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 != 6
std::span<int> iteratorSpan1(v1.begin(), v1.end());
std::span<int> iteratorSpan2(v1.begin()+1, v1.end());
std::span<int> iteratorsizeSpan3(v1.begin(), 6); // iterator + size
int 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);
}std::basic_string_view - cppreference.com
