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std::span<T,Extent>:: size_bytes

From cppreference.net
C++
Containers library
std::span
constexpr size_type size_bytes ( ) const noexcept ;
(since C++20)

Returns the size of the sequence in bytes.

Return value

size ( ) * sizeof ( element_type ) .

Example

Run this code 
#include <cstdint>
#include <iostream>
#include <span>
constexpr static std::int32_t a[]{1, 2, 3, 4, 5};
constexpr static std::span s{a};
static_assert
(
    sizeof(int32_t) == 4 &&
    std::size(a) == 5 &&
    sizeof a == 20 &&
    s.size() == 5 &&
    s.size_bytes() == 20
);
int main()
{
    // typically, a static span holds only a pointer:
    std::cout << sizeof(s) << '\n';
}

Possible output: 

8

See also

(C++20)
returns the number of elements in the sequence
(public member function)