Re: Multiple operator overloading

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Mon, 8 Nov 2010 04:41:04 -0800 (PST)
Message-ID:
<4ec82cb4-1e6b-4fe8-a523-1db1f681f975@r14g2000yqa.googlegroups.com>
On Nov 7, 4:21 pm, Andrea Crotti <andrea.crott...@gmail.com> wrote:

James Kanze <james.ka...@gmail.com> writes:

On Nov 7, 1:18 pm, Andrea Crotti <andrea.crott...@gmail.com> wrote:

Saeed Amrollahi <amrollahi.sa...@gmail.com> writes:

You can't overload two functions that are different just in return
type. It is
a general concept in "Function-name overloading":
  void f(int);
  int f(int); // error: only they are different in return types
Same applies to overloaded operators.


Ah true now I got it...
It would have been too easy of course, other ways to get the same target?


The classical solution is to have the function or operator
return a proxy with overloaded conversion operators, e.g.:

    class Toto
    {
    public:
        class Proxy
        {
            Toto const* myOwner;
            int myIndex;
        public:
            Proxy(Toto const* owner, int index)
                : myOwner(owner)
                , myIndex(index)
            {
            }
            operator int() const { myOwner-

getInt(myIndex); }

            operator std::string() const { myOwner-

getString(myIndex); }

        };
        Proxy operator[](int index) const
        {
            return Proxy(this, index);
        }
        // ...
    };


Last thing, now I rewrote it for my situation:

class Attribute
{
public:
    class Proxy
    {
        Attribute const *owner;
        string value;
    public:
        Proxy(Attribute const* _owner, string& _value)
            : owner(_owner), value(_value) {}

        operator int() const {
            owner->getInt(value);
        }
        operator std::string() const {
            owner->getString(value);
        }
    };

    Proxy operator[](string& index) const {
        return Proxy(this, index);
    }

    int getInt(const string& value) const {
        return atoi(value.c_str());
    }
    string getString(const string& value) const { return value; }
};

But how do I use it?


Like any other operator[]. Except that an additional user
defined conversion is involved (which can affect overload
resolution), it works like any other operator[].

Where am I supposed to give in the values I want

I thought something like this below but it's not correct, I have to call
the overloaded int which is in the class Proxy, not Attribute.

    Attribute x;
    x["new"] = int(10);


If you also want to use it for assignment, you have to add
assignment operators to the proxy, e.g.:

    class Attribute
    {
        // ...
        class Proxy
        {
            // ...
            Proxy const& operator=(int newValue) const
            {
                owner->setInt(newValue);
                return *this;
            }
            Proxy const& operator=(std::string const& newValue) const
            {
                owner->setString(newValue);
                return *this;
            }

    ...

(Note the use of an operator= which is const. That's a hallmark
of a proxy.)

--
James Kanze

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