Why is this function returning the ASCII value if there is no match in switch cases?

Question

int foo(char c)
{
  switch(c)
  {
    case '1':
     return(1);
    case '2':
    return(2);
  } 
}

int main()
{
cout << foo('a');
}

This program prints 97 .

** It prints the ASCII value as output if none of switch case matches. Why is the function returning the ASCII value if none of the cases match**

And what do you return from `foo` if none of the cases in the `switch` is matched? Not returning anything from a function declared to do that leads to *undefined behavior*, which makes the whole program *ill-formed* and invalid. — Some programmer dude, Jul 27 '17 at 08:55
Please enable compiler warnings. *warning C4715: 'foo': not all control paths return a value* — Weather Vane, Jul 27 '17 at 09:01
@SouravGhosh why? it's a one of the conventions (one I don't like, but still) — CIsForCookies, Jul 27 '17 at 09:04
@CIsForCookies Well, just because we can write `[5]array` and it's valid every bit, we don't use that, right? Similar here. Also, can you cite the source of the convention you mentioned, I'd like to see that. — Sourav Ghosh, Jul 27 '17 at 09:06
@SouravGhosh [5]array is confusing. return (1) is very simple to understand... I hope you don't think that using parentheses when not necessary is bad (for example: _sizeof 1.1_ vs. _sizeof(1.1)_ ) — CIsForCookies, Jul 27 '17 at 09:10
@CIsForCookies AFAIK there is a problem optimizing the return. But I don't remeber what exactly it was. Maybe RVO or something like that. — , Jul 27 '17 at 09:16
@CIsForCookies [a good read](https://stackoverflow.com/q/161879/2173917). — Sourav Ghosh, Jul 27 '17 at 09:18
@manni66 I looked around and found [this](https://softwareengineering.stackexchange.com/questions/238786/does-c-compiler-remove-optimize-useless-parentheses) — CIsForCookies, Jul 27 '17 at 11:22
@SouravGhosh I also saw that :). I guess there's no accounting for taste, so both are fine — CIsForCookies, Jul 27 '17 at 11:24

score 2 · Answer 1 · answered Jul 27 '17 at 08:58

2

Your program behaviour is undefined: you must have an explicit return value on any function returning an int (other than main).

(What you appear to be observing is a corruption of the stack; 97 is the ASCII value for the lower case letter a that you've passed as the function parameter).

answered Jul 27 '17 at 08:58

Bathsheba

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Iam running in DEV c++ ,it runs without any error and prints the output – Kishore Jul 27 '17 at 09:00
1

@Kishore: Absolutely: that's an acceptable manifestation of *undefined behaviour*. – Bathsheba Jul 27 '17 at 09:00
Now i got that output is due to corruption of stack.But should it not return an error as "reached the end of a non void function" – Kishore Jul 27 '17 at 09:05
2

@Kishore the *compiler* tells you that. C itself has no inherent error checking. – Weather Vane Jul 27 '17 at 09:07
@WeatherVane Now I have enabled the compiler warning . Thanks a lot – Kishore Jul 27 '17 at 09:15

Ihdina · Answer 2 · 2017-08-05T14:27:23.107

int foo(char c) {
    switch(c) {
        case '1':
          return(1);
        case '2':
          return(2);

        //code to execute if 'c' does not equal the value following any of the cases
        default:
          return(-1);
    } 
}

int main() {
    cout << foo('a');
}

OR

// simple char to int without switch
int foo(char c) {

   /*
   // reject unwanted char 
   if (c=='a') {
       return -1;
   }
   */

   // convert char to int
   return (int)c;
}

CIsForCookies · Answer 3 · 2017-07-27T09:05:51.043

What you have here is UB. The foo function gets an input it doesn't handle, since none of your cases support input 'a'. What's probably happening under the hood (if I may try to explain this UB in your specific case) is that foo returns the input it gets casted to int, meaning cout << (int)('a');

Your switch case should contain a default for such cases, for example:

int foo(char c)
{
  switch(c)
  {
    case '1':
      return(1);
    case '2':
      return(2);
    default:
      return (-1); // indicates error!!
  } 
}

int main()
{
int tmp = foo('a');
if (tmp != -1)
    cout << tmp;
else
    cout << "bad input";
}

Why is this function returning the ASCII value if there is no match in switch cases?

3 Answers3