RAII state management

Question

I need to change a state. Then do stuff. Then reset the state back to what it was - e.g:

auto oldActivationOrder = mdiArea->activationOrder();
mdiArea->setActivationOrder( QMdiArea::StackingOrder );
mdiArea->cascadeSubWindows();
mdiArea->setActivationOrder( oldActivationOrder );

How do I do this in a RAII way? (c++ 11 and/or 14)

Edit: Thanks for all the answers.

There are several suggestions to create a custom class for handling the state change (BoBTFish, mindriot, Mattias Johansson). This solution seems good and clear. However I think it is a drawback that it increases the line count from 4 to 20+. If used a lot this would bloat the code. Also it seems that some locality is lost by having a separate class.

Ami Tavory suggests using std::unique_ptr. This does not have the code bloat issue and maintains locality. However, as Ami also indicates, it may not be the most readable solution.

sp2danny suggests a generalized state-change class that can be reused. This avoids code bloat provided that it can replace several custom classes. I'm going to accept this answer - but I guess the right approach really depends on the context.

Answer 1

You can do a generic template:

template< typename Obj, typename Getter, typename Setter , typename StateType >
class ScopedStateChangeType
{
public:
    ScopedStateChangeType( Obj& o, Getter g, Setter s, const StateType& state )
        : o(o), s(s)
    {
        oldstate = (o.*g)();
        (o.*s)(state);
    }
    Obj* operator -> () { return &o; }
    ~ScopedStateChangeType()
    {
        (o.*s)(oldstate);
    }

private:
    Obj& o;
    Setter s;
    StateType oldstate;
};

template< typename Obj, typename Getter, typename Setter , typename StateType >
auto MakeScopedStateChanger( Obj& o, Getter g, Setter s, StateType state )
     -> ScopedStateChangeType<Obj,Getter,Setter,StateType>
{
    return { o, g, s, state };
}

use it like:

QMdiArea mdiArea;

{
    auto ref = MakeScopedStateChanger(
        mdiArea, &QMdiArea::activationOrder, &QMdiArea::setActivationOrder,
        QMdiArea::StackingOrder );
    ref->cascadeSubWindows();
}

maybe it's worth it if you use this pattern often