Why place headers in a separate directory? [duplicate]

Question

I know that it is common in C/C++ projects to place header files in a directory such as include and implementation in a separate directory such as src. I have been toying with different project structures and am wondering whether there any objective reasons for this or is it simply convention?

Answer 1

Convention is one of the reasons - most of the time, with effective abstraction, you only care about the interface and want to have it easy just looking at the headers.

It's not the only reason though. If your project is organised in modules, you most likely have to include some headers in different modules, and you want your include directory to be cleaned of other "noise" files in there.

Also, if you plan on redistributing your module, you probably want to hide implementation details. So you only supply headers and binaries - and distributing headers from a single folder with nothing else in it is simpler.

There's also an alternative which I actually prefer - public headers go in a separate folder (these contain the minimum interface - no implementation details are visible whatsoever), and private headers and implementation files are separate (possibly, but not necessarily, in separate folders).

Answer 2

I prefer putting them into the same directory. Reason:

The interface specification file(s), and the source file(s) implementing that interface belongs to the same part of the project. Say you have subsystemx. Then, if you put subsystemx files in the subsystemx directory, subsustemx is self-contained.

If there are many include files, sure you could do subsystemx/include and subsystemx/source, but then I argue that if you put the definition of class Foo in foo.hpp, and foo.cpp you certainly want to see both of them (or at least have the possibility to do so easily) together in a directory listing. Finding all files related to foo

ls foo*

Finding all implementation files:

ls *.cpp

Finding all declaration files:

ls *.hpp

Simple and clean.