C ++模板和访问命名空间

Question

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

template <class T> 
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!

Answer 1

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.

Answer 2

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.