This is called "shared data copy", or "copy-on-write", or "lazy copy" in various contexts. I will be producing a guide to all of this and uploading to the FEX "soon", but the short of it is:
A MATLAB variable is:
- A structure that contains information such as class, dimensions, and data pointers
- Data memory
When you do things that do not change the class or the data arrangement in memory, MATLAB will often produce what is called a "shared data copy" result. Meaning that a new information structure is created for the new variable, but the data pointers are exactly the same as the original variable (i.e., both variables point to the same data memory). An internal linked list keeps track of all variables that are sharing the same data memory (there can be any number of variables sharing the same data memory). If you alter the data or the arrangement of the data for one of the shared data copies, MATLAB will then copy the data to a new location in memory, remove it from the linked list, and then alter that. E.g., the following will produce a shared data copy result of X (for this discussion assume X is full):
format debug
X = [1 2 3 4 5 6];
Y = X;
Y = X(:) ;
Y = reshape(X,2,3);
Y = X.' ;
etc.
When you look at the data pointer pr you can see that they are all the same.
Unfortunately, MATLAB provides no official mex API functions to detect when a passed in variable is shared, and in addition there are multiple ways a variable can be shared (a shared data copy, a shared reference copy, a shared parent copy, a handle classdef property, etc) which I will not go into here. You will have to wait for me to upload my mex guide to the FEX in the next few weeks to learn all of these grisly details.
MATLAB passes shared data copies of all arguments to m-code and p-code functions, but it passes the original variable address to mex functions (except in the case of cell or struct references which are regarded as expressions and resolved as shared data copies of the original even when used as a mex function argument).
