C++ Virtual Constructor, without clone()
- by Julien L.
I want to perform "deep copies" of an STL container of pointers to polymorphic classes.
I know about the Prototype design pattern, implemented by means of the Virtual Ctor Idiom, as explained in the C++ FAQ Lite, Item 20.8.
It is simple and straightforward:
struct ABC // Abstract Base Class
{
virtual ~ABC() {}
virtual ABC * clone() = 0;
};
struct D1 : public ABC
{
virtual D1 * clone() { return new D1( *this ); } // Covariant Return Type
};
A deep copy is then:
for( i = 0; i < oldVector.size(); ++i )
newVector.push_back( oldVector[i]->clone() );
Drawbacks
As Andrei Alexandrescu states it:
The clone() implementation must follow the same pattern in all derived classes; in spite of its repetitive structure, there is no reasonable way to automate defining the clone() member function (beyond macros, that is).
Moreover, clients of ABC can possibly do something bad. (I mean, nothing prevents clients to do something bad, so, it will happen.)
Better design?
My question is: is there another way to make an abstract base class clonable without requiring derived classes to write clone-related code? (Helper class? Templates?)
Following is my context. Hopefully, it will help understanding my question.
I am designing a class hierarchy to perform operations on a class Image:
struct ImgOp
{
virtual ~ImgOp() {}
bool run( Image & ) = 0;
};
Image operations are user-defined: clients of the class hierarchy will implement their own classes derived from ImgOp:
struct CheckImageSize : public ImgOp
{
std::size_t w, h;
bool run( Image &i ) { return w==i.width() && h==i.height(); }
};
struct CheckImageResolution;
struct RotateImage;
...
Multiple operations can be performed sequentially on an image:
bool do_operations( std::vector< ImgOp* > v, Image &i )
{
std::for_each( v.begin(), v.end(),
/* bind2nd(mem_fun(&ImgOp::run), i ...) don't remember syntax */ );
}
int main( ... )
{
std::vector< ImgOp* > v;
v.push_back( new CheckImageSize );
v.push_back( new CheckImageResolution );
v.push_back( new RotateImage );
Image i;
do_operations( v, i );
}
If there are multiple images, the set can be split and shared over several threads. To ensure "thread-safety", each thread must have its own copy of all operation objects contained in v -- v becomes a prototype to be deep copied in each thread.
