Question about template's value_type usage in Stroustrup's book example

Question

In Stroustrup's "A Tour of C++" there is a code snippet

template<typename C>
using Value_type = typename C::value_type;  // the type of C’s elements

template<typename Container>
void algo(Container& c)
{
    /* (1) */ 
    Vector<Value_type<Container>> vec;  // keep results here
    // ...
}

Why we need this using, how it differs from writing in (1) just

Vector<Container::value_type> vec;

Answer 1

The reason is this declaration:

Vector<Container::value_type> vec;

is not actually valid, and is an error. Instead you need to write:

Vector<typename Container::value_type> vec;

which is more verbose.

The purpose of the alias template Value_type is to make it more convenient to use a member type alias of Container without having to say typename each time.