std::as_bytes, std::as_writable_bytes

 

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as_bytesas_writable_bytes (C++20)(C++20)

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dynamic_extent (C++20)

Deduction guides(C++20)

 

Defined in header <span>
template< class T, std::size_t N> std::span<const std::byte, S/* see below */>     as_bytes( std::span<T, N> s ) noexcept;	(1)	(since C++20)
template< class T, std::size_t N> std::span<std::byte, S/* see below */>     as_writable_bytes( std::span<T, N> s ) noexcept;	(2)	(since C++20)

Obtains a view to the object representation of the elements of the span s.

If N is std::dynamic_extent, the extent of the returned span S is also std::dynamic_extent; otherwise it is sizeof(T) * N.

as_writable_bytes only participates in overload resolution if std::is_const_v<T> is false.

Return value

1) A span constructed with {reinterpret_cast<const std::byte*>(s.data()), s.size_bytes()}.

2) A span constructed with {reinterpret_cast<std::byte*>(s.data()), s.size_bytes()}.

Example

Run this code

#include <cstddef>
#include <iomanip>
#include <iostream>
#include <span>
 
void print(float const x, std::span<const std::byte> const bytes)
{
    std::cout << std::setprecision(6) << std::setw(8) << x << " = { "
              << std::hex << std::uppercase << std::setfill('0');
    for (auto const b : bytes)
        std::cout << std::setw(2) << std::to_integer<int>(b) << ' ';
    std::cout << std::dec << "}\n";
}
 
int main()
{
    /* mutable */ float data[1]{3.141592f};
 
    auto const const_bytes = std::as_bytes(std::span{ data });
 
    print(data[0], const_bytes);
 
    auto const writable_bytes = std::as_writable_bytes(std::span{ data });
 
    // Change the sign bit that is the MSB (IEEE 754 Floating-Point Standard).
    writable_bytes[3] |= std::byte{ 0B1000'0000 };
 
    print(data[0], const_bytes);
}

Possible output:

 3.14159 = { D8 0F 49 40 }
-3.14159 = { D8 0F 49 C0 }

See also

start_lifetime_asstart_lifetime_as_array (C++23)	implicitly creates objects in given storage with the object representation reused (function template)
byte (C++17)	the byte type (enum)