Exceptions vs. errno

Question

As a C programmer, I don't have much experience with exceptions. I'm rather used to errno as a means of communicating errors across several function calls. That having said, I don't see the distinguishing feature of exceptions, so...

What is the fundamental difference between exceptions and using errno?

Answer 1

There are so many differences here it's hard to say where to start.

First of all, errno as used in C is a global variable; this means every routine that calls an errno-setting subroutine must check errno before performing any other work, if it cares about correctness. Luckily enough, errno is threadsafe.

C++ exceptions automatically unwind up the call stack until they find a function prepared to handle the fault. This means that in most cases users don't have to explicitly check every call for an error; instead they can collect error returns in one place. C++ exceptions can contain values other than integers, unlike errno.

Answer 2

You can casually ignore errno. Exceptions must be dealt with.

Of course I've seen my share of:

try {
   // something
}
catch( ... ) {
   // nothing
}
// continue as if nothing happened

and (Java)

try {
   // something
}
catch( Throwable t ) {
   // nothing
}
// continue as if nothing happened

BUT at least that kinda jumps out at you when you're plowing through someone else's mess.