C++ Templates and accessing namespaces

Let's say I'm using a templated class with something simple like:

template <class T开发者_Python百科> 
class MyClass

I want to use elements from T's namespace, for example T could be string, and I wanted to use

T::const_iterator myIterator; 

...or something like that. How do I achieve that? Probably, it's either not possible or very simple, but I have no idea.

Thanks for answers!

By default if T is a template parameter like in your example, the T::some_member is assumed not to name a type. You have to explicitly specify that it is, by prefixing it with typename:

typename T::const_iterator myIterator;

This resolves some parsing problems like in the following example

// multiplication, or declaration of a pointer?
T::const_iterator * myIterator;

So that the compiler can parse this even before instantiating the template, you have to give it a hand and use typename, including in those cases where it wouldn't be ambiguous, like in the first case above. The Template FAQ has more insight into this.

It is definitely possible.

template< typename T >
class Example
{
    void foo( const T& t )
    {
        typedef typename T::value_type Type;
        typedef typename T::const_iterator Iter;
        Iter begin = t.begin();
        Iter end = t.end();

        std::copy( begin, end, std::ostream_iterator<Type>(std::cout) );
    }
};

The key is the typename part of the typedef.