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Pimpl + RAII + Exceptions

The acronym RAII stands for Resource Acquisition Is Instanciation. Basicly this means that resources are allocated in the constructor of a class and released in the destructor. What the Pimpl idiom is I've described earlies this day. When using RAII, constructors that may throw exceptions are unavoidable. But this isn't a problem at all when RAII is applied consequently, for when an exception is being thrown before the constructor of an object isn't fully processed, the destructors of all statically allocated members will be called automatically. How to deal with dynamically allocated objects? Simply allocate a scoped pointer like std::auto_ptr statically and reference it to the dynamically allocated object. The problem about this method in combination with Pimpl is: The compiler will throw a warning for the code line "std::auto_ptr<MyClassImpl> pImpl;" that the destructor of MyClassImpl won't be invoked since the type of MyClassImpl is unknown. Once more, the solution is quite simple:

class __BaseImpl
{
public:
    virtual ~__BaseImpl()
    {
    }
}; // class __BaseImpl

Afterwards, simply use "std::auto_ptr<__BaseImpl> pImpl;". The only problem about this is that you will have to cast pImpl everytime you are going to access the object's implementation.

The Pimpl Pattern

Today I want to blog about a design pattern that is called "pImpl",which I am using widely. If used consequently, almost all inclusions inside of headers can be replaced by forward declarations, only except those which are required for inheritance. So, compile time reduction is one great advantage of that pattern. Much more significant is the improved abstraction: Classes that are defined in header files are reduced to what they originally were meant to be, namely interfaces, whose implementation is totally irrelevant. Anyway, note that excessive appliance of the "pImpl" pattern might lead to some disadvantages. It is at it has always have been concerning patterns, it is highly useful to keep them in mind, but you should never apply them just for the sake of appliance.