c++ – Simple username validation

Instead of the loop, we can use std::all_of() (or std::any_of() with the opposite condition).

And we have std::isalnum(), which is likely implemented as a lookup table like this (with the caveat that we need to be careful about signed char). It’s more portable, too – the code we have will be wrong if there are non-alphabetic characters between A and Z, as there are on EBCDIC systems, for example.

We could change the interface to just return the error string, with a null pointer indicating success, to avoid the “out” parameter.

#include <algorithm>
#include <cctype>
#include <string>
    
// return a null pointer if valid, else a pointer to the error message
const char *check_username_valid(const std::string& str)
{
    if (str.length() < MinUsernameLen) { return "Username Too Short"; }
    if (str.length() > MaxUsernameLen) { return "Username Too Long"; }
    auto const char_permitted
        = ()(unsigned char c){ return c == '_' || std::isalnum(c); };

    if (std::all_of(str.begin(), str.end(), char_permitted)) {
        return nullptr;
    }

    return "Invalid Character in Username";
}

If we’ll be adding more permitted username characters, we might want to use the lookup-table approach – but we don’t need a new type, and this static const can be built at compilation time:

#include <array>
#include <climits>

bool legal_username_char(unsigned char c)
{
    static auto const table
        = (){
              std::array<bool,UCHAR_MAX+1> a;
              for (unsigned char i = 0;  i++ < UCHAR_MAX; ) {
                  a(i) = std::isalnum(i);
              }
              // additional non-alnum chars allowed
              a('_') = true;
              return a;
          }();
    return table(c);
}

That idiom is an immediately-invoked lambda, and it’s very handy for creating complex constants like this. Note also the use of UCHAR_MAX rather than making assumptions about the size of the type.

Minor: no need for cast to int when indexing (a(int(((unsigned char)(x))))). An unsigned char is perfectly fine as array index.