NAME

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

Synopsis

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

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.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));

// 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 }