Does MATLAB call the double constructor on a=a+1?

Question

Jan Siegmund 2020-4-22

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/520006-does-matlab-call-the-double-constructor-on-a-a-1

评论： Jan Siegmund 2020-4-25

采纳的回答： Walter Roberson

在 MATLAB Online 中打开

I want to overload the double class and add some properties to it.

And i also have to override the plus function. A similar code could

look like this:

classdef myclass < double
    properties (Access = protected)
        foo
    end
    methods (Access = public)
        function obj = myclass(val,foo)
            obj     = obj@double(val);
            obj.foo = foo;
        end
        
        function out = plus(obj,in)
            out = myclass(plus@double(obj,in),obj.foo);
        end
    end
end

However, if out = in on plus, I could just obj.foo = in.foo and call plus@double(obj,in), could I?

How does MATLAB itself determine, if a plus is in fact an increment, so out = in?

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Answer 1

Walter Roberson 2020-4-22

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https://ww2.mathworks.cn/matlabcentral/answers/520006-does-matlab-call-the-double-constructor-on-a-a-1#answer_427749

在 MATLAB Online 中打开

How does MATLAB itself determine, if a plus is in fact an increment, so out = in?

Reference counting.

I do not know about class methods in this regard.

Outside of classes, in-place operations are only done inside a function of the form

function Variable = FunctionName(arguments that include Variable)

and only if the reference count for Variable is 1, and only if the outside output variable name is the same as the input variable name

In-place operations are not even done for simple code like

count = 0;
for K = 1 : 5
    count = count + 1;
end

it would have to be something like

count = 0;
for K = 1 : 5
    count = PlusPlus(count);
end
function var = PlusPlus(var)
  var = var + 1;
end

For more discussion of in-place operations, I recommend searching postings from James Tursa

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Walter Roberson 2020-4-24

Mind you, exactly what the Execution Engine optimizes to in the case where it detects built-in arithmetic types and non-overloaded operations is unknown.

Jan Siegmund 2020-4-25