CRC.Press A Guide to MATLAB Object Oriented Programming May.2007 Episode 1 Part 7 potx

94 A Guide to MATLAB Object-Oriented Programming

6.5 INDEPENDENT INVESTIGATIONS

1. In the ‘-full’ case, use class to inspect and assign the field’s type. What are you

going to do for object arrays?

2. Modify display.m to take advantage of fieldnames.

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7 struct.m

In the previous chapter, we patched a hole in MATLAB’s default encapsulation by tailoring

fieldnames. In this chapter, we patch another hole by tailoring struct. As we have already

seen, the built-in version of struct returns the names and values of private member variables.

In fact, struct’s default behavior represents a risky situation because clients can use it to gain

access to private data. We need to eliminate this possibility by developing a type-specific version

of struct.m.

As a bonus, a standard function that returns an object’s public structure is broadly useful. For

example, look back at the scalar case of the tailored version of display. The strategy of allowing

MATLAB to perform most of the formatting requires a structure of public variables. At that time,

public structure generation was coded directly in line and we could not easily make use of it in

developer view. Further proliferation of in-line structure-generation code makes class extensions

very tedious and error prone. Consolidating structure generation into one function makes a lot of

sense. We can even take advantage of the tailored version of fieldnames so that even public

names are not directly coded into struct.

7.1 STRUCT

While we could easily write a specific get function for this task (e.g., getPublicStructure),

MATLAB already defines a suitable function. The built-in version of struct already returns a

structure associated with the object. The built-in version will also operate on objects. Unlike the help

for fieldnames, help struct does not promise to return a structure of “public data fields.”

The help files for struct describe a function that converts an object into its equivalent

structure. Here our idea of “equivalent structure” and MATLAB’s idea are definitely not the same.

In our world, the structure should contain public member variables; however, the built-in version

of struct returns a structure made up of the private variables. The fieldnames function was

bad enough, but the struct function is even more despicable. It returns both the names and the

values of the private variables!

While it is still true that MATLAB prevents changes to the private data, that does not prevent clients

from using the values and even passing around the structures created from the object. Here is yet another

place where MATLAB seems very lax about preventing client access to private data. We need to

reinforce this area because the potential result of such permissiveness can be devastating. If left

unchecked, clients will use this back door to destroy most of the benefits of encapsulation. Once a

client ties code to the private structure returned by the default version of struct, later attempts to

reorganize or improve the class will result in broken code and angry clients. While it might indeed be

their fault, it becomes our problem. I have personally witnessed such chaos and can tell you it is no

easy chore to clean it up. Preventing it from happening in the first place is a much better plan.

Like always, if we don’t like what the built-in version gives us, we simply tailor the function

to suit our purposes. In this particular case, tailoring is not perfect because we can’t prevent clever

or devious clients from using builtin to tie their code to an object’s private data. A client can

bypass all our carefully crafted member functions by using, for example,

shape_struct = builtin(‘struct’, shape)

When a client uses builtin in this way, shape_struct is not an object, but rather a structure.

With a structure, the client loses the ability to call member functions, but in return gains the ability

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