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Difference between revisions of "cpp/container/span/operator="

From cppreference.com
< cpp‎ | container‎ | span
m (link to ja)
m (→‎See also: Use universal containers' dsc)
 
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{{cpp/container/span/navbar}}
 
{{cpp/container/span/navbar}}
  
−
{{ddcl | 1=
+
{{ddcl|since=c++20|1=
 
constexpr span& operator=( const span& other ) noexcept = default;
 
constexpr span& operator=( const span& other ) noexcept = default;
 
}}
 
}}
  
−
Assigns {{tt|other}} to {{c|*this}}. This defaulted assignment operator performs a shallow copy of the data pointer and the size, i.e., after a call to this function, {{c|1=data() == other.data()}} and {{c|1=size() == other.size()}}.
+
Assigns {{c|other}} to {{c|*this}}. This defaulted assignment operator performs a shallow copy of the data pointer and the size, i.e., after a call to this function, {{c|1=data() == other.data()}} and {{c|1=size() == other.size()}}.
  
−
=== Parameters ===
+
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | other | another span to copy from }}
+
{{par|other|another span to copy from}}
 
{{par end}}
 
{{par end}}
  
−
=== Return value ===
+
===Return value===
−
{{c|*this}}.
+
{{c|*this}}
  
−
=== See also ===
+
===Example===
 +
{{example
 +
|code=
 +
#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.empty())
 +
    {
 +
        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)
 +
    {
 +
        std::cout << " extent = ";
 +
        if (extent == std::dynamic_extent)
 +
            std::cout << "dynamic";
 +
        else
 +
            std::cout << extent;
 +
    }
 +
 
 +
    if (size_of)
 +
        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));
 +
 
 +
    // Check that assignment performs a shallow copy.
 +
    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 is also 'updated' since s1 and s2 point to the same data
 +
    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;  // ERROR: no match for ‘operator=’
 +
    static_1 = static_2;  // OK: same extents = 4
 +
//  static_1 = static_3;  // ERROR: different extents: 4 and 3
 +
}
 +
|output=
 +
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
 +
}}
 +
 
 +
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/container/span/dsc constructor}}
+
{{dsc inc|cpp/container/dsc constructor|span}}
−
{{dsc inc | cpp/container/span/dsc data}}
+
{{dsc inc|cpp/container/dsc data|span}}
−
{{dsc inc | cpp/container/span/dsc size}}
+
{{dsc inc|cpp/container/dsc size|span}}
 
{{dsc end}}
 
{{dsc end}}
  
−
{{langlinks|ja|zh}}
+
{{langlinks|de|es|ja|ru|zh}}

Latest revision as of 14:08, 21 December 2023

 
C++
 
Containers library
 
std::span
 
constexpr span& operator=( const span& other ) noexcept = default;
 (since C++20)

Assigns other to *this. This defaulted assignment operator performs a shallow copy of the data pointer and the size, i.e., after a call to this function, data() == other.data() and size() == other.size().

Contents

[edit] Parameters

other - another span to copy from

[edit] Return value

*this

[edit] Example

Run this code
#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.empty())
    {
        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)
    {
        std::cout << " extent = ";
        if (extent == std::dynamic_extent)
            std::cout << "dynamic";
        else
            std::cout << extent;
    }
 
    if (size_of)
        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));
 
    // Check that assignment performs a shallow copy.
    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 is also 'updated' since s1 and s2 point to the same data
    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;  // ERROR: no match for ‘operator=’
    static_1 = static_2;   // OK: same extents = 4
//  static_1 = static_3;   // ERROR: different extents: 4 and 3
}

Output: 

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

[edit] See also

 constructs the span
(public member function) [edit]
direct access to the underlying contiguous storage
(public member function) [edit]
returns the number of elements
(public member function) [edit]
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