英文:
What's the purpose of std::dynamic_extent in std::span
问题
我知道 std::span 是静态的。它只是一种查看一堆向量/数组等元素的视图。
我看到了 构造函数 中，似乎在第4-6个中使用了 std::dynamic_extent。但在这些构造函数中，还需要一个模板参数来表示大小 - std::size_t N。对我来说，这意味着大小/计数/长度在编译时已知。那么 std::dynamic_extent 到底是什么呢？
英文:
I know std::span is static. It is just view over bunch of vector/array/etc. elements.
I see constructors of span, and it seems like std::dynamic_extent is used in 4-6. But in those constructors, there is a required template parameter for the size - std::size_t N. To me this means that the size/count/len is known at compile time. So what really is std::dynamic_extent?
答案1
得分: 4
std::dynamic_extent 的定义是：
inline constexpr std::size_t dynamic_extent
= std::numeric_limits<std::size_t>::max();
它是一个 std::size_t 的特殊值，用于指示 std::span 具有动态范围。
大小需要的模板参数为
std::size_t N。对我来说，这意味着大小/计数/长度在编译时已知。
std::span 的“大小”仍然在编译时指定，只是当“大小”取特殊值时，它被视为动态范围。
英文:
The definition of is std::dynamic_extent is
inline constexpr std::size_t dynamic_extent
= std::numeric_limits<std::size_t>::max();
It's a special value of a std::size_t that's used to indicate that the std::span has a dynamic extent.
> There is a required template argument for the size - std::size_t N. To me this means that the size/count/len is known at compile time.
The "size" of the std::span is still specified at compile time, it's just that when the "size" takes on that special value, it's treated as a dynamic extent.
答案2
得分: 0
构造函数会设置初始大小，但当 span 具有 dynamic_extent 时，其大小可能会发生变化。
std::array<int, 4> arr{1, 2, 3, 4};
std::span<int> s{arr}; // 整个数组视图 // 大小为 4
s = s.subspan(1, 2)}; // 限制的数组视图 // 现在大小为 2
英文:
The constructor would set the initial size, but when span has dynamic_extent, its size might evolve
std::array<int, 4> arr{1, 2, 3, 4};
std::span<int> s{arr}; // whole array view // size is 4
s = s.subspan(1, 2)}; // restricted array view // size is 2 now
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论