c++ – *.h or *.hpp for your class definitions

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, Ive 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 youre in a C module, dont even try to include it. You wont 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 wont 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

c++ – *.h or *.hpp for your class definitions

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 Ive seen (and use) .hpp as an extension, there is no corresponding .cpp file. As others have said, this isnt a hard and fast rule, just how I tend to use .hpp files.