Optional Template parameter

Question

Is it possible to have optional template parameter in C++ , for example

template < class T, class U, class V>
class Test {
};

Here I

Answer 1

Sure, you can have default template parameters:

template 

template  >

The standard library does this all the time -- most containers take two to five parameters! For example, unordered_map is actually:

template<
    class Key,                        // needed, key type
    class T,                          // needed, mapped type
    class Hash = std::hash,      // hash functor, defaults to std::hash
    class KeyEqual = std::equal_to, // comparator, defaults to Key::operator==()
    class Allocator = std::allocator> // allocator, defaults to std::allocator
> class unordered_map;

Typicall you just use it as std::unordered_map without giving it any further thought.