Can someone help explain this function

[ssoffel]

Below is a function from learncpp.com chapter 13.5, compiles and works fine. I cant understand how or why it works. My confusion comes with the switch statement case labels.

we don’t have any (? _ @) symbols in our (###) ###-#### template so how came we expect our template’s indexes will match with those symbols?

example:

case '?': // match anything
break;






[code]

bool InputMatches(string strUserInput, string strTemplate)
{
if (strTemplate.length() != strUserInput.length())
return false;

// Step through the user input to see if it matches
for (unsigned int nIndex=0; nIndex < strTemplate.length(); nIndex++)
{
switch (strTemplate[nIndex])
{
case '#': // match a digit
if (!isdigit(strUserInput[nIndex]))
return false;
break;
case '_': // match a whitespace
if (!isspace(strUserInput[nIndex]))
return false;
break;
case '@': // match a letter
if (!isalpha(strUserInput[nIndex]))
return false;
break;
case '?': // match anything
break;
default: // match the exact character
if (strUserInput[nIndex] != strTemplate[nIndex])
return false;
}
}

return true;
}

int main()
{
string strValue;

while (1)
{
cout << "Enter a phone number (###) ###-####: ";
getline(cin, strValue); // get the entire line, including spaces
if (InputMatches(strValue, "(###) ###-####"))
break;
}

cout << "You entered: " << strValue << endl;
}

/code]