C or C++ Return Status

Question

What are the best practices for writing C or C++ functions that return an int that represents a status code?

Specifically, I want to know about the client usage but other tips are welcome.

For example, can I write something like this:

int foo() {
  return 0;  // because everything was cool
}

And then use it like this?

if (foo()) {
  // what to do if false, e.g. non-zero, e.g. not OK
} else {
  // what to do if true, e.g. zero, e.g. OK
}

This should work because best practices typically dictate that a status code of 0 means everything was OK and also 0 means false in a boolean statement.

However, this wouldn't be good, right:

if (!foo()) {
  // what to do if true
} else {
  // what to do if false
}

Answer 1

We use this in C where I work:

int err = foo();
if (err) {
    // armageddon
}

The assignment and if could be combined, but with more complicated function calls it gets more confusing and some people are confused by assignment in a conditional (and gcc hates it).

For C++, I would prefer exceptions if available, otherwise the above.

Edit: I would recommend returning 0 on success and anything else on error. This is what unix command line utilities do.

Answer 2

If you really want to use status codes that way, use them with an enum or block of #define statements that describe the intention of the status code.

For example:

enum
{
   kSuccess = 0,
   kFailure = -1,
}

function foo()
{
    return kSuccess;
}

if (kSuccess == foo())
{
    // Handle successful call to foo
}
else
{
    // Handle failed call to foo
}

This way, the intention is clear and there's no error-prone guesswork when someone wants to use or maintain your code in the future.

Answer 3

if (foo()) {
  // what to do if false
} else {
  // what to do if true
}

The problem with this approach is excess nesting. Suppose you have three functions you want to call:

if(foo1()) {
    if(foo2()) {
        if(foo3()) {
            // the rest of your code
        } else {
            // handle error
        }
    } else {
        // handle error
    }
} else {
    // handle error
}

To solve the excess nesting problem, invert the return value:

if(!foo1()) {
    // handle error
    return;
}

if(!foo2()) {
    // handle error
    return;
}

if(!foo3()) {
    // handle error
    return;
}

This solution suffers from another problem. It mixes the program logic with the error handling code. This complicates everything. Ideally, you want the program logic and error handling separated. This problem can be fixed with the goto

if(!foo1()) 
    goto error1;

if(!foo2())
    goto error2;

if(!foo3())
    goto error3;

return;

error1:
    // handle error
    return;
error2:
    // handle error
    return;
error3:
    // handle error
    return;

Much cleaner.

Also, the goto can solve the problem of resource deallocation. See Using goto for error handling in C by Eli Bendersky for more details.

Answer 4

The return statuses should be defined in your interface and known to the caller. Some return 0 on failure (because it's easy to check with !), some return 0 on success (because they have enum of error codes, with OK being the first item).

There's no law or standard, each interface defines its own conventions. In C++ - use exceptions.

Answer 5

Best practice is to document your code so that yourself and others can quickly look up what the return codes will be when doing error checking.

Answer 6

Just jumping on board with another option that may be appropriate in your circumstances:

enum fooret { GOOD, BAD, UGLY, WORSE };

fooret foo();  // defined elsewhere

switch(foo())
{
case BAD:
case UGLY:
   // maybe a recoverable failure(s)...
   // take appropriate actions
   break;
case WORSE:
   // maybe non-recoverable
   break;
case GOOD:
   // successful, take appropriate actions
   break;
}

Answer 7

int foo() {
   try{
    ...
   return 1
   }
   catch
   {
   return 0;  // because everything was cool
   }
}

I would start by wrapping everything in a try/catch block. Also instead of using and int it might make more scene to return a Boolean value. This is just a little more intuitive when testing in the if statement.