templates in C++

Question

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Why can templates only be implemented in the header file?

hello everyone I've just read that if I write some template I must write all code in header file (including constructor, desctructors...), but I can't understand why, can somebody please explain?

Answer 1

Because the templated code can be compiled only after it is know, which class/type replaces the template, so all code must be available then.

There is a very detailed explanation of the details in the C++ FAQ

Answer 2

When a template is used, it's much like the defined template class or function is used to create a new class by replacing the template parameters with the provided parameters.

The code for this new class is compiled right when it's needed, so all the source needs to be there at that time.

See this link at the bottom of the page for a bit more detail.

Answer 3

It's not necessary, but is recommended. Basically, every time you instantiate a template (declare an object with a different template parameter than before) the compiler has to know how to instantiate all its members. For instance, let's say you have this template in a .h:

template <class T> class MyTemplate
{
  public:
    void aMethod();
};

In x.cpp you define the method:

template <class T> void MyTemplate<T>::aMethod()
{
}

And after that you do:

void something()
{
  MyTemplate<int> foo;
  foo.aMethod();
}

This will compile and link, without errors. And, if you instantiate MyTemplate with <int> in another,say, y.cpp module, it will also compile and link. BUT if in a third z.cpp module you instantiate MyTemplate with <double>, this will compile but won't link, because aMethod for MyTemplate<double> will be missing. The code for aMethod is in x.cpp; when the compiler compiles x.cpp, it doesn't know a <double> instance should be generated. When it compiles z.cpp, it doesn't have the source code for the method.

With careful management of template instantiation, you could avoid completely the need to put the code in .h files. You just have to be sure that at least one module that uses a different template instance has the source code to generate the instance methods. But is it worth the trouble? In general, not.