variant.hpp

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/*
    Copyright 2005-2007 Adobe Systems Incorporated
   
    Use, modification and distribution are subject to the Boost Software License,
    Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
    http://www.boost.org/LICENSE_1_0.txt).

    See http://opensource.adobe.com/gil for most recent version including documentation.
*/

/*************************************************************************************************/

#ifndef GIL_DYNAMICIMAGE_VARIANT_HPP
#define GIL_DYNAMICIMAGE_VARIANT_HPP


#include "../../gil_config.hpp"
#include "../../utilities.hpp"
#include <cstddef>
#include <cassert>
#include <algorithm>
#include <typeinfo>
#include <boost/bind.hpp>

#include <boost/mpl/transform.hpp>
#include <boost/mpl/size.hpp>
#include <boost/mpl/sizeof.hpp>
#include <boost/mpl/max.hpp>
#include <boost/mpl/at.hpp>
#include <boost/mpl/fold.hpp>

namespace boost { namespace gil {

namespace detail { 
    template <typename Types, typename T> struct type_to_index;
    template <typename Op, typename T> struct reduce;
    struct destructor_op {
        typedef void result_type;
        template <typename T> result_type operator()(const T& t) const { t.~T(); }
    };
    template <typename T, typename Bits> void copy_construct_in_place(const T& t, Bits& bits);
    template <typename Bits> struct copy_construct_in_place_fn;
}
template <typename Types>    // models MPL Random Access Container
class variant {
    // size in bytes of the largest type in Types
    static const std::size_t MAX_SIZE  = mpl::fold<Types, mpl::size_t<0>, mpl::max<mpl::_1, mpl::sizeof_<mpl::_2> > >::type::value;
    static const std::size_t NUM_TYPES = mpl::size<Types>::value;
public:
    typedef Types                            types_t;

    typedef struct { char data[MAX_SIZE]; } base_t;    // empty space equal to the size of the largest type in Types

    // Default constructor - default construct the first type
    variant() : _index(0)    { new(&_bits) typename mpl::at_c<Types,0>::type(); }
    virtual ~variant()        { apply_operation(*this, detail::destructor_op()); }

    // Throws std::bad_cast if T is not in Types
    template <typename T> explicit variant(const T& obj){ _index=type_id<T>(); if (_index==NUM_TYPES) throw std::bad_cast(); detail::copy_construct_in_place(obj, _bits); }

    // When doSwap is true, swaps obj with the contents of the variant. obj will contain default-constructed instance after the call
    template <typename T> explicit variant(T& obj, bool do_swap);

    template <typename T> variant& operator=(const T& obj) { variant tmp(obj); swap(*this,tmp); return *this; }
    variant& operator=(const variant& v)                   { variant tmp(v  ); swap(*this,tmp); return *this; }

    variant(const variant& v) : _index(v._index)           { apply_operation(v, detail::copy_construct_in_place_fn<base_t>(_bits)); }
    template <typename T> void move_in(T& obj)             { variant tmp(obj, true); swap(*this,tmp); }

    template <typename TS> friend bool operator==(const variant<TS>& x, const variant<TS>& y);
    template <typename TS> friend bool operator!=(const variant<TS>& x, const variant<TS>& y);

    template <typename T> static bool has_type()           { return type_id<T>()!=NUM_TYPES; }

    template <typename T> const T& _dynamic_cast()   const { if (!current_type_is<T>()) throw std::bad_cast(); return *gil_reinterpret_cast_c<const T*>(&_bits); }
    template <typename T>       T& _dynamic_cast()         { if (!current_type_is<T>()) throw std::bad_cast(); return *gil_reinterpret_cast  <      T*>(&_bits); }

    template <typename T> bool current_type_is()     const { return type_id<T>()==_index; }

private:
    template <typename T> static std::size_t type_id()     { return detail::type_to_index<Types,T>::value; }

    template <typename Cs> friend void swap(variant<Cs>& x, variant<Cs>& y);
    template <typename Types2, typename UnaryOp> friend typename UnaryOp::result_type apply_operation(variant<Types2>& var, UnaryOp op);
    template <typename Types2, typename UnaryOp> friend typename UnaryOp::result_type apply_operation(const variant<Types2>& var, UnaryOp op);
    template <typename Types1, typename Types2, typename BinaryOp> friend typename BinaryOp::result_type apply_operation(const variant<Types1>& arg1, const variant<Types2>& arg2, BinaryOp op);

    base_t      _bits;
    std::size_t    _index;
};

namespace detail {

    template <typename T, typename Bits>
    void copy_construct_in_place(const T& t, Bits& bits) {
        T& b=*gil_reinterpret_cast<T*>(&bits);
        new(&b)T(t);     // default-construct
    }

    template <typename Bits>
    struct copy_construct_in_place_fn {
        typedef void result_type;
        Bits& _dst;
        copy_construct_in_place_fn(Bits& dst) : _dst(dst) {}

        template <typename T> void operator()(const T& src) const { copy_construct_in_place(src,_dst); }
    };

    template <typename Bits>
    struct equal_to_fn {
        const Bits& _dst;
        equal_to_fn(const Bits& dst) : _dst(dst) {}
        
        typedef bool result_type;
        template <typename T> result_type operator()(const T& x) const {
            return x==*gil_reinterpret_cast_c<const T*>(&_dst);
        }
    };
}

// When doSwap is true, swaps obj with the contents of the variant. obj will contain default-constructed instance after the call
template <typename Types> 
template <typename T> variant<Types>::variant(T& obj, bool do_swap) {
    _index=type_id<T>(); 
    if (_index==NUM_TYPES) throw std::bad_cast(); 

    if (do_swap) {
        new(&_bits) T();    // default construct
        swap(obj, *gil_reinterpret_cast<T*>(&_bits));
    } else 
        detail::copy_construct_in_place(const_cast<const T&>(obj), _bits);
}

template <typename Types> 
void swap(variant<Types>& x, variant<Types>& y) {
    std::swap(x._bits,y._bits); 
    std::swap(x._index, y._index);
}

template <typename Types>
inline bool operator==(const variant<Types>& x, const variant<Types>& y) {
    return x._index==y._index && apply_operation(x,detail::equal_to_fn<typename variant<Types>::base_t>(y._bits));
}

template <typename C>
inline bool operator!=(const variant<C>& x, const variant<C>& y) {
    return !(x==y);
}

} }  // namespace boost::gil

#endif