Overload -> operator to forward member-access through Proxy

问题

I'm trying to wrap a Python PyObject* in an Object class. In Python, everything is a PyObject*. A list is a PyObject*, and each item in the list is itself a PyObject*. Which could even be another list. etc.

I'm trying to allow fooList[42] = barObj style syntax by means of a Proxy pattern (here).

Now that I have that working, I want to extend it so that fooList[42] can be used as an Object. Specifically I want to be able to handle...

fooList[42].myObjMethod()
fooList[42].myObjMember = ...

Object has a lot of methods, and currently fooList[42].myObjMethod() is going to first resolve fooList[42] into a Proxy instance, say tmpProxy, and then attempt tmpProxy.myObjMethod().

This means I would have to do

void Proxy::myObjMethod(){ return wrapped_ob.myObjMethod(); }

i.e. manually relay each of Object's methods through Proxy, which is ugly.

I can't see any perfect solution (see the above linked answer), but I would be happy to use:

fooList[42]->myObjMethod()

... as a compromise, seeing as -> can be overloaded (as opposed to . which cannot).

However, I can't find any documentation for overloading operator->.

My best guess is that it must return a pointer to some object (say pObj), and C++ will invoke pObj->whatever.

Below is my attempted implementation. However, I'm running into a 'taking the address of a temporary object of type Object' warning.

I have, within my Object class:

const Object operator[] (const Object& key)     const { 
    return Object{ PyObject_GetItem( p, key.p ) }; 
}

NOTE that 'const Object&' runs into 'taking the address of a temporary object of type Object' warning.

class Proxy {
private:
    const Object& container;
    const Object& key;

public:
    // at this moment we don't know whether it is 'c[k] = x' or 'x = c[k]'
    Proxy( const Object& c, const Object& k ) : container{c}, key{k}
    { }

    // Rvalue
    // e.g. cout << myList[5] hits 'const Object operator[]'
    operator Object() const {
        return container[key];
    }

    // Lvalue
    // e.g. (something = ) myList[5] = foo
    const Proxy&  operator= (const Object& rhs_ob) {
        PyObject_SetItem( container.p, key.p, rhs_ob.p );
        return *this; // allow daisy-chaining a = b = c etc, that's why we return const Object&
    }

    const Object* operator->() const { return &container[key]; }
    // ^ ERROR: taking the address of a temporary object of type Object
};

The idea is to allow myList[5]->someMemberObj = ... style syntax.

myList[5] resolves as a Proxy instance, which is wrapping an Object (the sixth element of myList). Let's call it myItem.

Now I want someProxy->fooFunc() or someProxy->fooProperty to invoke myItem.fooFunc() or myItem.fooProperty respectively.

I'm running into a 'taking the address of a temporary object of type Object' warning.

回答1:

If you can change Object, you may add

class Object {
public:
    // other code
    const Object* operator -> () const { return this; }
    Object* operator -> () { return this; }
};

And for your Proxy

Object operator->() { return container[key]; }

So, for example

myObj[42]->myFoo = ...

is mostly equivalent to

Proxy proxy = myObj[42];
Object obj = proxy.operator ->();
Object* pobj = obj.operator ->(); // so pobj = &obj;
pobj->myFoo = ...

回答2:

I find the Proxy class that you wrote as an example a bit confusing so i took the liberty to change it a little: Here is a simple example:

//object with lots of members:
class my_obj
{
public:
    std::string     m_text;

    void foo()
    {
        std::cout << m_text << std::endl;
    }
    void bar(std::string t)
    {
        m_text = t;
    }
};
//proxy object
class proxy_class
{
private:
    friend class CustomContainer;
    my_obj* px;

    proxy_class(my_obj * obj_px)
        :px(obj_px)
    {
    }
    proxy_class() = delete;
    proxy_class(const proxy_class &) = delete;
    proxy_class& operator =(const proxy_class &) = delete;
public:

    my_obj* operator ->()
    {
        return px;
    }
};
//custom container that is the only one that can return proxy objects
class CustomContainer
{
public:
    std::map<std::size_t, my_obj> stuff;

    proxy_class     operator [](const std::size_t index)
    {
        return proxy_class(&stuff[index]);
    }
};

example usage:

CustomContainer cc;
cc[0]->foo();
cc[0]->bar("hello world");
cc[0]->foo();

As a design consideration the proxy class should be create in a controlled environment so constructors are removed from preventing miss-usage.

CustomContainer has to only return proxy_class with a reference to my_obj so it can use anything, std::map, std::vector, etc

回答3:

After several hours of coaxing coliru, I have a working testcase.

Please refer to: https://codereview.stackexchange.com/questions/75237/c11-proxy-pattern-for-supporting-obidx-someobjmember-type-acc

Many thanks to Jarod, for supplying the correct syntax and understanding for -> overload.

来源：https://stackoverflow.com/questions/27689105/overload-operator-to-forward-member-access-through-proxy

标签

c++

c++11

operator-overloading

proxy-pattern