stdspan ---C++20

`std::span` —C++20

std::span的定义

template<class T,std::size_t Extent = std::dynamic_extent
> class span;

std::span是指向一组连续的对象的对象, 是一个视图view, 不是一个拥有者owner

一组连续的对象可以是 C 数组, 带着大小的指针, std::array, 或者 std::string

std::span可以有两种范围:

静态范围static extend
动态范围dynamic extend

静态范围, 动态范围

**静态范围static extent: **编译期就可以确定大小

**动态范围dynamic extend: ** 由指向第一个对象的指针和连续对象的大小组成

#include <ranges>
#include <vector>
#include <iostream>
#include <span>
#include <format>void printSpan(std::span<int> container)
{std::cout << std::format("container size: {} \n", container.size());for (auto ele : container){std::cout << ele << " ";}std::cout << std::endl;std::cout << std::endl;
}int main()
{std::vector v1{1, 2, 3, 4, 5, 8};std::vector v2{9, 2, 4, 2, 6, 78};std::span<int> dynamicSpan(v1);std::span<int, 6> staticSpan(v2);printSpan(dynamicSpan);printSpan(staticSpan);
}

构造

默认构造 template< class T, std::size_t Extent = std::dynamic_extent > class span;
使用迭代器(std::contiguous_iterator)
迭代器 + 大小
C-stye数组


int main()
{std::vector v1{1, 2, 3, 4, 5, 8};std::vector v2{9, 2, 4, 2, 6, 78};std::span<int> dynamicSpan(v1); // 默认构造std::span<int, 6> staticSpan(v2);std::span<int, 2> undefineSpan(v2); // 未定义行为 2 != 6std::span<int> iteratorSpan1(v1.begin(), v1.end());std::span<int> iteratorSpan2(v1.begin()+1, v1.end());std::span<int> iteratorsizeSpan3(v1.begin(), 6); // iterator + sizeint cArray[5]{ 1, 2, 3, 4, 5 };std::array<int, 5> cppArray{ 1, 2, 3, 4, 5 };std::span<int> cArrSpan(cArray);std::span<int> cppArrSpan{cppArrSpan};std::span<int,5> cppArrSpan1{cppArrSpan};
}

std::span 在构造时不支持隐式类型转换
std::span可以推断C-stye数组的大小

修改

可以修改std::span的指向,或者修改子集

#include <ranges>
#include <vector>
#include <iostream>
#include <span>
#include <format>
#include <array>void printSpan(std::span<int> container)
{std::cout << std::format("container size: {} \n", container.size());for (auto ele : container){std::cout << ele << " ";}std::cout << std::endl;std::cout << std::endl;
}int main()
{std::cout << '\n';std::vector vec{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};printSpan(vec);std::span span1(vec);std::span span2{span1.subspan(1, span1.size() - 2)};std::transform(span2.begin(), span2.end(),span2.begin(),[](int i) { return i * i; });printSpan(vec);printSpan(span1);
}