为什么这个case语句需要一个“if else”而不仅仅是一个“if”？

Question

So I am new to C++, and I am working through a pdf tutorial that is getting me started with basic stuff. I was writing a simple case program, and I experienced something weird.

#include "pch.h"
#include <iostream>
#include <string>

using namespace std;

enum string_feeling {
    eGood,
    eBad,
    eOk,
};

string_feeling hashit(string const& feeling) {
    if (feeling == "Good" || feeling == "good" || feeling == "GOOD") {
    return eGood;
    }
    if (feeling == "Bad" || feeling == "bad" || feeling == "BAD") {
        return eBad;
    }
    if (feeling == "Ok" || feeling == "ok" || feeling == "OK") {
        return eOk;
    } 
    else cout << "";
}

int main() {
    string username;
    cout << "Hello! Please enter your first name here: \n";
    cin >> username;
    cout << "Hello, " << username << "!\n";
    cout << "How are you today? ";
    string feeling;
    cin >> feeling;
    cout << endl;

    switch (hashit(feeling)) {
    case eGood:
        cout << "That's great!";
        break;
    case eBad:
        cout << "I hope you are happy soon!";
        break;
    case eOk:
        cout << "That's good.";
        break;
    default:
        cout << "Ok.";
    }
}

Whenever I didn't have the "else" after the "if (feeling == ok)" stuff, the default case would never be called and if I entered something random it would give me the text from the eGood case. I was wondering why this is happening and since I'm learning C++ I didn't want to just brush it off not ever knowing why it worked after I put the else statement in there. So, if anyone could explain this to me that would be great. Sorry for my bad grammar.

Answer 1

Compile your program with warnings enabled like g++ -Wall -Wextra -Werror and it won't even compile, because string_feeling hashit(string const& feeling) does not return a value in all cases.

Compiling code without warnings enabled is a surefire way to waste time.

Answer 2

When none of conditions in the three if statements in hashit function become true, no return statement is executed in the function and undefined behavior is invoked.

(Quote from N3337 6.6.3 The return statement)

Flowing off the end of a function is equivalent to a return with no value; this results in undefined behavior in a value-returning function.

To avoid this, you should add one more kind to the enum

enum string_feeling {
    eGood,
    eBad,
    eOk,
    eOther // add this
};

and return it when no conditions are met.

string_feeling hashit(string const& feeling) {
    if (feeling == "Good" || feeling == "good" || feeling == "GOOD") {
    return eGood;
    }
    if (feeling == "Bad" || feeling == "bad" || feeling == "BAD") {
        return eBad;
    }
    if (feeling == "Ok" || feeling == "ok" || feeling == "OK") {
        return eOk;
    } 
    else cout << "";
    return eOther; // add this
}

Answer 3

You always have to return a value else the behavior is undefined

If you cannot modify your enum to add a case for an unknown feeling you can modify hashit to return true if feeling is valid and in that case to set the output parameter with the corresponding enum value, else to return false without setting the output parameter:

#include <iostream>
#include <string>

using namespace std;

enum string_feeling {
    eGood,
    eBad,
    eOk,
};

bool hashit(string const& feeling, string_feeling & r) {
  if (feeling == "Good" || feeling == "good" || feeling == "GOOD") {
    r = eGood;
  }
  else if (feeling == "Bad" || feeling == "bad" || feeling == "BAD") {
    r = eBad;
  }
  else if (feeling == "Ok" || feeling == "ok" || feeling == "OK") {
    r = eOk;
  } 
  else
    return false;
  
  return true;
}

int main() {
    string username;
    cout << "Hello! Please enter your first name here: \n";
    cin >> username;
    cout << "Hello, " << username << "!\n";
    cout << "How are you today? ";
    string feeling;
    cin >> feeling;
    cout << endl;
    
    string_feeling f;
    
    if (! hashit(feeling, f))
      cout << "I do not understand how you are" << endl;
    else {
      switch (f) {
      case eGood:
        cout << "That's great!" << endl;
        break;
      case eBad:
        cout << "I hope you are happy soon!" << endl;
        break;
      case eOk:
        cout << "That's good." << endl;
        break;
      }
    }
}

Compilation and execution:

pi@raspberrypi:/tmp $ g++ -Wall c.cc
pi@raspberrypi:/tmp $ ./a.out
Hello! Please enter your first name here: 
bruno
Hello, bruno!
How are you today? good

That's great!
pi@raspberrypi:/tmp $ ./a.out
Hello! Please enter your first name here: 
bruno
Hello, bruno!
How are you today? aze

I do not understand how you are
pi@raspberrypi:/tmp $ 

---

Out of that:

• to name your enum string_feeling is not very clear, whatever the feeling was input as a string, better to just name it Feeling

• it can be practical in hashit to get the string by value to change it to lowercase then to just compare it with with "good" "bad" and "ok" or to use strcasecmp on its .c_str() , allowing to also manage "gOoD" etc

Answer 4

If no if conditions will meet, hashit went to

else cout << "";

Since you didn't explicitly write the return statement, the function returns the default value 0 , which is equal to eGood .

However, the default return value is not always 0 . This is an undefined behaviour .

If you runs this code with a different compiler, you may get different results.