Re: how do you write proxies for rvalues?

From:
=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Tue, 17 Aug 2010 11:37:12 CST
Message-ID:
<afda88cd-7b69-4026-b9a1-1fb68641344d@s9g2000yqd.googlegroups.com>
On 17 Aug., 09:56, Gene Bushuyev <publicfil...@gbresearch.com> wrote:

As an exercise to familiarize myself with rvalues I decided to write a
proxy class for plus operator. It turned out to be more difficult than
I thought.
Here is what I got (tell me if I did it wrong):


In the following I assume that at least the headers <string>,
<iostream>, <utility>, and <type_traits> are included.

    template<class T1, class T2>
    class PlusProxy
    {
        T1 t1;
        T2 t2;
    public:
        PlusProxy(T1&& t1, T2&& t2)
            : t1(std::forward<T1>(t1)),
            t2(std::forward<T1>(t2))
        {}

        PlusProxy(PlusProxy&& other)
            : t1(std::forward<T1>(other.t1)),
            t2(std::forward<T1>(other.t2))
        {}

        PlusProxy&& operator = (PlusProxy&& other)
        {
            swap(std::forward<T1>(*this), other);
        }

        friend
        void swap(PlusProxy<T1,T2>&& p1, PlusProxy<T1,T2>&& p2)
        {
            swap(std::forward<T1>(p1.t1), std::forward<T1>(p2.t1));
            swap(std::forward<T1>(p1.t2), std::forward<T1>(p2.t2));
        }


Note that above operator= implementation will never consider
your above free swap function. This is so, because operator=
provides lvalues to swap, but your swap acts only on rvalues.
Either replace your above free swap by

friend void swap(PlusProxy<T1,T2>& p1, PlusProxy<T1,T2>& p2);

or add such an overload to enable proper swapping.

        decltype(typename std::remove_reference<T1>::type() +
            typename std::remove_reference<T2>::type())
        operator ()() const { return t1 + t2; }

        //decltype(typename std::remove_reference<T1>::type()[0] +
        // typename std::remove_reference<T2>::type()[0])
        //operator [](std::size_t i) const { return t1[i] + t2[i]; }

    };

    template<class T1, class T2>
    PlusProxy<T1, T2> MakePlus (T1&& t1, T2&& t2)
    {
        return PlusProxy<T1,T2>(std::forward<T1>(t1),
            std::forward<T2>(t2));
    }

    int main()
    {
        int a = 1, b = 2;
        std::cout << "int 1: " << MakePlus(a, b)() << std::endl;
        std::cout << "int 2: " << MakePlus(a, 1)() << std::endl;
        std::cout << "int 3: " << MakePlus(5, 2)() << std::endl;

        std::string s1("-string1-");
        std::string s2("-string2-");
        std::cout << "str 1: " << MakePlus(s1, s2)() << std::endl;
        std::cout << "str 2: "
            << MakePlus(s1, std::string("hello"))() << std::endl;
        //std::cout << "str 3: "
        // << MakePlus(s1, "world")() << std::endl;
        //std::cout << "str 4: "
        // << MakePlus("hello", "world")() << std::endl;
    }

I compiled this example with VC2010 and gcc 4.5 and got the same
results.
The first problem I encountered was with operator[] in this example.
It turns out if I try to instantiate PlusProxy on int type, compiler
would complain about decltype(...) for operator[]. Why? There is no
instantiation of operator[], why doesn't compiler defer this check to
the point of instantiation as it would be the case if I used return
type without decltype.


If a specialization of PlusProxy is instantiated, this will cause the
instantiation of all member *declarations*. But your last two member
declaration of operator ()() and operator[] can be ill-formed
depending
on the type.

To solve this problem, you need to make them constrained members.
This is possible in C++0x by taking advantage of default template
arguments in function templates. E.g. change

decltype(typename std::remove_reference<T1>::type()[0] +
    typename std::remove_reference<T2>::type()[0])
operator [](std::size_t i) const { return t1[i] + t2[i]; }

to

template<class U = T1, class V = T2>
decltype(typename std::remove_reference<U>::type()[0] +
    typename std::remove_reference<V>::type()[0])
operator [](std::size_t i) const { return t1[i] + t2[i]; }

and similarly for your operator() overload.

The second problem was with instantiation on string types. The
expression MakePlus(s1, "world") wouldn't compile because
remove_reference did something to the string literal in decltype
expression, so I'm getting this error:
conversion from 'int' to non-scalar type 'const char [6]' requested.
Is there anything I can do to handle both string and non-string
literals in the same way?


Using std::remove_reference is a bad idea. In fact, you use it
as a work-around for a proper solution, which should use
std::declval. Given this tool, the recommended declarations
of both members could be

template<class U = T1, class V = T2>
decltype(std::declval<U>() + std::declval<V>())
operator ()() const { return t1 + t2; }

template<class U = T1, class V = T2>
decltype(std::declval<U>()[std::declval<std::size_t>()] +
std::declval<V>()[std::declval<std::size_t>()])
operator [](std::size_t i) const { return t1[i] + t2[i]; }

HTH & Greetings from Bremen,

Daniel Kr?gler

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