How it works...

What we just did was a simple recursive application of a binary operator (+) to its parameters. This is generally called folding. C++17 comes with fold expressions, which help expressing the same idea with less code.

This kind of expression is called unary fold. C++17 supports folding parameter packs with the following binary operators: +, -, *, /, %, ^, &, |, =, <, >, <<, >>, +=, -=, *=, /=, %=, ^=, &=, |=, <<=, >>=, ==, !=, <=, >=, &&, ||, ,, .*, ->*.

By the way, in our example code, it does not matter if we write (ts + …) or (… + ts); both work. However, there is a difference that may be relevant in other cases--if the … dots are on the right-hand side of the operator, the fold is called a right fold. If they are on the left-hand side, it is a left fold.

In our sum example, a unary left fold expands to 1 + (2 + (3 + (4 + 5))), while a unary right fold will expand to (((1 + 2) + 3) + 4) + 5. Depending on the operator in use, this can make a difference. When adding numbers, it does not.