compiler).

For example:

#ifndef MY_HEADER_H
#define MY_HEADER_H

   #ifdef __cplusplus
      extern "C"
      {
   #endif

   void myCFunction() ;

   #ifdef __cplusplus
      } // extern "C"
   #endif

#endif // MY_HEADER_H

Or it could be included indirectly by the corresponding .hpp header enclosing it with the extern "C" declaration.

For example:

#ifndef MY_HEADER_HPP
#define MY_HEADER_HPP

extern "C"
{
#include "my_header.h"
}

#endif // MY_HEADER_HPP

and:

#ifndef MY_HEADER_H
#define MY_HEADER_H

void myCFunction() ;

#endif // MY_HEADER_H

I always considered the .hpp header to be a sort of portmanteau of .h and .cpp files...a header which contains implementation details as well.

Typically when I've seen (and use) .hpp as an extension, there is no corresponding .cpp file. As others have said, this isn't a hard and fast rule, just how I tend to use .hpp files.

It does not matter which extension you use. Either one is OK.

I use *.h for C and *.hpp for C++.