std::span<T,Extent>::operator=

 

C++

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技术规范

 

容器库

序列

array (C++11)

vector

deque

forward_list (C++11)

list

关联

set

multiset

map

multimap

无序关联

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

适配器

stack

queue

priority_queue

视图

span (C++20)

 

std::span

成员函数

span::span

span::operator=

元素访问

span::front

span::back

span::operator[]

span::data

迭代器

span::begin

span::end

span::rbegin

span::rend

观察器

span::empty

span::size

span::size_bytes

子视图

span::first

span::last

span::subspan

非成员函数

as_bytesas_writable_bytes

get

非成员常量

dynamic_extent

推导指引

 

constexpr span& operator=( const span& other ) noexcept = default;

赋值 other 给 *this 。此默认化的赋值运算符进行数据指针和大小的浅复制，即调用此函数后， data() == other.data() 且 size() == other.size() 。

参数

other

-

要复制的另一 span

返回值

*this 。

示例

运行此代码

#include <algorithm>
#include <array>
#include <cassert>
#include <cstddef>
#include <iostream>
#include <span>
#include <string_view>
 
void print(std::string_view info = "", std::span<const int> span = {},
           std::size_t extent = 0, std::size_t size_of = 0) {
    if (span.size() == 0) {
        std::cout << info << '\n';    
        return;
    }
    std::cout << info << '[' << span.size() << "] {";
    std::ranges::for_each(span, [](const int x) { std::cout << ' ' << x; });
    std::cout << " }";
    if (extent != 0) {
        std::cout << " extent = ";
        if (extent == std::dynamic_extent)
            std::cout << "dynamic";
        else
            std::cout << extent;
    }
    if (size_of != 0) {
        std::cout << ", sizeof = " << size_of;
    }
    std::cout << '\n';
}
 
int main()
{
    std::array<int,6> a1;
    std::array<int,6> a2;
    a1.fill(3);
    a2.fill(4);
 
    auto s1 = std::span(a1);
    auto s2 = std::span(a2);
    print("s1", s1, s1.extent, sizeof(s1));
    print("s2", s2, s2.extent, sizeof(s2));
 
    // 检查赋值是否进行浅复制
    s1 = s2;
    (s1.data() == s2.data() && s1.size() == s2.size())
        ? print("s1 = s2; is a shallow copy!")
        : print("s1 = s2; is a deep copy!");
    print("s1", s1);
 
    print("Fill s1 with 5:");
    std::ranges::fill(s1, 5);
    // s2 亦被“更新”因为 s1 与 s2 引用同一数据
    assert(std::ranges::equal(s1, s2));
    print("s1", s1);
    print("s2", s2);
    print();
 
    int a3[] {1, 2, 3, 4};
    int a4[] {2, 3, 4, 5};
    int a5[] {3, 4, 5};
 
    std::span<int, std::dynamic_extent> dynamic_1 {a3};
    std::span<int, std::dynamic_extent> dynamic_2 {a4, 3};
    std::span<int, 4> static_1 {a3};
    std::span<int, 4> static_2 {a4};
    std::span<int, 3> static_3 {a5};
 
    print("dynamic_1", dynamic_1, dynamic_1.extent, sizeof(dynamic_1));
    print("dynamic_2", dynamic_2, dynamic_2.extent, sizeof(dynamic_2));
    print("static_1", static_1, static_1.extent, sizeof(static_1));
    print("static_2", static_2, static_2.extent, sizeof(static_2));
    print("static_3", static_3, static_3.extent, sizeof(static_3));
 
    dynamic_1 = dynamic_2; // OK
    dynamic_1 = static_1;  // OK
 // static_1  = dynamic_1; // 错误：无匹配的 ‘operator=’
    static_1  = static_2;  // OK ：相同长度 = 4
 // static_1  = static_3;  // 错误：不同长度： 4 与 3
}

输出：

s1[6] { 3 3 3 3 3 3 } extent = 6, sizeof = 8
s2[6] { 4 4 4 4 4 4 } extent = 6, sizeof = 8
s1 = s2; is a shallow copy!
s1[6] { 4 4 4 4 4 4 }
Fill s1 with 5:
s1[6] { 5 5 5 5 5 5 }
s2[6] { 5 5 5 5 5 5 }
 
dynamic_1[4] { 1 2 3 4 } extent = dynamic, sizeof = 16
dynamic_2[3] { 2 3 4 } extent = dynamic, sizeof = 16
static_1[4] { 1 2 3 4 } extent = 4, sizeof = 8
static_2[4] { 2 3 4 5 } extent = 4, sizeof = 8
static_3[3] { 3 4 5 } extent = 3, sizeof = 8

参阅

(构造函数)	构造 span (公开成员函数)
data	返回指向元素序列起始的指针 (公开成员函数)
size	返回序列中的元素数 (公开成员函数)