Exceptions - Boost.Outcome documentation

# Exceptions

Exceptions are the default mechanism in C++ for reporting, propagating and processing the information about function failures. Their main advantage is the ability to describe the “success dependency” between functions: if you want to say that calling function `g()` depends on the successful execution of function `f()`, you just put `g()` below `f()` and that’s it:

    int a()
    {
      f();
      g();        // don't call g() and further if f() fails
      return h(); // don't call h() if g() fails
    }

In the C++ Standard terms this means that `f()` is sequenced before `g()`. This makes failure handling extremely easy: in a lot of cases you do not have to do anything.

Also, while next operations are being canceled, the exception object containing the information about the initial failure is kept on the side. When at some point the cancellation cascade is stopped by an exception handler, the exception object can be inspected. It can contain arbitrarily big amount of data about the failure reason, including the entire call stack.

### Downsides

There are two kinds of overheads caused by the exception handling mechanism. The first is connected with storing the exceptions on the side. Stack unwinding works independently in each thread of execution; each thread can be potentially handling a number of exceptions (even though only one exception can be active in one thread). This requires being prepared for storing an arbitrary number of exceptions of arbitrary types per thread. Additional things like jump tables also need to be stored in the program binaries.

Second overhead is experienced when throwing an exception and trying to find the handler. Since nearly any function can throw an exception of any time, this is a dynamic memory allocation. The type of an exception is erased and a run-time type identification (RTTI) is required to asses the type of the active exception object. The worst case time required for matching exceptions against handlers cannot be easily predicted and therefore exceptions are not suitable for real-time or low-latency systems.

Another problem connected with exceptions is that while they are good for program flows with linear “success dependency”, they become inconvenient in situations where this success dependency does not occur. One such notable example is releasing acquired resources which needs to be performed even if previous operations have failed. Another example is when some function `c()` depends on the success of at least one of two functions `a()` and `b()` (which try, for instance, to store user data by two different means), another example is when implementing a strong exception safety guarantee we may need to apply some fallback actions when previous operations have failed. When failures are reported by exceptions, the semantics of canceling all subsequent operations is a hindrance rather than help; these situations require special and non-trivial idioms to be employed.

For these reasons in some projects using exceptions is forbidden. Compilers offer switches to disable exceptions altogether (they refuse to compile a `throw`, and turn already compiled `throw`s into calls to `std::abort()`).

Last revised: March 22, 2019 at 13:58:05 -0700