std::ranges::transform, std::ranges::unary_transform_result, std::ranges::binary_transfor_result
来自cppreference.com
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| 在标头 <algorithm> 定义
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template< std::input_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O, std::copy_constructible F, class Proj = std::identity > requires std::indirectly_writable<O, std::indirect_result_t<F&, std::projected<I, Proj>>> constexpr unary_transform_result<I, O> transform( I first1, S last1, O result, F op, Proj proj = {} ); |
(1) | (C++20 起) |
template< ranges::input_range R, std::weakly_incrementable O, std::copy_constructible F, class Proj = std::identity > requires std::indirectly_writable<O, std::indirect_result_t<F&, std::projected<ranges::iterator_t<R>, Proj>>> constexpr unary_transform_result<ranges::borrowed_iterator_t<R>, O> transform( R&& r, O result, F op, Proj proj = {} ); |
(2) | (C++20 起) |
template< std::input_iterator I1, std::sentinel_for<I1> S1, std::input_iterator I2, std::sentinel_for<I2> S2, std::weakly_incrementable O, std::copy_constructible F, class Proj1 = std::identity, class Proj2 = std::identity > requires std::indirectly_writable<O, std::indirect_result_t<F&, std::projected<I1, Proj1>, std::projected<I2, Proj2>>> constexpr binary_transform_result<I1, I2, O> transform( I1 first1, S1 last1, I2 first2, S2 last2, O result, F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {} ); |
(3) | (C++20 起) |
template< ranges::input_range R1, ranges::input_range R2, std::weakly_incrementable O, std::copy_constructible F, class Proj1 = std::identity, class Proj2 = std::identity > requires std::indirectly_writable<O, std::indirect_result_t<F&, std::projected<ranges::iterator_t<R1>, Proj1>, std::projected<ranges::iterator_t<R2>, Proj2>>> constexpr binary_transform_result<ranges::borrowed_iterator_t<R1>, ranges::borrowed_iterator_t<R2>, O> transform( R1&& r1, R2&& r2, O result, F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {} ); |
(4) | (C++20 起) |
| 辅助类型 |
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template< class I, class O > using unary_transform_result = ranges::in_out_result<I, O>; |
(5) | (C++20 起) |
template< class I1, class I2, class O > using binary_transform_result = ranges::in_in_out_result<I1, I2, O>; |
(6) | (C++20 起) |
对范围应用给定的函数并将结果存储于始于 result 的另一范围。
1) 应用一元运算
op 到 [first1, last1) 所定义的范围（在以投影 proj 投影后）。2) 同 (1)，但以
r 为源范围，如同以 ranges::begin(r) 为 first 并以 ranges::end(r) 为 last。3) 应用二元运算
binary_op 到来自两个范围的元素：一个由 [first1, last1) 定义而另一个由 [first2, last2) 定义（在分别以投影 proj1 与 proj2 投影后）。4) 同 (3)，但以
r1 为第一源范围，如同以 ranges::begin(r1) 为 first1 并以 ranges::end(r1) 为 last1，而对 r2 也类似。此页面上描述的函数式实体是算法函数对象（非正式地称为 niebloid），即：
参数
| first1, last1 | - | 要变换的第一元素范围的迭代器-哨位对 |
| r, r1 | - | 要变换的元素范围 |
| first2, last2 | - | 要变换的第二元素范围的迭代器-哨位对 |
| r2 | - | 要变换的第二元素范围 |
| result | - | 目标范围的起始，可以等于 first1 或 first2
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| op, binary_op | - | 应用到投影后元素的操作 |
| proj1 | - | 应用到第一范围中的元素的投影 |
| proj2 | - | 应用到第二范围中的元素的投影 |
返回值
1,2)
unary_transform_result，含有等于 last 的输入迭代器与指向最后变换元素后一元素的输出迭代器。3,4)
binary_transform_result，含有分别作为 in1 与 in2 的指向来自范围 [first1, last1) 与 [first2, last2) 的最后变换元素的输入迭代器，与作为 out 的指向最后变换元素后一元素的输出迭代器。复杂度
1,2) 准确应用
ranges::distance(first1, last1) 次 op 与 proj。3,4) 准确应用
ranges::min(ranges::distance(first1, last1), ranges::distance(first2, last2)) 次 binary_op 与投影。可能的实现
struct transform_fn
{
// 版本 (1)
template<std::input_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O,
std::copy_constructible F, class Proj = std::identity>
requires std::indirectly_writable<O, std::indirect_result_t<F&,
std::projected<I, Proj>>>
constexpr ranges::unary_transform_result<I, O>
operator()(I first1, S last1, O result, F op, Proj proj = {}) const
{
for (; first1 != last1; ++first1, (void)++result)
*result = std::invoke(op, std::invoke(proj, *first1));
return {std::move(first1), std::move(result)};
}
// 版本 (2)
template<ranges::input_range R, std::weakly_incrementable O,
std::copy_constructible F, class Proj = std::identity>
requires std::indirectly_writable<O,
std::indirect_result_t<F&, std::projected<ranges::iterator_t<R>, Proj>>>
constexpr ranges::unary_transform_result<ranges::borrowed_iterator_t<R>, O>
operator()(R&& r, O result, F op, Proj proj = {}) const
{
return (*this)(ranges::begin(r), ranges::end(r), std::move(result),
std::move(op), std::move(proj));
}
// 版本 (3)
template<std::input_iterator I1, std::sentinel_for<I1> S1,
std::input_iterator I2, std::sentinel_for<I2> S2,
std::weakly_incrementable O,
std::copy_constructible F,
class Proj1 = std::identity, class Proj2 = std::identity>
requires std::indirectly_writable<O,
std::indirect_result_t<F&,
std::projected<I1, Proj1>,
std::projected<I2, Proj2>>>
constexpr ranges::binary_transform_result<I1, I2, O>
operator()(I1 first1, S1 last1, I2 first2, S2 last2, O result,
F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
for (; first1 != last1 && first2 != last2;
++first1, (void)++first2, (void)++result)
*result = std::invoke(binary_op,
std::invoke(proj1, *first1),
std::invoke(proj2, *first2));
return {std::move(first1), std::move(first2), std::move(result)};
}
// 版本 (4)
template<ranges::input_range R1, ranges::input_range R2,
std::weakly_incrementable O, std::copy_constructible F,
class Proj1 = std::identity, class Proj2 = std::identity>
requires std::indirectly_writable<O,
std::indirect_result_t<F&,
std::projected<ranges::iterator_t<R1>, Proj1>,
std::projected<ranges::iterator_t<R2>, Proj2>>>
constexpr ranges::binary_transform_result<ranges::borrowed_iterator_t<R1>,
ranges::borrowed_iterator_t<R2>, O>
operator()(R1&& r1, R2&& r2, O result,
F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
return (*this)(ranges::begin(r1), ranges::end(r1),
ranges::begin(r2), ranges::end(r2),
std::move(result), std::move(binary_op),
std::move(proj1), std::move(proj2));
}
};
inline constexpr transform_fn transform;
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注解
std::ranges::transform 不保证按顺序应用 op 或 binary_op。为按顺序应用函数导序列或应用修改元素的函数到序列，请用 std::ranges::for_each。
示例
下列代码用 ranges::transform 以 std::toupper 函数原地转换为大写，然后变换每个 char 为其序数值。然后以一个投影用 ranges::transform 将 std::vector<Foo> 的元素变换为 char，并填充一个 std::string。
运行此代码
#include <algorithm>
#include <cctype>
#include <functional>
#include <iostream>
#include <string>
#include <vector>
int main()
{
std::string s{"hello"};
auto op = [](unsigned char c) -> unsigned char { return std::toupper(c); };
namespace ranges = std::ranges;
// 字符串就地改为大写
ranges::transform(s.begin(), s.end(), s.begin(), op );
std::vector<std::size_t> ordinals;
// 将每个 char 转换为 size_t
ranges::transform(s, std::back_inserter(ordinals),
[](unsigned char c) -> std::size_t { return c; });
std::cout << s << ':';
for (auto ord : ordinals)
std::cout << ' ' << ord;
// 使每个序数倍增
ranges::transform(ordinals, ordinals, ordinals.begin(), std::plus{});
std::cout << '\n';
for (auto ord : ordinals)
std::cout << ord << ' ';
std::cout << '\n';
struct Foo { char bar; };
const std::vector<Foo> f = {{'h'},{'e'},{'l'},{'l'},{'o'}};
std::string result;
// 投影，然后转大写
ranges::transform(f, std::back_inserter(result), op, &Foo::bar);
std::cout << result << '\n';
}
输出：
HELLO: 72 69 76 76 79
144 138 152 152 158
HELLO
参阅
(C++20) |
应用一元函数对象到范围中元素 (算法函数对象) |
对序列的每个元素应用某个变换函数的 view (类模板) (范围适配器对象) | |
| 应用函数到元素范围，并在目标范围存储结果 (函数模板) |