} cout << i << endl; // ordinary read, 1729 }21 span22 span Encapsulates Ptr-Len Pairs • span is a non-owning view of contiguous elements • span is like a pair of T* and size_t, but better • spans accidental pointer arithmetic • span  span; span 🐞 span • Shallow const, like T* • span can be reassigned, can't modify elements • const span can't be reassigned, • MSVC's std::span checks debug; gsl::span always23 Contiguous Range Constructor • GH-142, GH-500, GH-587 implemented by miscco #include #include #include <span> #include

Initializer  Non-Type Template Parameters  [[likely]] and [[unlikely]]  Calendars & Timezones  std::span  Feature Test Macros   Immediate Functions – consteval  constinit  Class Enums system_clock::now() } };  Output: cout << localt << endl; // 2020-09-15 09:35:10.15365 std::span  <span>  Provides a “view” over some contiguous data  Does not own the data  Never allocates/deallocates support strides66 std::span  Can be dynamic-sized (run time) or fixed-sized (compile time)  Examples: int data[42]; span a {data}; // fixed-size: 42 ints span b {data}; //

i < n; i++) y[i] = x[i] + y[i]; } __global__ void add_gpu( int n, gsl::span x, gsl::span y) { for (int i = 0; i < n; i++) y[i] = x[i] + y[i]; } 22 |Avoid auto operator()(index i) const { return a_[i] + b_[i]; } }; struct add_span { gsl::span a_; gsl::span b_; auto operator()(index i) const { return a_[i] + + b_[i]; } }; template void add(int n, callable f, gsl::span y) { for (int i = 0; i < n; i++) y[i] += f(i); } 23 |“Unified Memory creates a pool of managed memory that is shared

Defined in  std::mdspan  Multidimensional array view  Multidimensional extension of std::span (from C++20)  Supports different layout policies  std::submdspan  An mdspan viewing a subset data[] = "11 22"; std::ispanstream s { std::span{data} }; int a, b; s >> a >> b;  E.g. output: char data[32] {}; std::ospanstream s { std::span{data} }; s << 22 << 11;96 Agenda  C++23

char *ptr; • size_t len; }; • void cihou_array(FatPtr fptr); • 这就是 rust 炫耀已久的数组胖指针。 C++20 中的 span 也是这个思想。 • 他提倡把 ptr 和 len 这两个逻辑上相关的参数绑在一起，避免程序员犯错。 用胖指针表示字符串 • 其实胖指针的思想既不是 rust 原创的，也不是 C++20 以后才出现的。 c_str 时）。 • 长度已经存储在 string 的成员里， size() 是 O(1) 的。 • 在尾部切片可以用 resize() 修改长度，无需写入字符串本身。 • string_view 和 span 无非是个弱引用版本，额外增加了在头部切片的能力而 已。 强引用胖指针： string • 刚刚说的 string 容器，是掌握着字符串生命周期（ lifespan ）的胖指针。 • 这种掌 string_view 以后，不要改写原字符串。 举例：常见容器及其相应的弱引用 强引用 弱引用 string string_view wstring wstring_view vector span unique_ptr T * shared_ptr weak_ptr 字符串用 substr 切片 • 熟悉 Python 的同学对切片 (slice) 操作肯定不陌生，例如：

};42 SENDERS AND COROUTINES43 // This is a coroutine: unifex::task read_socket_async(socket, span); int main() { socket s = /*...*/; char buff[1024]; auto [cbytes] = std::this_thread::sync_wait( a coroutine type trivially. // This is a coroutine: unifex::task read_socket_async(socket, span); unifex::task concurrent_read_async(socket s1, socket s2) { char buff1[1024];