一个any的实现

gistfile1.cpp

    class any
    {
    public: // structors

        any()
            : content(0)
        {
        }

        template<typename ValueType>
        any(const ValueType & value)
            : content(new holder<ValueType>(value))
        {
        }

        any(const any & other)
            : content(other.content ? other.content->clone() : 0)
        {
        }

        ~any()
        {
            delete content;
        }

    public: // modifiers

        any & swap(any & rhs)
        {
            std::swap(content, rhs.content);
            return *this;
        }

        template<typename ValueType>
        any & operator=(const ValueType & rhs)
        {
            any(rhs).swap(*this);
            return *this;
        }

        any & operator=(any rhs)
        {
            rhs.swap(*this);
            return *this;
        }

    public: // queries

        bool empty() const
        {
            return !content;
        }

        const std::type_info & type() const
        {
            return content ? content->type() : typeid(void);
        }

    public: // types (public so any_cast can be non-friend)

        class placeholder
        {
        public: // structors

            virtual ~placeholder()
            {
            }

        public: // queries

            virtual const std::type_info & type() const = 0;

            virtual placeholder * clone() const = 0;

        };

        template<typename ValueType>
        class holder : public placeholder
        {
        public: // structors

            holder(const ValueType & value)
                : held(value)
            {
            }

        public: // queries

            virtual const std::type_info & type() const
            {
                return typeid(ValueType);
            }

            virtual placeholder * clone() const
            {
                return new holder(held);
            }

        public: // representation

            ValueType held;

        private: // intentionally left unimplemented
            holder & operator=(const holder &);
        };

    public: // representation (public so any_cast can be non-friend)

        placeholder * content;

    };

    template<typename ValueType>  
    ValueType any_cast(const any& operand)  
    {  
        assert(operand.type() == typeid(ValueType));  
        return static_cast<any::holder<ValueType> *>(operand.content)->held;
    }