Re: [boost] RFC: type erasure

Subject: Re: [boost] RFC: type erasure
From: Fernando Pelliccioni (fpelliccioni_at_[hidden])
Date: 2011-07-20 15:11:41

On Wed, Jul 20, 2011 at 12:14 PM, Steven Watanabe <watanabesj_at_[hidden]>wrote:

> AMDG
>
> On 07/20/2011 05:49 AM, Fernando Pelliccioni wrote:
> > On Sun, May 22, 2011 at 7:18 PM, Steven Watanabe <watanabesj_at_[hidden]
> >wrote:
> >
> >> The code is available from the Vault:
> >> http://tinyurl.com/3z9jcwp
> >>
> >>
> > The link is broken.
> > I want to test your library, is there somewhere else where I can download
> > it?
> >
>
> The vault no longer exists.  All the existing
> content has been moved to github.  You can
> get the file from:
> https://github.com/boost-vault/Miscellaneous
>
>
Hi Steven,

I want to write something like this ...

void test_iostream_concept()
{
    typedef mpl::vector<
                ostreamable<>,
                destructible<>
    > requirements;

    typedef any< requirements, _self&> any_type;

    typedef std::vector<any_type> ostr_vec;
    ostr_vec vec;

    //boost iostreams
    io::stream<writer_1> writer1;
    io::stream<writer_2> writer2;
    io::stream<io::file_sink> file_writer;
    io::stream<io::null_sink> null_writer;

    std::ofstream f1("test.txt");

    vec.push_back( any_type(writer1) );
    vec.push_back( any_type(std::cout) );
    vec.push_back( any_type(f1) );

    ostr_vec::const_iterator it = vec.begin();
    ostr_vec::const_iterator end = vec.end();

    for ( ; it != end; ++it )
    {
        std::cout << *it << std::endl;
        //(*it) << "hello " << "world!";        //(1) compile-time error.
any_type do not support operator<<
    }

}

The line (1) do not compile because *any_type* do not has *operator<<.*

I could create my *custom concept*, like this...

template<class C, class T>
struct bitwise_left_shift : primitive_concept<bitwise_left_shift<C, T>,
void(C&, const T&)>
{
    static void apply( C& ostr, const T& arg )
    {
        ostr.operator<<(arg);
        //ostr << arg;
    }
};

I used in this way ... it works!!!

void test_custom_concept_bitwise_left_shift()
{
    any<bitwise_left_shift<_self, *int*>, _self&> ostr( std::cout );

    int i = 10;
    bitwise_left_shift<_self, int>()(ostr, i);

}

BUT!.... *ostr* object only works for *int's.*
*
*
How do I get *ostr* works generically?
*
*

Something like this...

void test_custom_concept_bitwise_left_shift()
{
    //pseudo-code, do not compiles

    any<bitwise_left_shift<_self, *T*>, _self&> ostr( std::cout );

    int i = 10;
    bitwise_left_shift<_self, *T*>()(ostr, i);

    float ff = 10.98;
    bitwise_left_shift<_self, *T*>()(ostr, ff);

}

How I define *T* ?

Many thanks!
Fernando Pelliccioni.