How does copy-on-modify operate on structs and classes?

Question

Ian 2019-4-29

2
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https://ww2.mathworks.cn/matlabcentral/answers/459251-how-does-copy-on-modify-operate-on-structs-and-classes

评论： Ian 2019-4-30

I understand that Matlab uses "copy-on-modify" so matrices, structs and class objects are only copied inside a function when the item passed in is modified.

Question: When matlab modifies a member of a struct or class in a function, is the entire struct or class object copied, or only the member which was modified?

Why is this important? Because if a struct or class has some very large matrices embedded, then a function which modifies a small part of the entire object would either copy/modify only the small part it works with (good!), or it might duplicate the entire object, even if it only works with a small piece (bad!).

A somewhat trivial example:

s.owner="charlie";
s.data=rand(10000,10000);
s=change_owner(s,"charlene");
. . .
function t = change_owner(t,new_owner);
    t.owner = new_owner;
end

If change_owner(...) sees the entire object as modified, then it has to duplicate the nearly 1 GB data matrix, in order to change the owner field, which makes this function very inefficient. But if matlab recognizes that only the "owner" member is modified, then it can avoid duplicating the data matrix, and thus operate efficiently on the struct.

The same question obviously would apply to a class object as to a struct...

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Answer 1

James Tursa 2019-4-29

3
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https://ww2.mathworks.cn/matlabcentral/answers/459251-how-does-copy-on-modify-operate-on-structs-and-classes#answer_372873

编辑：James Tursa 2019-4-29

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Question: When matlab modifies a member of a struct or class in a function, is the entire struct or class object copied, or only the member which was modified?

Only the member being modified is deep copied. The other members are typically reference copied only. E.g., in the following you can see that the data pointer value pr did not change ... i.e. the memory of the data field was not deep copied.

>> s.owner = 'charlie';
>> s.data = rand(1,5);
>> format debug
>> s.data
ans =
Structure address = 54747f90
m = 1
n = 5
pr = 11c661dc0
pi = 0
    0.8147    0.9058    0.1270    0.9134    0.6324
>> s = change_owner(s,'charlene')
s = 
    owner: 'charlene'
     data: [0.8147 0.9058 0.1270 0.9134 0.6324]
>> s.data
ans =
Structure address = 54747f90
m = 1
n = 5
pr = 11c661dc0
pi = 0
    0.8147    0.9058    0.1270    0.9134    0.6324