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

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Hakob Bazoyan 2019-8-7

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评论： Hakob Bazoyan 2019-8-9

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?

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Answer 1

Walter Roberson 2019-8-7

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在 MATLAB Online 中打开

 % 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

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Steven Lord 2019-8-9

在 MATLAB Online 中打开

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.

Steven Lord 2019-8-9

在 MATLAB Online 中打开

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 2019-8-9

Steven, Guillaume,

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

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Answer 2

Guillaume 2019-8-7

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在 MATLAB Online 中打开

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.

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