I have a string defined as std::string header = \"00110033\";
now I need the string to hold the byte values of the digits as if its constructed like this
Assuming you're using a "fairly normal" system where the numeric values of '0' to '9' are consecutive, you can just iterate over each element and subtract '0':
for(int i = 0; i < header.size(); ++i)
{
header[i] -= '0';
}