std::ranges::uninitialized_move, std::ranges::uninitialized_move_result

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ranges::uninitialized_move
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在标头 <memory> 定义
调用签名
template< std::input_iterator I, std::sentinel_for<I> S1,

          no-throw-forward-iterator O, no-throw-sentinel-for<O> S2 >
requires std::constructible_from<std::iter_value_t<O>,
         std::iter_rvalue_reference_t<I>>
uninitialized_move_result<I, O>

uninitialized_move( I ifirst, S1 ilast, O ofirst, S2 olast );
(1) (C++20 起)
template< ranges::input_range IR, no-throw-forward-range OR >

requires std::constructible_from<ranges::range_value_t<OR>,
         ranges::range_rvalue_reference_t<IR>>
uninitialized_move_result<ranges::borrowed_iterator_t<IR>,
                          ranges::borrowed_iterator_t<OR>>

uninitialized_move( IR&& in_range, OR&& out_range );
(2) (C++20 起)
辅助类型
template<class I, class O>
using uninitialized_move_result = ranges::in_out_result<I, O>;
(3) (C++20 起)
1) 从输入范围 [ifirst, ilast) 移动 N 元素到输出范围 [ofirst, olast) (为未初始化内存区域),其中 Nmin(ranges::distance(ifirst, ilast), ranges::distance(ofirst, olast))
效果等价于:
for (; ifirst != ilast && ofirst != olast; ++ofirst, ++ifirst)
    ::new (const_cast<void*>(static_cast<const volatile void*>(std::addressof(*first))))
        std::remove_reference_t<std::iter_reference_t<O>>(ranges::iter_move(ifirst));
若在初始化期间抛出异常,则以未指定顺序销毁 [ofirst, olast) 中已构造的元素。而且 [ifirst, ilast) 中已被移动的元素被置于合法但未指定的状态。
2)(1) ,但以 in_range 为第一范围并以 out_range 为第二范围,如同以 ranges::begin(in_range)ifirst ,以 ranges::end(in_range)ilast ,以 ranges::begin(out_range)ofirst ,并以 ranges::end(out_range)olast

此页面上描述的仿函数实体是 niebloid,即:

实际上,它们能以函数对象,或者某些特殊编译器扩展实现。

参阅

ifirst, ilast - 要移动的输入元素范围
ofirst, olast - 要初始化的输出范围
in_range - 要移动的输入元素范围
out_range - 要初始化的输出范围

返回值

{ifirst + N, ofirst + N}

复杂度

N 成线性。

异常

构造目标范围中的元素时抛出的异常,若存在。

注解

若输出范围的值类型为平凡类型 (TrivialType) ,则实现可能提升 ranges::uninitialized_move 的效率,例如用 ranges::copy_n

可能的实现

struct uninitialized_move_fn {
    template <std::input_iterator I, std::sentinel_for<I> S1,
             no-throw-forward-iterator O, no-throw-sentinel-for<O> S2>
    requires std::constructible_from<std::iter_value_t<O>,
             std::iter_rvalue_reference_t<I>>
    ranges::uninitialized_move_result<I, O>
    operator()( I ifirst, S1 ilast, O ofirst, S2 olast ) const {
        O current {ofirst};
        try {
            for (; !(ifirst == ilast or current == olast); ++ifirst, ++current)
                ::new (const_cast<void*>(static_cast<const volatile void*>
                    (std::addressof(*current)))) std::remove_reference_t<
                        std::iter_reference_t<O>>(ranges::iter_move(ifirst));
            return {std::move(ifirst), std::move(current)};
        } catch (...) { // 回滚:析构已构造的元素
            for (; ofirst != current; ++ofirst)
                ranges::destroy_at(std::addressof(*ofirst));
            throw;
        }
    }
 
    template <ranges::input_range IR, no-throw-forward-range OR>
    requires std::constructible_from<ranges::range_value_t<OR>,
             ranges::range_rvalue_reference_t<IR>>
    ranges::uninitialized_move_result<ranges::borrowed_iterator_t<IR>,
             ranges::borrowed_iterator_t<OR>>
    operator()( IR&& in_range, OR&& out_range ) const {
        return (*this)(ranges::begin(in_range), ranges::end(in_range),
                       ranges::begin(out_range), ranges::end(out_range));
    }
};
 
inline constexpr uninitialized_move_fn uninitialized_move{};

示例

#include <cstdlib>
#include <iomanip>
#include <iostream>
#include <memory>
#include <string>
 
void print(auto rem, auto first, auto last) {
    for (std::cout << rem; first != last; ++first)
        std::cout << std::quoted(*first) << ' ';
    std::cout << '\n';
}
 
int main() {
    std::string in[] { "Home", "World" };
    print("initially, in: ", std::begin(in), std::end(in));
 
    if (
        constexpr auto sz = std::size(in);
        void* out = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz)
    ) {
        try {
            auto first {static_cast<std::string*>(out)};
            auto last {first + sz};
            std::ranges::uninitialized_move(std::begin(in), std::end(in), first, last);
 
            print("after move, in: ", std::begin(in), std::end(in));
            print("after move, out: ", first, last);
 
            std::ranges::destroy(first, last);
        }
        catch (...) {
            std::cout << "Exception!\n";
        }
        std::free(out);
    }
}

可能的输出:

initially, in: "Home" "World"
after move, in: "" ""
after move, out: "Home" "World"

参阅

移动一定量对象到未初始化的内存区域
(niebloid)
移动一个范围的对象到未初始化的内存区域
(函数模板)