Weak scope exit

ON_SCOPE_EXIT is great for pretending that C++ has a finally or defer keyword, and cleaning up things that don’t support RAII (looking at you, UObjects).
In the far future, there will be a std::scope_exit, but it’s not even close to standardization right now.

Usage

In case you’re not familiar, it’s used like so:

void ProcessAnything()
{
    Something->Enable();
    ON_SCOPE_EXIT
    {
        Something->Disable();
    };
    ProcessEnabledSomething(Something);
}   // Something gets disabled here

Its implementation is nothing more than the usual local variable with a destructor, its main benefit is that it’s already there in the engine.

In case you’re working with exceptions enabled, this is even exception-safe and will always run, even if your stack is getting unwound due to an exception.

Issues

ON_SCOPE_EXIT always running is its main strength, but also one of its greatest weaknesses.
What if you don’t want to run if this is a UObject that’s not valid anymore?

FWeakObjectPtr WeakThis(this);
ON_SCOPE_EXIT
{
    if (!WeakThis.IsValid())
        return;
    // Do the thing
};

Although this is relatively rare to encounter in synchronous game-thread-based code, it needs to be considered if you’re, e.g., multithreading, using callbacks, or coroutines.

Doing this every single time will get old quick.

Not getting this right can also result in very sneaky, hard-to-track-down bugs, since you’ll need to get it wrong at just the right moment for the GC to cause you to crash, and even that is not 100%. Usually it will just deal with data it shouldn’t be dealing with, possibly leading to silent corruption.

Implementation

With just a little tweak, you can make a weak version of ON_SCOPE_EXIT. The following code block is usable as is in a .h file.

I’m using the += operator instead of the engine’s +, because it has much lower precedence.
+ is a weird choice. If Epic wanted strong precedence, * would’ve been stronger.

#define ON_SCOPE_EXIT_WEAK auto PREPROCESSOR_JOIN(_weakScopeExit_, __LINE__) = \
                           ::WeakScopeExit::FHelper(this) += [&]
namespace WeakScopeExit
{
template<typename F>
struct TWeakScopeExit
{
    FWeakObjectPtr Weak;
    F Finally;
    explicit TWeakScopeExit(const UObject* Object, F&& Callback)
        : Weak(Object), Finally(MoveTemp(Callback)) { }
    ~TWeakScopeExit() { if (Weak.IsValid()) Finally(); }
};

struct FHelper
{
    const UObject* Object;
    explicit FHelper(const UObject* Object) : Object(Object) { }
    template<typename F>
    TWeakScopeExit<F> operator+=(F Callback)
    {
        return TWeakScopeExit(Object, MoveTemp(Callback));
    }
};
}

This will only run if this is valid, which is great to use from lambda callbacks in a member function, or directly in member coroutines.

Other variants

  • It’s trivial to modify the macro to work with any other object instead: 
    #define ON_SCOPE_EXIT_WEAK_OBJ(Obj) \
  auto PREPROCESSOR_JOIN(_weakScopeExit_, __LINE__) = \
  ::WeakScopeExit::FHelper(Obj) += [&]

  • If you wrap the entire code block in #ifndef ON_SCOPE_EXIT_WEAK/#endif, you can automatically let any future engine version “win” if this feature gets added. Of course, you can also use a custom name for the macro to prevent collisions.

  • If you are in an environment with exceptions enabled, you can easily modify this to only run if there is/isn’t currently an exception being thrown. std::uncaught_exceptions(), when called from ~TWeakScopeExit(), will return a nonzero number in case there’s currently an exception being processed. For UE4, you’ll want the deprecated std::uncaught_exception() without the s.

Updated: