NAME¶

std::span::operator= - std::span::operator=

Synopsis¶

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().

Parameters¶

other - another span to copy from

Return value¶

*this

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

See also¶

constructor constructs the span
(public member function)
data direct access to the underlying contiguous storage
(public member function)
size returns the number of elements
(public member function)