Modern C++ Programming Cookbook

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.