std::ranges::begin 和 std::begin 有什么区别? [英] What is the difference between std::ranges::begin and std::begin?

问题描述

std::begin 和新的 std::ranges::begin?(end、size 等也一样)

两者似乎工作相同:

#include <iostream>
#include <vector>
#include <array>
#include <ranges>

template<std::ranges::range R>
void printInfo(const R &range)
{
    std::cout << (std::ranges::begin(range) == std::begin(range));
}

template<class T>
struct X
{
    std::vector<T> v;

    auto begin() const { return v.begin(); }
    auto end() const { return v.end(); }
};

int main()
{
    printInfo(std::vector{1, 2, 3, 4});
    printInfo(std::array{1, 2, 3, 4});
    printInfo(X<int>{{1, 2, 3, 4}});

    int oldSchool[]{1, 2, 3, 4};
    printInfo(oldSchool);
}

按预期编译并打印 1111.

ranges::begin 是否会使 std::begin 过时?还是两者有不同的用例?

Does ranges::begin make std::begin obsolete? Or do the two have different use-cases?

推荐答案

有一些区别.

首先，ranges::begin(x) 适用于所有范围，而 std::begin(x) 不适用.后者不会对 begin 进行 ADL 查找，因此指定的范围如下:

First, ranges::begin(x) works on all ranges while std::begin(x) does not. The latter will not do ADL lookup on begin, so ranges specified like:

struct R {
    ...
};
auto begin(R const&);
auto end(R const&);

行不通，这就是为什么你必须写这样的东西:

won't work, which is why you have to write something like:

using std::begin, std::end;
auto it = begin(r);

你不必用 ranges::begin 做两步.

其次，ranges::begin(x) 更安全一些.Ranges 引入了这个借用范围的概念，这是一个可以安全保留其迭代器的范围.vector 例如不是一个借用的范围 - 因为一旦 vector 死亡，数据就会死亡.ranges::begin 防止这种情况:

Second, ranges::begin(x) is a little safer. Ranges introduces this notion of a borrowed range, which is a range whose iterators that you can hold onto safely. vector<int> for instance is not a borrowed range - since once the vector dies the data dies. ranges::begin guards against that:

auto get_data() -> std::vector<int>;

auto a = std::begin(get_data());    // ok, but now we have a dangling iterator
auto b = ranges::begin(get_data()); // ill-formed

第三，ranges::begin 和 ranges::end 有额外的类型检查.ranges::begin(r) 需要 r.begin() 或 begin(r) 的结果来建模 input_or_output_iterator.ranges::end(r) 要求 ranges::begin(r) 有效并且需要 r.end() 或 >end(r) 对 sentinel_for 进行建模.也就是说 - 我们从 begin 和 end 得到的实际上是一个范围.

Third, ranges::begin and ranges::end have extra type checks. ranges::begin(r) requires the result of either r.begin() or begin(r) to model input_or_output_iterator. ranges::end(r) requires ranges::begin(r) to be valid and requires either r.end() or end(r) to model sentinel_for<decltype(ranges::begin(r))>. That is - that whatever we get from begin and end is actually a range.

这意味着，例如:

struct X {
    int begin() const { return 42; }
};

X x;
auto a = std::begin(x);    // ok, a == 42
auto b = ranges::begin(x); // ill-formed, int is not an iterator

虽然更烦人的是你有一个迭代器类型，它可能是可增加的、可取消引用的、可比较的等等......但没有默认的构造函数.这不符合 C++20 的 input_or_output_iterator 所以 ranges::begin 会失败.

Although more annoyingly is a case where you have an iterator type that might be incrementable, dereferenceable, comparable, etc... but fail to have a default constructor. That does not meet the requirements of C++20's input_or_output_iterator so ranges::begin will fail.

四、ranges::begin是一个函数对象，而std::begin是一组重载的函数模板:

Fourth, ranges::begin is a function object, while std::begin is a set of overloaded function templates:

auto f = ranges::begin; // ok
auto g = std::begin;    // error: which std::begin did you want?

第五，一些范围自定义点对象除了只调用该名称的函数外，还有其他回退行为.std::size(r) 总是调用一个名为 size 的函数(除非 r 是一个原始数组).std::empty(r) 总是调用一个名为 empty 的函数(除非 r 是一个原始数组，在这种情况下它只是 false，或者 r 是一个 initializer_list，在这种情况下 r.size() == 0).但是 ranges::size 在某些情况下可以 a> 执行 ranges::end(r) - Ranges::begin(r)(如果 size(r) 和 r.size() 不存在)就像 ranges::empty 可以 在某些情况下要么做 ranges::size(r) == 0 或 ranges::begin(r) == 范围::end(r).

Fifth, some of the ranges customization point objects have other fallback behavior besides just calling a function of that name. std::size(r) always invokes a function named size (unless r is a raw array). std::empty(r) always invokes a function named empty(unless r is a raw array, in which case it's just false, or r is an initializer_list, in which case r.size() == 0). But ranges::size could under certain circumstances perform ranges::end(r) - ranges::begin(r) (as a fallback if size(r) and r.size() don't exist) just like ranges::empty could under certain circumstances either do ranges::size(r) == 0 or ranges::begin(r) == ranges::end(r).

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