# fold expression

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Reduces (folds) a parameter pack over a binary operator.

### Syntax

`(` pack op `...` `)` | (1) | (since C++17)
`(` `...` op pack `)` | (2) | (since C++17)
`(` pack op `...` op init `)` | (3) | (since C++17)
`(` init op `...` op pack `)` | (4) | (since C++17)

1) unary right fold
2) unary left fold
3) binary right fold
4) binary left fold

op | - | any of the following 32 binary operators: + - * / % ^ & | = < > << >> += -= *= /= %= ^= &= |= <<= >>= == != <= >= && || , .* ->*. In a binary fold, both ops must be the same.
pack | - | an expression that contains an unexpanded parameter pack and does not contain an operator with precedence lower than cast at the top level (formally, a cast-expression)
init | - | an expression that does not contain an unexpanded parameter pack and does not contain an operator with precedence lower than cast at the top level (formally, a cast-expression)

Note that the open and closing parentheses are part of the fold expression.

### Explanation

The instantiation of a fold expression expands the expression `e` as follows:

1) Unary right fold (E op ...) becomes E
1 op (... op (E
N-1 op E
N))
2) Unary left fold (... op E) becomes ((E
1 op E
2) op ...) op E
N
3) Binary right fold (E op ... op I) becomes E
1 op (... op (E
N−1 op (E
N op I)))
4) Binary left fold (I op ... op E) becomes (((I op E
1) op E
2) op ...) op E
N

(where N is the number of elements in the pack expansion)

For example,

    template<typename... Args>
    bool all(Args... args) { return (... && args); }

    bool b = all(true, true, true, false);
     // within all(), the unary left fold expands as
     //  return ((true && true) && true) && false;
     // b is false

When a unary fold is used with a pack expansion of length zero, only the following operators are allowed:

1) Logical AND (&&). The value for the empty pack is true
2) Logical OR (||). The value for the empty pack is false
3) The comma operator (,). The value for the empty pack is void()

### Note

If the expression used as init or as pack has an operator with precedence below cast at the top level, it can be parenthesed:

    template<typename ...Args>
    int sum(Args&&... args) {
    //    return (args + ... + 1 * 2); // Error: operator with precedence below cast
        return (args + ... + (1 * 2)); // OK
    }

### Example

Run this code

    #include <iostream>
    #include <vector>
    #include <climits>
    #include <cstdint>
    #include <type_traits>
    #include <utility>

    template<typename ...Args>
    void printer(Args&&... args) {
        (std::cout << ... << args) << '\n';
    }

    template<typename T, typename... Args>
    void push_back_vec(std::vector<T>& v, Args&&... args)
    {
        (v.push_back(args), ...);
    }

    // compile-time endianness swap based on http://stackoverflow.com/a/36937049
    template<class T, std::size_t... N>
    constexpr T bswap_impl(T i, std::index_sequence<N...>) {
      return (((i >> N*CHAR_BIT & std::uint8_t(-1)) << (sizeof(T)-1-N)*CHAR_BIT) | ...);
    }
    template<class T, class U = std::make_unsigned_t<T>>
    constexpr U bswap(T i) {
      return bswap_impl<U>(i, std::make_index_sequence<sizeof(T)>{});
    }

    int main()
    {
        printer(1, 2, 3, "abc");

        std::vector<int> v;
        push_back_vec(v, 6, 2, 45, 12);
        push_back_vec(v, 1, 2, 9);
        for (int i : v) std::cout << i << ' ';

        static_assert(bswap<std::uint16_t>(0x1234u)==0x3412u);
        static_assert(bswap<std::uint64_t>(0x0123456789abcdefULL)==0xefcdab8967452301ULL);
    }

Output:

    123abc
    6 2 45 12 1 2 9

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