examples/tutorial/dynatype.cpp

/**************************************************************************
 *
 * dynatype.cpp - Example program of map. See Class Reference Section
 *
 * $Id: //stdlib/dev/examples/stdlib/tutorial/dynatype.cpp#11 $
 *
 ***************************************************************************
 *
 * Copyright (c) 1994-2005 Quovadx,  Inc., acting through its  Rogue Wave
 * Software division. Licensed under the Apache License, Version 2.0 (the
 * "License");  you may  not use this file except  in compliance with the
 * License.    You    may   obtain   a   copy   of    the   License    at
 * http://www.apache.org/licenses/LICENSE-2.0.    Unless   required    by
 * applicable law  or agreed to  in writing,  software  distributed under
 * the License is distributed on an "AS IS" BASIS,  WITHOUT WARRANTIES OR
 * CONDITIONS OF  ANY KIND, either  express or implied.  See  the License
 * for the specific language governing permissions  and limitations under
 * the License.
 * 
 **************************************************************************/

#include <iostream>
#include <map>
#include <typeinfo>

#include <examples.h>


// ordinary class whose objects are dynamically typed at runtime
class dynatype
{
    // member template class that encapsulates a per-specialization copy
    // of an associative container (map) used to bind a specific instance
    // of dynatype to its value
    template <class T>
    struct map {
        typedef std::map<const dynatype*, T> map_type;
        
        static map_type& get () {
            static map_type m;
            return m;
        }
    };

    // helper: removes this instance of dynatype from map
    template <class T>
    void remove () {
        map<T>::get ().erase (this);
    }

    // helper: copies one instance of dynatype to another
    template <class T>
    void copy (const dynatype &rhs) {
        *this = static_cast<T>(rhs);
    }

    // pointers to the helpers (do not depend on a template parameter)
    void (dynatype::*p_remove)();
    void (dynatype::*p_copy)(const dynatype&);

public:

    dynatype ();

    // construct a dynatype object from a value of any type
    template <class T>
    dynatype (const T&);

    // copy one dynatype object to another
    dynatype (const dynatype &rhs)
        : p_remove (rhs.p_remove),
          p_copy (rhs.p_copy) {
        (this->*p_copy)(rhs);
    }

    // assign one dynatype object to another
    dynatype& operator= (const dynatype &rhs);

    // remove `this' from the map destroy the object
    ~dynatype () {
        (this->*p_remove)();
    }

    // retrieve a reference to the concrete type from an instance of
    // dynatype throws std::bad_cast if the types don't match exactly
    template <class T>
    operator T& () {
        if (map<T>::get ().end () == map<T>::get ().find (this))
            throw std::bad_cast ();
        return map<T>::get () [this];
    }

    // retrieve a value of the concrete type from an instance of
    // dynatype throws std::bad_cast if the types don't match exactly
    template <class T>
    operator T () const {
        return static_cast<T&> (*const_cast<dynatype*>(this));
    }

    // assign a value of any type to an instance of dynatype
    template <class T>
    dynatype& operator= (const T &t);
};


// 14.7.3, p6 - explicit specializations must be defined before first use
template <>
inline void dynatype::remove<dynatype::map<void> >()
{ /* no-op */ }


template <>
inline void dynatype::copy<dynatype::map<void> >(const dynatype&)
{ /* no-op */ }


template <>
inline dynatype& dynatype::operator= (const dynatype::map<void>&)
{
    // no-op
    return *this;
}


// initialize with pointers to no-ops
inline dynatype::dynatype ()
    : p_remove (&dynatype::remove<map<void> >),
      p_copy (&dynatype::copy<map<void> >)
{
}


// construct a dynatype object from a value of any type
template <class T>
inline dynatype::dynatype (const T &t)
    : p_remove (&dynatype::remove<T>),
      p_copy (&dynatype::copy<T>)
{
    *this = t;
}


// assign one dynatype object to another
inline dynatype& dynatype::operator= (const dynatype &rhs)
{
    if (this != &rhs) {
        // remove `this' from the associated map
        (this->*p_remove)();

        // copy pointers to helpers from the right-hand-side
        p_remove = rhs.p_remove;
        p_copy   = rhs.p_copy;

        // copy value of unknown type from rhs to `*this'
        (this->*p_copy)(rhs);
    }
    return *this;
}


// assign a value of any type to an instance of dynatype
template <class T>
inline dynatype& dynatype::operator= (const T &t)
{
    // remove `this' from the map of the corresponding type
    (this->*p_remove)();

    // insert `t' into the map specialized on `T'
    map<T>::get () [this] = t;

    // assign pointers to the helpers
    p_remove = &dynatype::remove<T>;
    p_copy   = &dynatype::copy<T>;

    return *this;
}


int main ()
{
    try {
        std::cout << "dynatype v1 = 1\n";

        // create an instance of dynatype an initialized it with an int
        dynatype v1 = 1;

        std::cout << "int (v1) = "
                  << int (v1) << std::endl;

        // assign a double to an instance of dynatype
        v1 = 3.14;

        std::cout << "double (v1 = 3.14) = "
                  << double (v1) << std::endl;

        // copy construct an instance of dynatype
        dynatype v2 = v1;

        std::cout << "double (v2 = v1) = "
                  << double (v2) << std::endl;

        // assign a const char* literal to an instance of dynatype
        const char* const literal = "abc";
        v2 = literal;

        std::cout << "(const char*)(v2 = \"abc\") = "
                  << (const char*)v2 << std::endl;

        // assign one instance of dynatype to another
        v1 = v2;

        std::cout << "(const char*)(v1 = v2) = "
                  << (const char*)v1 << std::endl;

        // create uninitialized (untyped) instances of dynatype
        dynatype v3, v4;

        // assign one uninitialized instance to another
        v3 = v4;

        // attempt to extract any value from an unitialized dynatype fails
        std::cout << "char (v3) = "
                  << char (v1) << std::endl;
        
    }
    catch (...) {
        std::cerr << "exception\n";
    }
}