CODEWITHSUNDEEP
c++templatestypes
David
David

Reputation: 6162

How can I refer to the type in a nested template for a parameter argument?

Suppose I have the following definitions

template <class T>
class Sequence
{
}

E.g Sequence<string> might be an array of strings, similar to vector<string>

// Now define iterator template
template <class T>
class SequenceIterator
{
}

The idea here of course is to be able to create an iterator over some sequence E.g.

SequenceIterator< Sequence<string> > iter1;
SequenceIterator< Sequence<int> > iter2;

The question I now have is how to define the member function that would exist inside SequenceIterator and whose purpose is to return the next value in the sequence. Typically I would expect to write that as

bool Next(T1 & value); // If the iterator has not finished, set value to the next item

However, the SequenceIterator has been passed in a templated name already, i.e, Sequence<string> or Sequence<int>

So the question is, how do I generically refer to that underlying type (string or int) so that I can define the Next member function.

Thanks, David

Upvotes: 1

Views: 240

Answers (2)

rcollyer
rcollyer

Reputation: 10695

There are three ways: change the definition of Sequence to include

typedef T type;

or, change the template parameters for SequenceIterator to explicitly recognize that Sequence is a template itself

template< template < class > class Seq, class T >
class SequenceIterator< Seq< T > >

and while the instantiation of SequenceIterator does not change, you can now access T directly. Thirdly, you can use a container traits class that handles the type deduction for you. The third option provides the least coupling between Sequence and SequenceIterator, but, like Mark said, the standard containers tend to use the first method.

Upvotes: 2

Mark B
Mark B

Reputation: 96311

The standard library solves this by having typedefs within every container. In this case Sequence<T> would have a typedef T value_type; so you can then use Sequence<T>::value_type to refer to that type.

Also, I would highly consider using operator++ and operator* like the standard library so you don't confuse people with a non-standard-like iterator interface.

Upvotes: 2

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