How to specify the type of the argument of the function?

I have a class of a type BlockInfo containing several attributes. I need to pass an object of the BlockInfo type to the function. I cannot find a suitable syntax for this.

in blockinfo.m file

classdef BlockInfo
    %BlockInfo Summary of this class goes here
    %   Detailed explanation goes here 
    properties (Access = public)
        HueDev double
        HueAvg double
        HueMin double
        HueMax double
        SatDev double
        SatAvg double
        SatMin double
        SatMax double
        ValDev double
        ValAvg double
        ValMin double
        ValMax double
        iPosX  int32
        iPosY  int32
    end    
end

in function definition below the "Block" argument needs to be of a type "BlockInfo" above so I can access properties of the object

 % detecting the blocks with Green Screen hue and 
        function isGS = DetectGSBlocks(obj, Block)
            % Block = BlockInfo;
            if 0.39 < Block.HueAvg < 0.43
                isGS = 1
            end
        end

Pretty basic question, I could not find a simple answer for.

Could you please help?

Answer 1

[Walter Roberson]

 % detecting the blocks with Green Screen hue and 
        function isGS = DetectGSBlocks(obj, Block)
            assert(isa(Block, 'BlockInfo'))
            isGS = 0.39 < Block.HueAvg && Block.HueAvg < 0.43;
        end

[Guillaume]

I would add some text to the assert to make it clearer what the problem is:

assert(isa(Block, 'BlockInfo'), 'Block input must be an object of class BlockInfo');

Or you could use validateattributes which would construct more or less the same error message and do a bit more validation:

validateattributes(Block, {'BlockInfo'}, {'scalar'}, mfilename, 'Block');

[Hakob Bazoyan]

Walter, Guillaume, Thanks very much for quick reply. I might have not correctly formulated the question. The problem here is not the validation of the variable passed to the function after the fact.

Rather, I was getting an exeption while trying to call the function with the call like this:

varBlock = BlockInfo;
....
isGS = DetectGSBlocks(varBlock);

Exception was thrown "Undefined function 'HasGSColor' for input arguments of type 'BlockInfo'.", so Matlab thinks that the variable 'Block' in function definition is of a type double most probably, not of the type BlockInfo.

If I would be writing the definition of the function in C/C++, I would simply write

int DetectGSBlocks(BlockInfo Block);

So the compiler would explicitly know that I am expecting the type of the argument Block to be BlockInfo.

The question is - how can I do the same type of declaration in Matlab.

Thanks in Advance, Hakob

[Guillaume]

"Undefined function 'HasGSColor' for input arguments of type 'BlockInfo'."

I see no mention of double variable, so I don't know where you get that matlab thinks Block is double. In fact, matlab as correctly identified its type.

The most likely reason for this type of error is that the HasGSColor function is not on matlab path. The bit to focus on is the undefined function xxx, not the for input of type yyy.

What does

which HasGSColor -all

return?

[Hakob Bazoyan]

Actually function that calls HasGSColor and HasGSColor are both metods of the same class. Below is the more detailed code snippet

classdef GS
    properties
        hsvImage
        ...
        ...
    end
    methods (Access = public)
        function retval = HasGSColor(obj, Block)
            assert(isa(Block, 'BlockInfo'));
            if 0.39 < Block.HueAvg < 0.43
                retval = 1;
            end
        end
        function obj = runEstimation(obj, Chart, Histogram)
            Block = BlockInfo;
            ....
            if HasGSColor(Block) == 1 % Exception is thrown here
        
            end
        end
    end
end

K>> which HasGSColor -all

C:\Users\Hakob\MATLAB Drive\Green Screen\GreenScreen.m % GreenScreen method

and exact exception is:

Undefined function 'HasGSColor' for input arguments of type 'BlockInfo'.

Error in GreenScreen/runEstimation (line 61)

s = HasGSColor(bl);

[Steven Lord]

function retval = HasGSColor(obj, Block)

    
if HasGSColor(Block); % Exception is thrown here

What is Block? If it is an instance of the GS class, the HasGSColor method of the GS class will be called. Inside that method, the variable obj will contain that instance of the GS class and the Block variable won't exist.

Perhaps you meant to define HasGSColor differently:

function retval = HasGSColor(Block)

If Block is not an instance of the GS class but is an instance of another class (or is a double array) then MATLAB is correct in throwing an error. See this documentation page that talks about how MATLAB determines which class's method to invoke. You would be trying to call a GS method without passing a GS object into it. That can be done if the method is a Static method but HasGSColor isn't (and if my guess about what it's doing is correct it can't be, since it references a property of the instance of the GS class inside it. It requires a class instance on which to operate.)

Two other issues with your HasGSColor method:

1) This piece of code "0.39 < Block.HueAvg < 0.43" doesn't do what you think it does, and Code Analyzer should have warned you about that. I believe there's an auto-fix for that and has been for several releases.

2) What should that method return if Block.HueAvg is outside that range? Right now it doesn't, and MATLAB will error if that situation occurs.

[Hakob Bazoyan]

Hi Steven,

Thanks for attention and help.

Block is an object of a different class defined by classdef BlockInfo which is casing exception. Definition of the class BlockInfo is at the top of this thread - in the question itself.

For other issues - I'll deal with them later, I cannot even get inside the function, so passing an object of the class BlockInfo as an argument to the function HasGSColor of the class GS is a showstopper....

[Guillaume]

As a member of function of the GS class, the first argument of HasGSColor must be an instance of the GS class. The function must be called with either:

result = someGSObject.HasGSColor(varBlock);

or

result = HasGSColor(someGSObject, varBlock);

So, yes as Steven said, the error message that matlab issue is correct. The problem has nothing to do with the block object, it's because you haven't passed an object of class GS as the first input.

And as Steven also pointed out, a < b < c has the exact same meaning in matlab as it does in C/C++ and produce exactly the same result. It's equivalent to logical(a < b) < c

[Steven Lord]

A cat object knows how to meow. Asking a dog object to meow is probably going to get the dog object staring at you confused, since it doesn't know how to meow.

The BlockInfo class does not have a method HasGSColor. Asking it to call GS's HasGSColor method is like asking that dog to meow. If the BlockInfo class has a property that is a GS object, you could call that property's HasGSColor method. The metaphor gets a bit strained here, but if the dog object has a toy object (that contains a cat object that is a stuffed animal) the dog could ask its toy to meow.

meow(mydog.toy) % mydog.toy contains a cat object

Looking more closely, your HasGSColor method doesn't actually do anything with a GS object. That suggests to me that either it should be a Static method with one input that you could call as:

GS.HasGSColor(Block)

or it should be a method of the BlockInfo class (since that's the object on which it acts), or it should be a function (not a method.) If it's needed only inside the BlockInfo class it could be a class-related function defined inside the file but after the classdef block.

[Hakob Bazoyan]

Steven, Guillaume,

Thanks much for help, I think I know how to fix the problem.

Answer 2

[Guillaume]

If you're asking how to construct a BlockInfo object, with the current class definition, it has to be:

block = BlockInfo;
block.HueDev = ...;
block.HueAvg = ...;
block.HueMin = ...;
%etc.

You could create a constructor to the class definition to help with the creation. However, objects with so many properties are always a pain to construct. If all these properties are scalar, you may want to consider grouping them into vectors.