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Can MATLAB pass by reference?

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Doug Hull
Doug Hull on 18 Jan 2011
Commented: James Tursa on 10 Jan 2019
How does MATLAB deal with this?
  1 Comment
E65EnyOwC
E65EnyOwC on 14 Sep 2018
Check out: "How MATLAB Allocates Memory" in the Matlab help files

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Accepted Answer

Doug Hull
Doug Hull on 18 Jan 2011
If you are attempting to use pass-by-reference to modify the input argument passed into a function, the answer to the question depends on whether the input is a handle object or a value object. The two types of objects are described in the Object-Oriented Programming
http://www.mathworks.com/access/helpdesk/help/techdoc/matlab_oop/brfylq3.html and Programming Fundamentals http://www.mathworks.com/access/helpdesk/help/techdoc/matlab_prog/brtm8si.html sections of the documentation. By default, objects (including matrices, arrays, etc. of the built-in data types) are value objects.
Handle objects do exhibit reference behavior when passed as function arguments; value objects do not. When you pass a handle object to a function, MATLAB still copies the value of the argument to the parameter variable in the function (with one bit of subtlety; see below.) However, all copies of a handle object refer to the same underlying object.
If a function modifies a handle object passed as an input argument, the modification affects the object referenced by both the original and copied handles. In this case, the function does not need to return the result to be reassigned.
If instead you are attempting to use pass-by-reference to avoid unnecessary copying of data into the workspace of the function you're calling, you should be aware that MATLAB uses a system commonly called "copy-on-write" to avoid making a copy of the input argument inside the function workspace until or unless you modify the input argument. If you do not modify the input argument, MATLAB will avoid making a copy. For instance, in this code:
function y = functionOfLargeMatrix(x)
y = x(1);
MATLAB will not make a copy of the input in the workspace of functionOfLargeMatrix, as x is not being changed in that function. If on the other hand, you called this function:
function y = functionOfLargeMatrix2(x)
x(2) = 2;
y = x(1);
then x is being modified inside the workspace of functionOfLargeMatrix2, and so a copy must be made.
For more information on this behavior, read this posting http://blogs.mathworks.com/loren/2006/05/10/memory-management-for-functions-and-variables/ on Loren Shure's blog.
[From the MATLAB FAQ of Ancient Times]
  4 Comments
Richard Crozier
Richard Crozier on 18 Dec 2017
Thanks for the thorough answers to my comment!

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More Answers (4)

Marco
Marco on 18 Jul 2012
Is this also true for nested function calls? Cuz it seems like it only works for direct function calls. In my GUIDE gui I have a function (call it Fn_takesHandles) that takes in the figure handles and updates the axes, when called by callback functions directly works just as intended. However, I run into problems when the stack frame is not one-to-one. A callBack function calls a helper function called Auto which in turn calls Fn_takesHandles. In this case it does not work as intended. Any changes made to the figure handles by Fn_takesHandles is not persistent. How would you go about solving this problem?
Is there any way to declare pointers in matlab? Any help would be appreciated!!
  1 Comment
Walter Roberson
Walter Roberson on 19 Jul 2012
There is no way to declare pointers in MATLAB (at least not for calling MATLAB functions; there are ways to copy around pointers that have been created at the C / C++ level.)

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Jason Climer
Jason Climer on 20 Jun 2017
Is this determined during the JIT compilation or as needed, i.e., is the copy of x made upon the function call or when the program executes
x(2) = 2;
?
  1 Comment
Walter Roberson
Walter Roberson on 20 Jun 2017
It is generally determined when the program executes, as it is not always possible to tell ahead of time whether a value is shared or not. Especially when you consider the hidden effect of assignin() .
Also, overloading can happen at run-time: the current directory or path of an operation at the time a routine is first JIT'd is not necessarily going to be the same as during a later operation.
There are cases where MATLAB does enough analysis to establish that "update in place" can happen; I do not know enough about the mechanics of that to say how it is done taking into account overloading.

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Mandeguz
Mandeguz on 22 Jun 2017
How about when writing MEXs? Can one pass by reference within the computational routine and pull those values back to the main MATLAB code that called the MEX?
  4 Comments
James Tursa
James Tursa on 10 Jan 2019
And as of R2018a, complex variable arguments to mex routines essentially get deep data copied in both directions if you use the -R2017a compilation model.

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Edgar Sanchez
Edgar Sanchez on 25 Dec 2018
Maybe you can use global variables

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