Assign Unicode values to char - c++

Question

My code includes a loop in which checks every character of a std::string, assign it to a char variable, which fails its assertion when -1 >= c >= 255.
It is a method from a JSON parser class that it's not mine:
static std::string UnescapeJSONString(const std::string& str)
{
std::string s = "";
for (int i = 0; i < str.length(); i++)
{
char c = str[i]; // << HERE FAILS WHEN 'É' CHARACTER
if ((c == '\\') && (i + 1 < str.length()))
{
int skip_ahead = 1;
unsigned int hex;
std::string hex_str;
switch (str[i+1])
{
case '"' : s.push_back('\"'); break;
case '\\': s.push_back('\\'); break;
case '/' : s.push_back('/'); break;
case 't' : s.push_back('\t'); break;
case 'n' : s.push_back('\n'); break;
case 'r' : s.push_back('\r'); break;
case 'b' : s.push_back('\b'); break;
case 'f' : s.push_back('\f'); break;
case 'u' : skip_ahead = 5;
hex_str = str.substr(i + 4, 2);
hex = (unsigned int)std::strtoul(hex_str.c_str(), nullptr, 16);
s.push_back((char)hex);
break;
default: break;
}
i += skip_ahead;
}
else
s.push_back(c);
}
return Trim(s);
}
How can I assign a Unicode value to a char? In this case the value is É, and the code is not ready to receive such a characters.
This is included into a dll library, and is giving me this error:

Answer 1

The std::string doesn't use Unicode. This is evident because there is a method c_str that lets you get a char array from the std::string.
Answering your question, your test is wrong:
-1 >= c && c >= 255
It should be:
-1 <= c && c <= 255
But you can't get c anywhere near 255 since the char is signed.
If you want to get 255 out a char it needs to be
unsigned char
This will not let you reach -1 though.
Read about char*'s here:
http://www.cplusplus.com/doc/tutorial/variables/
see char arrays here:
http://www.cplusplus.com/doc/tutorial/ntcs/
see std::string here:
http://www.cplusplus.com/reference/string/string/