General-use object copy

When using Matlab objects, either a Matlab class (MCOS) or any other (e.g., Java, COM, C# etc.), it is often useful to create a copy of the original object, complete with all internal property values. This enables modification of the new copy without affecting the original object. This is not important for MCOS value-class objects, since value objects use the COW (Copy-on-Write/Update, a.k.a. Lazy Copy) and this is handled automatically by the Matlab interpreter when it detects that a change is made to the copy reference. However, it is very important for handle objects, where modifying any property of the copied object also modifies the original object.

Most OOP languages include some sort of a copy constructor, which enables programmers to duplicate a handle/reference object, internal properties included, such that it becomes entirely separate from the original object. Unfortunately, Matlab did not include such a copy constructor until R2011a (matlab.mixin.Copyable.copy()).

On Matlab R2010b and older, as well as on newer releases, we do not have a readily-available solution for handle object copy. Until now, that is.

There are several ways by which we can create such a copy function. We might call the main constructor to create a default object and then override its properties by iterating over the original object’s properties. This might work in some cases, but not if there is no default constructor for the object, or if there are side-effects to object property modifications. If we wanted to implement a deep (rather than shallow) copy, we’d need to recursively iterate over all the properties of the internal objects as well.

A simpler solution might be to save the object to a temporary file (tempname, then load from that file (which creates a copy), and finally delete the temp file. This is nice and clean, but the extra I/O could be relatively slow compared to in-memory processing.

Which leads us to today’s chosen solution, where we use Matlab’s builtin functions getByteStreamFromArray and getArrayFromByteStream, which I discussed last year as a way to easily serialize and deserialize Matlab data of any type. Specifically, getArrayFromByteStream has the side-effect of creating a duplicate of the serialized data, which is perfect for our needs here (note that these pair of function are only available on R2010b or newer; on R2010a or older we can still serialize via a temp file):

% Copy function - replacement for matlab.mixin.Copyable.copy() to create object copies
function newObj = copy(obj)
    try
        % R2010b or newer - directly in memory (faster)
        objByteArray = getByteStreamFromArray(obj);
        newObj = getArrayFromByteStream(objByteArray);
    catch
        % R2010a or earlier - serialize via temp file (slower)
        fname = [tempname '.mat'];
        save(fname, 'obj');
        newObj = load(fname);
        newObj = newObj.obj;
        delete(fname);
    end
end

This function can be placed anywhere on the Matlab path and will work on all recent Matlab releases (including R2010b and older), any type of Matlab data (including value or handle objects, UDD objects, structs, arrays etc.), as well as external objects (Java, C#, COM). In short, it works on anything that can be assigned to a Matlab variable:

obj1 = ... % anything really!
 
obj2 = obj1.copy();  % alternative #1
obj2 = copy(obj2);   % alternative #2

Alternative #1 may look “nicer” to a computer scientist, but alternative #2 is preferable because it also handles the case of non-object data (e.g., [] or ‘abc’ or magic(5) or a struct or cell array), whereas alternative #1 would error in such cases.

In any case, using either alternatives, we no longer need to worry about inheriting our MCOS class from matlab.mixin.Copyable, or backward compatibility with R2010b and older (I may possibly be bashed for this statement, but in my eyes future compatibility is less important than backward compatibility). This is not such a wild edge-case. In fact, I came across the idea for this post last week, when I developed an MCOS project for a consulting client that uses both R2010a and R2012a, and the same code needed to run on both Matlab releases.

Using the serialization functions also solves the case of creating copies for Java/C#/COM objects, which currently have no other solution, except if these objects happen to contain their own copy method.

In summary, using Matlab’s undocumented builtin serialization functions enables easy implementation of a very efficient (in-memory) copy constructor, which is expected to work across all Matlab types and many releases, without requiring any changes to existing code – just placing the above copy function on the Matlab path. This is expected to continue working properly until Matlab decides to remove the serialization functions (which should hopefully never happen, as they are so useful).

Sometimes, the best solutions lie not in sophisticated new features (e.g., matlab.mixin.Copyable), but by using plain ol’ existing building blocks. There’s a good lesson to be learned here I think.

p.s. – I do realize that matlab.mixin.Copyable provides the nice feature of enabling users to control the copy process, including implementing deep or shallow or selective copy. If that’s your need and you have R2011a or newer then good for you, go ahead and inherit Copyable. Today’s post was meant for the regular Joe who doesn’t need this fancy feature, but does need to support R2010b, and/or a simple way to clone Java/C#/COM objects.

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