C++17 STL Cookbook

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In case someone calls sum() with no arguments, the variadic parameter pack contains no values that could be folded. For most operators, this is an error (for some, it is not; we will see this in a minute). We then need to decide if this should stay an error or if an empty sum should result in a specific value. The obvious idea is that the sum of nothing is 0.

This is how it’s done:

template <typename ... Ts>auto sum(Ts ... ts){    return (ts + ... + 0);}

This way, sum() evaluates to 0, and sum(1, 2, 3) evaluates to (1 + (2 + (3 + 0))). Such folds with an initial value are called binary folds.

Again, it works if we write (ts + ... + 0), or (0 + ... + ts), but this makes the binary fold a binary right fold or a binary left fold ...