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I have the following code with a variadic template copied from:
https://www.youtube.com/watch?v=iWvcoIKSaoc @41:30

auto sum() return 0; 

template<typename Head, typename... Tail>
auto sum(Head head, Tail... tail)

 return head+sum(tail...);


int main() 
 cout<< sum(1,2.4) << endl;
 //cout<< sum("hello ", "world") << endl;
 return 0;

I have two questions:
1. The sum() function is required here so that I can have a return value for a void passed in when processing the last variadic member - Is it possible to avoid writing this sum() function and have the same functionality?

2. Returning a integer '0' from the sum() function restricts the entire template to be used by integers - Can I extend the same template to concatenate strings?

Thank You

The trick is to never allow empty sum() calls, and treat the sum(last) as the last recursion:

template<typename Last>
auto sum(Last last) 
 return last;


template<typename Head, typename Second, typename... Tail>
auto sum(Head head, Second second, Tail... tail)

 return head + sum(second, tail...);


int main() 
 cout<< sum(1,2.4) << endl;
 cout<< sum("hello ", "world") << endl;
 return 0;

Live example

To complement @GuillaumeRacicot answer I prefer to end a recursion with if constexpr which is a c++17 feature.

template<typename Head, typename Second, typename... Tail>
auto sum(Head head, Second second, Tail... tail)
 
 if constexpr(sizeof...(tail) > 0)
 return head + sum(second, tail...);
 return head + second;

You can also consider fold expressions:

template<typename ...Pack>
auto sum(Pack... args) 
 return (args + ...);

1. The sum() function is required here so that I can have a return value for a void passed in when processing the last variadic member - Is it possible to avoid writing this sum() function and have the same functionality?

Every recursion needs a stop condition. In the typical use of recursion with variadic templates (such as in this code), the stop condition is a different overload of the primary template. So you cannot get rid of this entirely.

You can of course replace the stop condition with a different one. Perhaps this one, which will also work for summing things which are not default-constructible:

template <class T>
auto sum(T last) return last; 

Of course, there are other approaches to this than recursive variadic templates; such approaches may not need a stop condition.

2. Returning a integer '0' from the sum() function restricts the entire template to be used by integers - Can I extend the same template to concatenate strings?

No, because the non-template function has no knowledge of which type the previous recursive invocations were dealing with. This can be solved by using the "last item" stop condition I suggested above.