CRC.Press A Guide to MATLAB Object Oriented Programming May.2007 Episode 1 Part 5 pdf

54 A Guide to MATLAB Object-Oriented Programming

Modify the constructor by replacing all occurrences of mColorRgb with mColorHsv. Also

in the constructor, set the value for mColorHsv to rgb2hsv([0 0 1])’. The modified

constructor code is shown in Code Listing 15. Line 5 replaces mColorRgb with mColorHsv

and assigns blue as the default color. Line 8 also represents an addition over the earlier constructor.

Here we increase the superiority of cShape with respect to double because the interface

definition overloads the associative operator, mtimes.

The change to subsref is almost as simple and is completely isolated inside the ColorRgb

case. The modified ColorRgb case code is shown in Code Listing 16. Line 2 uses hsv2rgb

Code Listing 14, Public Variable Names in subref’s Dot-Reference Case

1 switch index(1).type

2 case '.'

3 switch index(1).subs

4 case 'Size'

5 varargout = {this.mSize};

6 case 'ColorRgb'

7 varargout = {this.mColorRgb};

8 otherwise

9 error(['??? Reference to non-existent field ' ...

10 index(1).subs '.']);

11 end

12 case '()'

13 % code to deal with cell array of index values

14 case '{}'

15 % code to deal with cell array of index values

16 otherwise

17 error(['??? Unexpected index.type of ' index(1).type]);

18 end

Code Listing 15, Modified Constructor Using mColorHsv Instead of mColorRgb

1 function this = cShape

2 this = struct( ...

3 'mSize', ones(2,1), ... % scaled [width height]’ of

bounding box

4 'mScale', ones(2,1), ... % [width height]’ scale factor

5 'mColorHsv', rgb2hsv([0 0 1])' ... % [H S V]’ of border,

default, blue

6 );

7 this = class(this, 'cShape');

8 superiorto('double')

Code Listing 16, Converting HSV Values to RGB Values

1 case 'ColorRgb'

2 rgb = hsv2rgb([this.mColorHsv]')';

3 varargout = mat2cell(rgb, 3, ones(1, size(rgb,2)));

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Changing the Rules … in Appearance Only 55

to convert private HSV values into public RGB values. The function will convert multiple HSV

vectors by packaging each HSV 3-tuple as a row of the input matrix. Similarly, each output RGB

3-tuple is returned as a row of the output matrix. In Line 2, [this.mColorHsv] supports a

nonscalar this by concatenating HSV columns. The concatenated columns are transposed before

they are passed into hsv2rgb, and the result is transposed so that each column contains an RGB

3-tuple. Line 3 converts the combined RGB array into a cell array of 3 × 1 RGB vectors and assigns

the cell array into varargout. Now, just like all the other cases, a nonscalar this returns multiple

arguments.

To a client familiar with dot-reference and structures, dot-reference and objects looks identical.

While the outward appearance is the same, inside the private implementation we can do whatever

we want. As with Size, the public name might refer to a private member variable, but the public

name can easily refer to a data conversion or a combination of several private variables. The public

names are included in the interface specification and the client doesn’t need to know what is really

happening behind the interface.

4.1.2.6 subsref Dot-Reference, Attempt 4: Multiple Indexing Levels

If the length of the substruct index array is greater than one, index includes reference

operators beyond the initial dot-reference operator. The length is unlimited; however, certain

combinations of nested reference operators are illegal. For example, when the length of the indexed

variable is greater than one, a second dot-reference operator generates an error. That is, when a is

nonscalar, a.level_1 is allowed but a.level_1.level_2 is not. MATLAB already lives by

these rules so it would be very convenient to coerce MATLAB to handle all of these details.

Code Listing 17 shows an improved version of the dot-reference case that can handle multiple

indexing levels. This version is not as compact as before primarily due to the addition of error￾checking code. Each public name case adds a check for an empty object… If the object is empty

the function’s return value is also empty. Lines 4–5 and 10–11 are examples. Exactly how an empty

object can occur is discussed in the array-reference-operator section. When an empty object does

appear, the correct return is an empty cell. The nonempty else code is identical to the code already

discussed. Lines 20–35 implement multiple-level indexing.

Code Listing 17, An Improved Version of the subsref Dot-Reference Case

1 case '.'

2 switch index(1).subs

3 case 'Size'

4 if isempty(this)

5 varargout = {};

6 else

7 varargout = {this.mSize};

8 end

9 case 'ColorRgb'

10 if isempty(this)

11 varargout = {};

12 else

13 rgb = hsv2rgb([this.mColorHsv]')';

14 varargout = mat2cell(rgb, 3, ones(1, size(rgb,2)));

15 end

16 otherwise

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