What is the meaning of a C++ Wrapper Class?

Question

I have a little trouble in understanding a wrapper class. It would be great if some one could help providing apt examples.

1. What is a C++ Wrapper Class and what are the circumstances of writing it ?
2. What is it's use any way ?

Thanks.

Answer 1

A "wrapper class" is a de facto term meaning a class that "wraps around" a resource; i.e, that manages the resource. When people write a wrapper, then, they are doing something like this:

class int_ptr_wrapper
{
public:
    int_ptr_wrapper(int value = 0) :
    mInt(new int(value))
    {}

    // note! needs copy-constructor and copy-assignment operator!

    ~int_ptr_wrapper()
    {
        delete mInt;
    }

private:
    int* mInt;
};

This class manages ("wraps") a pointer to an int. All resources should be wrapped in some fashion, for cleanliness (no explicit clean up code or noise) and correctness (destructor is guaranteed to run; cannot forget to clean up, and safe with exceptions).

This pattern is called Scoped-bound Resource Management (SBRM), though a far more common (but most esoteric) name is Resource-Acquisition is Initialization (RAII). The idea is to bind a resource's clean-up to a destructor, for the reasons given above: the scope handles the rest.

Note that I said it was missing a copy-constructor and copy-assignment operator. This is due to the Rule of Three. (See linked question for detailed explanation.) The simplest way to correctly implement this rule is with the copy-and-swap idiom, explained here.

---

Sometimes, it's not pragmatic to write wrapper class for resource clean-up, usually when the resource is unique or used once. (Or with transactional programming.) The solution to this is called scope guard, a way of writing clean-up code inside the function that needs it.

You may find more information by searching for it in your favorite search provider (that is, Google), or going to the "primary" document here. Note that Boost provides a utility for this, as it usually does for good idioms.