Separating C++ Class Code into Multiple Files, what are the rules?

Question

Thinking Time - Why do you want to split your file anyway?

As the title suggests, the end problem I have is multiple definition linker errors. I have actually fixed the

Answer 1

Your three points sound about right. That's the standard way to do things (although I've not seen .tpp extension before, usually it's .inl), although personally I just put inline functions at the bottom of header files rather than in a separate file.

Here is how I arrange my files. I omit the forward declare file for simple classes.

myclass-fwd.h

#pragma once

namespace NS
{
class MyClass;
}

myclass.h

#pragma once
#include "headers-needed-by-header"
#include "myclass-fwd.h"

namespace NS
{
class MyClass
{
    ..
};
}

myclass.cpp

#include "headers-needed-by-source"
#include "myclass.h"

namespace
    {
    void LocalFunc();
}

NS::MyClass::...

Replace pragma with header guards according to preference..

The reason for this approach is to reduce header dependencies, which slow down compile times in large projects. If you didn't know, you can forward declare a class to use as a pointer or reference. The full declaration is only needed when you construct, create or use members of the class.

This means another class which uses the class (takes parameters by pointer/reference) only has to include the fwd header in its own header. The full header is then included in the second class's source file. This greatly reduces the amount of unneeded rubbish you get when pulling in a big header, which pulls in another big header, which pulls in another...

The next tip is the unnamed namespace (sometimes called anonymous namespace). This can only appear in a source file and it is like a hidden namespace only visible to that file. You can place local functions, classes etc here which are only used by the the source file. This prevents name clashes if you create something with the same name in two different files. (Two local function F for example, may give linker errors).