How to convert between a Unicode/UCS codepoint and a UTF16 surrogate pair?

How to convert back and forth between a Unicode/UCS codepoint and a UTF16 surrogate pair in C++14 and later?

EDIT: Removed mention of UCS-2 surrogates, as there is no such thing. Thanks @remy-lebeau!

In C++11 and later, you can use std::wstring_convert to convert between various UTF/UCS encodings, using the following std::codecvt types:

• UTF-8 <-> UCS-2:
  std::codecvt_utf8<char16_t>

• UTF-8 <-> UTF-16:
  std::codecvt_utf8_utf16

• UTF-8 <-> UTF-32/UCS-4:
  std::codecvt_utf8<char32_t>

• UCS-2 <-> UTF-16:
  std::codecvt_utf16<char16_t>

• UTF-16 <-> UTF-32/UCS-4:
  std::codecvt_utf16<char32_t>

• UCS-2 <-> UTF-32/UCS-4:
  no standard conversion, but you can write your own std::codecvt class for it if needed. Otherwise, use one of the above conversions in between:
  UCS-2 <-> UTF-X <-> UTF-32/UCS-4

You don't need to handle surrogates manually.

You can use std::u32string to hold your codepoint(s), and std::u16string to hold your UTF-16/UCS-2 codeunits.

For example:

using convert_utf16_uf32 = std::wstring_convert<std::codecvt_utf16<char32_t>, char16_t>;

std::u16string CodepointToUTF16(const char32_t codepoint)
{
    const char32_t *p = &codepoint;
    return convert_utf16_uf32{}.from_bytes(
        reinterpret_cast<const char*>(p),
        reinterpret_cast<const char*>(p+1)
    );
}

std::u16string UTF32toUTF16(const std::u32string &str)
{
    return convert_utf16_uf32{}.from_bytes(
        reinterpret_cast<const char*>(str.data()),
        reinterpret_cast<const char*>(str.data()+str.size())
    );
}

char32_t UTF16toCodepoint(const std::u16string &str)
{
    std::string bytes = convert_utf16_uf32{}.to_bytes(str);
    return *(reinterpret_cast<const char32_t*>(bytes.data()));
}

std::u32string UTF16toUTF32(const std::u16string &str)
{
    std::string bytes = convert_utf16_uf32{}.to_bytes(str);
    return std::u32string(
       reinterpret_cast<const char32_t*>(bytes.data()),
       bytes.size() / sizeof(char32_t)
    );
}