Including .cpp at end of template header file

Question

I was reading an older data-structures book and it said that when you're doing template class programming you should include the .cpp at the end of the .h file.

As far as I know you have to do full function implementations in your .h file for any template class member functions - it's due to the way the template compiler works.

The only functions I was taught could be put in a implementation file for a template class were template specialization functions i.e.: template<> Class<Type>::function_name().

Why would this book to suggest to include the .cpp at the end of the .h? is this just a way of separating the implementations into different files while getting them to compile with the header? and if so, where would you put real specializations - I'm guessing they couldn't go in the .cpp included by the header.

score 11 · Accepted Answer · answered Sep 28 '11 at 13:11

11

Most likely the author prefers having declaration and definition in different files, I can guess this is because it makes it easier to jump between declarations and definitions.

But having "cpp" file extension is a bit confusing. Usually those files are called "ipp" for "Inline C++".

answered Sep 28 '11 at 13:11

3

I have never in my life seen these files named `ipp` – John Dibling Sep 28 '11 at 13:48
@JohnDibling: It is all over boost. Questions about that were even raised here on SO before. – Sep 28 '11 at 14:02
see http://stackoverflow.com/questions/543507/in-the-c-boost-libraries-why-is-there-a-ipp-extension-on-some-header-files – Benedikt Waldvogel Nov 02 '12 at 13:16

score 3 · Answer 2 · answered Sep 28 '11 at 13:10

This is probably old parlance, and I think your analysis of separation of implementation and declaration is correct. Back when this particular book was written, the author probably thought of a cpp file as the file where definitions resided, and h files as the files where declarations resided. Putting real explicit specializations into the former file would, naturally, be lethal (or at least useless) to the linker because of repeated definitions. Nowadays, I'd avoid naming the definitions file .cpp.

score 2 · Answer 3 · answered Sep 28 '11 at 13:37

First, there's no rule that you have to have the implementations of the templates in your .h (or .hpp, or .hh) file; in fact, for anything but the simplest templates, I would recommend against doing so. You do have to include the implementation, regardless of the file. What the author probably had in mind was to put the implementation in a .cpp file, and include that. I'd recommend finding a different name, however, since most people (and some IDE's) will suppose that you should compile all .cpp files. A common convention where .cc and .hh are the usual extensions for sources and headers is .tcc for template implementations; in a Windows world (where .cpp is almost universal), I'd recommend something like .tpp.

Note that the earliest implementations of templates required the implementation to be in a .cpp. These implementations didn't require (or allow) it to be included, however; the compiler searched for the .cpp (or .cc) corresponding to the .hpp (or .hh) file in which the template class definition or function declaration appeared, and generated a dummy source file which included it (and anything else which was necessary).

score 0 · Answer 4 · answered Sep 28 '11 at 13:13

As far as I know you have to do full function implementations in your .h file for any template class member functions - it's due to the way the template compiler works.

May be the book assumed you have the full function implementations in your .cpp file.

If full specializations are in cpp file, they should not be included in header file. Most likely it wont compile if you did so. Because compiler will see multiple definitions of the same function as header file will generally be included in multiple source files.

score 0 · Answer 5 · answered Sep 28 '11 at 13:15

If you want to use a template, the entire template definition has to be visible in the TU that instantiates the template. So you could simply put full definitions in the header file.

It's only if you really want to keep the class definition and class member function bodies separate, then you might be tempted to put those function definitions into a separate file. However, the above rule still applies, so you'll have to include the separate file together with the header in order for anyone to be able to use the template.

It's a matter of taste. If the member function bodies are short, you might as well define them inline. Also bear in mind that functions that are defined inside the class definition are implicitly declared inline, while you'd have to specify that explicitly if you write the bodies separately.

A partially specialised templated is still a template, so the same rules apply.

A fully specialized class template is just an ordinary class, so you treat is as such:

// FooInt.hpp
template <typename> class Foo;
template <> class Foo<int>
{
  // your class here
};

One thing you may have had in mind is "explicit template instantiation", a la template class Foo<int>;. That's something else; leave a comment if you're interested in using that.

Including .cpp at end of template header file

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