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Fold expressions (since C++17)

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

Syntax

( pack op ... ) (1)
( ... op pack ) (2)
( pack op ... op init ) (3)
( init op ... op pack ) (4)
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 opening and closing parentheses are a required 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 (E1 op (... op (EN-1 op EN)))
2) Unary left fold (... op E) becomes (((E1 op E2) op ...) op EN)
3) Binary right fold (E op ... op I) becomes (E1 op (... op (EN−1 op (EN op I))))
4) Binary left fold (I op ... op E) becomes ((((I op E1) op E2) op ...) op EN)

(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().

Notes

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

template<typename... Args>
int sum(Args&&... args)
{
//  return (args + ... + 1 * 2);   // Error: operator with precedence below cast
    return (args + ... + (1 * 2)); // OK
}
Feature-test macro Value Std Feature
__cpp_fold_expressions 201603L (C++17) Fold expressions

Example

#include <climits>
#include <concepts>
#include <cstdint>
#include <iostream>
#include <type_traits>
#include <utility>
#include <vector>
 
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)
{
    static_assert((std::is_constructible_v<T, Args&&> && ...));
    (v.push_back(std::forward<Args>(args)), ...);
}
 
template<class T, std::size_t... dummy_pack>
constexpr T bswap_impl(T i, std::index_sequence<dummy_pack...>)
{
    T low_byte_mask = (unsigned char)-1;
    T ret{};
    ([&]
    {
        (void)dummy_pack;
        ret <<= CHAR_BIT;
        ret |= i & low_byte_mask;
        i >>= CHAR_BIT;
    }(), ...);
    return ret;
}
 
constexpr auto bswap(std::unsigned_integral auto i)
{
    return bswap_impl(i, std::make_index_sequence<sizeof(i)>{});
}
 
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 << ' ';
    std::cout << '\n';
 
    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

References

  • C++23 standard (ISO/IEC 14882:2023):
    • 7.5.6 Fold expressions [expr.prim.fold]
  • C++20 standard (ISO/IEC 14882:2020):
    • 7.5.6 Fold expressions [expr.prim.fold]
  • C++17 standard (ISO/IEC 14882:2017):
    • 8.1.6 Fold expressions [expr.prim.fold]

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 2611 C++17 the expansion results of fold expressions were not enclosed with parentheses enclosed with parentheses

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