Reduces (folds) a pack over a binary operator.
[edit] 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 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 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.
[edit] ExplanationThe 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 (ENâ1 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().
[edit] NotesIf 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
}[edit] Example
#include <climits>
#include <concepts>
#include <cstdint>
#include <iostream>
#include <limits>
#include <type_traits>
#include <utility>
#include <vector>
// Basic usage, folding variadic arguments over operator<<
template<typename... Args>
void printer(Args&&... args)
{
(std::cout << ... << args) << '\n';
}
// Folding an expression that uses the pack directly over operator,
template<typename... Ts>
void print_limits()
{
((std::cout << +std::numeric_limits<Ts>::max() << ' '), ...) << '\n';
}
// Both a fold over operator&& using the pack
// and over operator, using the variadic arguments
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)), ...);
}
// Using an integer sequence to execute an expression
// N times by folding a lambda over operator,
template<class T, std::size_t... dummy_pack>
constexpr T bswap_impl(T i, std::index_sequence<dummy_pack...>)
{
T low_byte_mask = static_cast<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");
print_limits<uint8_t, uint16_t, uint32_t>();
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 255 65535 4294967295 6 2 45 12 1 2 9[edit] References
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 parenthesesRetroSearch is an open source project built by @garambo | Open a GitHub Issue
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