c++ adding "\u" to string - c++

Question

Learning c++, trying to find a way to display UTF-16 characters by adding the 4 digits after the "\u". But, for example, if I try to directly add 0000:
string temp = "\u" + "0000";
I get the error: incorrectly formed universal character name. So is there a way to get these two to form one Unicode character? Also I realize that the end four numbers range from 0-F but for now I just want to focus on the 0-9 characters.
How can I add"\u" with a different string
Edit: I was looking for the C++ equivalent of the JavaScript function:
String.fromCharCode()

Answer 1

You can't say "\u" + "0000", because the parsing of escape sequences happens early in the process, before the actual compilation begins. By the time the strings would be tacked together, escape sequences are already parsed and won't be again. And since \u is not a valid escape sequence on its own, you get an error about it.

Answer 2

You can't separate a string literal like that. The special sequence inside the quotes is a directive to the compiler to insert the relevant Unicode character at compile time so if you break it into two pieces it is no longer recognized as a directive.
To programatically generate a UTF-16 character based on its Unicode codepoint number you could use the Standard Library Unicode converson functions. Unfortunately there is no direct conversion between UTF-32 (Unicode codepoints) and UTF-16 so you have to go through UTF-8 as an intermediate value:
// UTF-16 may contain either one or two char16_t characters so
// we return a string to potentially contain both.
///
std::u16string codepoint_to_utf16(char32_t cp)
{
// convert UTF-32 (standard unicode codepoint) to UTF-8 intermediate value
char utf8[4];
char* end_of_utf8;
{
char32_t const* from = &cp;
std::mbstate_t mbs;
std::codecvt_utf8<char32_t> ccv;
if(ccv.out(mbs, from, from + 1, from, utf8, utf8 + 4, end_of_utf8))
throw std::runtime_error("bad conversion");
}
// Now convert the UTF-8 intermediate value to UTF-16
char16_t utf16[2];
char16_t* end_of_utf16;
{
char const* from = nullptr;
std::mbstate_t mbs;
std::codecvt_utf8_utf16<char16_t> ccv;
if(ccv.in(mbs, utf8, end_of_utf8, from, utf16, utf16 + 2, end_of_utf16))
throw std::runtime_error("bad conversion");
}
return {utf16, end_of_utf16};
}
int main()
{
std::u16string s; // can hold UTF-16
// iterate through some Greek codepoint values
for(char32_t u = 0x03b1; u < 0x03c9; ++u)
{
// append the converted UTF-16 characters to our string
s += codepoint_to_utf16(u);
}
// do whatever you want with s here...
}

Answer 3

What you're trying to do is not possible. C++ parsing is split into multiple phases. Per [lex.phases], escape sequences (in phase 5) are escaped before adjacent string literals are concatenated (phase 6).