Issues with using isalpha() C++

Question

Very much a newbie to C++ here. I'm writing a very simple program that checks to see if a character in a string containing two characters is alphabetical. If a character is alphabetical, I want to replace it with the character '_', however, there seems to be an issue with my if statement and I cannot figure it out for the life of me. I feel like the fix is simple, but cannot think of the answer. Any help would be appreciated.

#include <iostream>
#include <string>
#include <cctype>
using namespace std;

int main() {
   string passCode;

   cin >> passCode;

   if (isalpha(passCode.at(0)) == true){
      passCode.at(0) = '_';
   }
   if (isalpha(passCode.at(1)) == true){
      passCode.at(1) = '_';
   }


   cout << passCode << endl;
   return 0;
}

Answer 1

The issue is that std::isalpha does not return bool, it returns an int -- and it is only guaranteed to return a nonzero value on success.

When you compare:

if (isalpha(passCode.at(0)) == true)

You are implicitly converting true to an integral value (in this case, 1) -- which is not the result that std::isalpha is returning.

You can fix this easily by removing the == true and allowing the int value to be implicitly converted to bool instead:

if (isalpha(passCode.at(0))){
   passCode.at(0) = '_';
}
if (isalpha(passCode.at(1))){
   passCode.at(1) = '_';
}

Live Example