Question

I am developing a fairly large C++ support library, and have found myself moving towards a header-only approach. In C++ this almost works because you can implement where you define in classes. For templated methods, the implementation has to be in the same file anyway, so I find that it is much easier to just keep the implementation with the definition.

However, there are several times where “sources” must be used. As just one example, circular dependencies sometimes occur and the implementation has to be written outside the class definition. Here is how I am handling it:

//part of libfoo.h
class Bar
{
  void CircularDependency(void);
};

#ifdef LIBFOO_COMPILE_INLINE
void Bar::CircularDependency(void)
{
  //...
}
#endif

Then the project that uses libfoo would do the following in main.cpp:

//main.cpp
#define LIBFOO_COMPILE_INLINE
#include "libfoo.h"

And in any other .cpp:

//other.cpp
#include "libfoo.h"

The point is that the compile-inline section only gets compiled once (in main.cpp).

And finally my question: is there a name for this idiom or any other projects that work this way? It just seems to be a natural outcome of the implementation and definition being blurred by templating and class methods. And: are there any reasons why this is a bad idea or why it would potentially not scale well?

Quick aside: I know that many coders, with good reason, prefer their headers to resemble interfaces and not implementations, but IMHO documentation generators are better suited to describing interfaces because I like to hide private members all together 🙂

Answer 1

You should be able to use the inline keyword if the circular dependency problem is resolved by the time the definition of Bar::CircularDependency() would show up in the libfoo.h header:

//part of libfoo.h
class Bar
{
  void CircularDependency(void);
};


// other stuff that 'resolves' the circular dependency
// ...
// ...

inline void Bar::CircularDependency(void)
{
  //...
}

That would make your library easier to use (the user wouldn’t need to deal with the fact that LIBFOO_COMPILE_INLINE needed to be defined in exactly one place where the header is included).