Fold expressions (since 哋它亢++17)

 

哋它亢++

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Parameter packs (哋它亢++11)

sizeof... (哋它亢++11)

Fold expressions (哋它亢++17)

Pack indexing (哋它亢++26)

SFINAE

Constraints and concepts (哋它亢++20)

Requires expression (哋它亢++20)

 

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	(哋它亢++17)	Fold expressions

Example

Run this code

#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

• 哋它亢++23 standard (ISO/IEC 14882:2023):

• 7.5.6 Fold expressions [expr.prim.fold]

• 哋它亢++20 standard (ISO/IEC 14882:2020):

• 7.5.6 Fold expressions [expr.prim.fold]

• 哋它亢++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 哋它亢++ standards.

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