Either your function can complete its task successfully, in which case it satisfies whatever invariants are required of it and returns accordingly, or it can’t, in which case it can throw an exception to indicate that inability to return successfully. In the success case, the calling code can then continue based on whatever returned value and/or side effects the function was supposed to provide. In the failure case, the calling code may well fail itself if it can also no longer complete its task successfully, and so on up the stack until you reach a level in your design that knows how to recover from the failure or stop the entire program gracefully, which is where you catch the exception.
I’ve never quite understood the idea that exceptions are similar to goto. Other than transferring control over potentially long “distances” within a program, they don’t seem to have much in common at all. To me, a much closer analogy would be returning early from a function on success, if you reach a point in your algorithm where you already know the required answer to whatever question your calling code was asking. Another similar situation would be using a break or continue statement to finish an iteration early within a loop, or adding extra guard cases to stop a recursion early, again if you already know the outcome and continuing with the full algorithm has no advantage.
That is the point, but my specific objection is to the dynamic binding and dynamic typing, not the returning. With “return”, even early return, you don’t really need to reason about where you’ll end up at all—it’s always the end of the current function. And the same goes for functions that throw exceptions directly.
The problem is that when you have unchecked exceptions, any function can throw, so every function call in your code is now also a potential early return, and for correct code you need to account for that.
This means that writing state-modifying code becomes unreasonably difficult, as you can no longer just write:
// A and B must be kept in sync and updated together
A = computeA()
B = computeB()
Yes, there are ways round this involving temporary variables. So you write:
tmpA = computeA()
tmpB = computeB()
A = tmpA, B = tmpB;
Personally I don't think that exceptions work very well in the presence of mutable state. Without mutable state, they're exactly equivalent to
Exceptions with mutable state needs to be coded transactionally, yes, and this is an excellent reason to try and avoid mutable state and use persistent data structures where possible.
.. which makes using the STL surprisingly hard, because you can get an out of memory exception all over the place.
In practice, I think this issue gets far more attention than it deserves, particularly within the C++ community. If you’re running in a particularly resource-constrained environment where running out is a relevant problem, you probably already take precautions about how you allocate those resources and adjust how you structure your program accordingly. You’re trying to avoid any possibility of those errors occurring, which means you don’t then have to worry about dealing with them unexpectedly all over your code.
For me, the more interesting discussing is around how we deal with errors that can be properly recognised and handled within normal execution. That means the interesting exceptions are those we throw ourselves and those deliberately thrown by other modules that we depend on, which we hope will be documented with each module’s interface. These things don’t just go off arbitrarily in just about any line of code we write, and they can be systematically structured, and we can write code that controls where mutations and other effects happen and where exceptions happen and how they interact.
It's even harder to get right concurrency with exceptions :D
No it isn't.