问题:

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

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

Here I want user to use this class either with V or without V

Is following possible

Test<int,int,int> WithAllParameter
Test<int,int> WithOneMissing

If Yes how to do this.

回答1:

You can have default template arguments, which are sufficient for your purposes:

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

Now the following work:

Test<int> a;           // Test<int, int, int>
Test<double, float> b; // Test<double, float, float>

回答2:

Sure, you can have default template parameters:

template <typename T, typename U, typename V = U>

template <typename T, typename U = int, typename V = std::vector<U> >

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<Key>,      // hash functor, defaults to std::hash<Key>
    class KeyEqual = std::equal_to<Key>, // comparator, defaults to Key::operator==()
    class Allocator = std::allocator<std::pair<const Key, T>> // allocator, defaults to std::allocator
> class unordered_map;

Typicall you just use it as std::unordered_map<std::string, double> without giving it any further thought.