dynatype.cpp

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#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 ();
    };
 
    // 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) {
        // cast to <const T&> instead of <T> to avoid error on gcc 3.4.4/cygwin:
        // error: invalid static_cast from type `const dynatype' to type `int'
        *this = static_cast<const T&>(rhs);
    }
 
    // pointers to the helpers (do not depend on a template parameter)
    void (dynatype::*p_remove)();
    void (dynatype::*p_copy)(const dynatype&);
 
    // provides access to the mapped data value or throws bad_cast
    template <class T>
    T& retrieve (const T*) {
        if (map<T>::get ().end () == map<T>::get ().find (this))
            throw std::bad_cast ();
        return map<T>::get () [this];
    }
 
    // overload throws away const on template parameter
    template <class T>
    T& retrieve (const T*) const {
        return const_cast<dynatype*>(this)->retrieve ((T*)0);
    }
 
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& () {
        return retrieve ((T*)0);
    }
 
    // retrieve a const 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 const T& () const {
        return retrieve ((T*)0);
    }
 
    // 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 <>
void dynatype::
remove<void> ()
{ /* no-op */ }
 
 
template <>
void dynatype::
copy<void> (const dynatype&)
{ /* no-op */ }
 
 
// initialize with pointers to no-ops
dynatype::
dynatype ()
    : p_remove (&dynatype::remove<void>),
      p_copy (&dynatype::copy<void>)
{
}
 
 
template <class T>
typename dynatype::map<T>::map_type&
dynatype::map<T>::
get ()
{
    static map_type m;
 
    return m;
}
 
 
// construct a dynatype object from a value of any type
template <class T>
dynatype::
dynatype (const T &t)
    : p_remove (&dynatype::remove<T>),
      p_copy (&dynatype::copy<T>)
{
    *this = t;
}
 
 
// assign one dynatype object to another
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>
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 {
 
        // create an instance of dynatype an initialized it with an int
        dynatype v1 = 1;
 
        std::cout << "static_cast<const int&>(v1 = 1) = ";
        std::cout <<  static_cast<const int&>(v1) << '\n';
 
        // assign a double to an instance of dynatype
        v1 = 3.14;
 
        std::cout << "static_cast<const double&>(v1 = 3.14) = ";
        std::cout <<  static_cast<const double&>(v1) << '\n';
 
        // copy construct an instance of dynatype
        dynatype v2 = v1;
 
        std::cout << "static_cast<const double&>(v2 = v1) = ";
        std::cout <<  static_cast<const double&>(v2) << '\n';
 
        // assign a const char* literal to an instance of dynatype
        const char* const literal = "abc";
        v2 = literal;
 
        std::cout << "static_cast<const char* const&>(v2 = \"abc\") = ";
        std::cout <<  static_cast<const char* const&>(v2) << '\n';
 
        // assign one instance of dynatype to another
        v1 = v2;
 
        std::cout << "static_cast<const char* const&>(v1 = v2) = ";
        std::cout <<  static_cast<const char* const&>(v1) << '\n';
 
        // 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 << "static_cast<const char&>(v3) = ";
        std::cout <<  static_cast<const char&>(v3) << '\n';
        
    }
    catch (...) {
        std::cerr << "exception\n";
    }
 
    return 0;
}