How to do it...

To fold a parameter pack over a binary operator, use one of the following forms:

• Left folding with a unary form (... op pack):

        template <typename... Ts> 
        auto add(Ts... args) 
        { 
          return (... + args); 
        }

• Left folding with a binary form (init op ... op pack):

        template <typename... Ts> 
        auto add_to_one(Ts... args) 
        { 
          return (1 + ... + args); 
        }

• Right folding with a unary form (pack op ...):

        template <typename... Ts> 
        auto add(Ts... args) 
        { 
          return (args + ...); 
        }

• Right folding with a binary form (pack op ... op init):

        template <typename... Ts> 
        auto add_to_one(Ts... args) 
        { 
          return (args + ... + 1); 
        }

The parentheses shown above are part of the fold expression and cannot be omitted.