boost::ptr_map instantiating virtual base class

I think I might be misusing ptr_map. We'll see. I want to avoid the visual headache of shared_ptr<base> everywhere.

Here's some scaled-down problematic code. First what I'm doing, and then what boost:tr_map is doing.

Code:

/usr/local/include/boost/ptr_container/ptr_map_adapter.hpp|213|error: cannot allocate an object of abstract type ‘drawing’

Code:

typedef unsigned int color_t;

class drawing
{
    public:
        class bad_argument {};

        drawing(int n_int_args_in, int n_color_args_in) : n_int_args(n_int_args_in), n_color_args(n_color_args_in) {}
        virtual void draw(BMP & bitmap) = 0;
        virtual drawing* create(const std::vector<int> & int_args, const std::vector<color_t> & color_args) = 0;
        virtual ~drawing() {}
    protected:
        const unsigned int n_int_args;
        const unsigned int n_color_args;
};

class circle : public drawing
{
    public:
        circle() : drawing(3,1) {}
        circle(const std::vector<int> & int_args, const std::vector<color_t> & color_args)
            : drawing(3,1)
        {
            // . . .
        }
        void draw(BMP & bitmap);
        circle* create(const std::vector<int> & int_args, const std::vector<unsigned int> & color_args)
        {
            return new circle(int_args, color_args);
        }
    protected:
        unsigned int radius;
        unsigned int centerx;
        unsigned int centery;
        color_t color;
};

In main()...

Code:

    boost::ptr_map< std::string, drawing > clone_zoo;
    boost::ptr_vector<drawing> todo_list;

    std::string circle_name = "circle";
    clone_zoo.insert(circle_name, new circle());
    std::vector<int> intargs;
    intargs.push_back(5); intargs.push_back(7); intargs.push_back(2);
    std::vector<color_t> colorargs;
    colorargs.push_back(0xFFAA33);

    todo_list.push_back( clone_zoo[circle_name].create(intargs, colorargs) );

And here is what happens in the map:

Code:

mapped_reference operator[]( const key_type& key )
        {
            return insert_lookup( key );
        }  

// . . .

     mapped_reference insert_lookup( const key_type& key )
        {
            void*& ref = this->base()[key];
            if( ref )
            {
                return *static_cast<mapped_type>(ref);
            }
            else
            {
                eraser e(&this->base(),key);      // nothrow
                mapped_type res = new T();        // strong 
                ref = res;                        // nothrow
                e.release();                      // nothrow
                return *res;
            }
          }

So I guess that the reference-ness of the ptr_map is not allowing me to avoid instantiating the base class? Or have I done something wrong?

Like std::map, it's operator[] requires that the stored type is default constructible. If you know the key is there, you might be able to use find, which doesn't (potentially) insert new items.

Code:

todo_list.push_back( clone_zoo.find(circle_name)->second.create(intargs, colorargs) );  //or something like that

Better make that a separate functions that also checks whether the key was found.

I see. shared_ptr it is.