Any good solutions for C++ string code point and code unit?

Question

In Java, a String has methods:

length()/charAt(), codePointCount()/codePointAt()

C++11 has std::string a = u8\"很烫烫的一锅汤\";

Answer 1

I generally convert the UTF-8 string to a wide UTF-32/UCS-2 string before doing character operations. C++ does actually give us functions to do that but they are not very user friendly so I have written some nicer conversion functions here:

// This should convert to whatever the system wide character encoding 
// is for the platform (UTF-32/Linux - UCS-2/Windows)
std::string ws_to_utf8(std::wstring const& s)
{
    std::wstring_convert<std::codecvt_utf8<wchar_t>, wchar_t> cnv;
    std::string utf8 = cnv.to_bytes(s);
    if(cnv.converted() < s.size())
        throw std::runtime_error("incomplete conversion");
    return utf8;
}

std::wstring utf8_to_ws(std::string const& utf8)
{
    std::wstring_convert<std::codecvt_utf8<wchar_t>, wchar_t> cnv;
    std::wstring s = cnv.from_bytes(utf8);
    if(cnv.converted() < utf8.size())
        throw std::runtime_error("incomplete conversion");
    return s;
}

int main()
{
    std::string s = u8"很烫烫的一锅汤";

    auto w = utf8_to_ws(s); // convert to wide (UTF-32/UCS-2)

    // now we can use code-point indexes on the wide string

    std::cout << s << " is " << w.size() << " characters long" << '\n';
}

Output:

很烫烫的一锅汤 is 7 characters long

If you want to convert to and from UTF-32 regardless of platform then you can use the following (not so well tested) conversion routines:

std::string utf32_to_utf8(std::u32string const& utf32)
{
    std::wstring_convert<std::codecvt_utf8<char32_t>, char32_t> cnv;
    std::string utf8 = cnv.to_bytes(utf32);
    if(cnv.converted() < utf32.size())
        throw std::runtime_error("incomplete conversion");
    return utf8;
}

std::u32string utf8_to_utf32(std::string const& utf8)
{
    std::wstring_convert<std::codecvt_utf8<char32_t>, char32_t> cnv;
    std::u32string utf32 = cnv.from_bytes(utf8);
    if(cnv.converted() < utf8.size())
        throw std::runtime_error("incomplete conversion");
    return utf32;
}

NOTE: As of C++17 std::wstring_convert is deprecated.

However I still prefer to use it over a third party library because it is portable, it avoids external dependencies, it won't be removed until a replacement is provided and in all cases it will be easy to replace the implementations of these functions without having to change all the code that uses them.