How to modify a TypedArray in a structure without modifying the underlying pointer in a mex file?

Question

Simon Müller 2022-1-29

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1638825-how-to-modify-a-typedarray-in-a-structure-without-modifying-the-underlying-pointer-in-a-mex-file

回答： Karan Singh 2024-1-30

I have a mex file which receives a structure which has several fields as input and returns the same structure. One of the fields is a 2D double array.

I can read the data on the specific field, cast is to a TypedArrayRef<double> and modify it. But everytime I look at the structure address behind data (using format debug) in successive executions of the MEX file, the pointer to the array changes. Why is this?

I am following this example: https://de.mathworks.com/help/matlab/matlab_external/avoid-copies-of-large-arrays.html

Does this mean the data is being copied? I want to avoid copying the data.

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Simon Müller 2022-1-31

编辑：Walter Roberson 2022-1-31

在 MATLAB Online 中打开

Hi @James Tursa, thank you for your answer. I am using exactly the example from here: https://de.mathworks.com/help/matlab/matlab_external/avoid-copies-of-large-arrays.html

So basically I use the following code:

#include "mex.hpp"
#include "mexAdapter.hpp"
using namespace matlab::data;
using matlab::mex::ArgumentList;
class MexFunction : public matlab::mex::Function {
public:
void operator()(ArgumentList outputs, ArgumentList inputs) {
checkArguments(inputs);
TypedArray<double> largeArray = std::move(inputs[0]);
for (auto& elem : largeArray) {
    if (elem < 0) {
        elem = 0;
        }
        }
        outputs[0] = largeArray;
        }
        
        void checkArguments(ArgumentList inputs) {
        std::shared_ptr<matlab::engine::MATLABEngine> matlabPtr = getEngine();
        ArrayFactory factory;
        
        if (inputs[0].getType() != ArrayType::DOUBLE ||
            inputs[0].getType() == ArrayType::COMPLEX_DOUBLE) {
            matlabPtr->feval(u"error", 0, 
            std::vector<Array>({ factory.createScalar("Incorrect input") }));
            }
            }
            };

I compile it with the mex command and execute it as follows:

format debug

testMatrix = magic(3)

testMatrix =

Structure address = f6d0cf00

m = 3

n = 3

pr = 1193c2e60

8 1 6

3 5 7

4 9 2

Then I run the mex as follows:

testMatrix = removeNegativeNumbers(testMatrix)

testMatrix =

Structure address = f6d0bf20

m = 3

n = 3

pr = 13df6e6e0

8 1 6

3 5 7

4 9 2

So the addresses change. The title of the help suggests there is a way of avoiding copies. Or am I missunderstanding something here?

Simon Müller 2022-2-11

@James Tursa, Is this possible, or does the MATLAB API not allow something like it?

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Answer 1

Karan Singh 2024-1-30

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1638825-how-to-modify-a-typedarray-in-a-structure-without-modifying-the-underlying-pointer-in-a-mex-file#answer_1399586

在 MATLAB Online 中打开

Hi Simon,

The MATLAB Data API, which you're using in the provided C++ MEX example, is designed to be safe and to prevent accidental misuse of memory. When you pass data from MATLAB to a MEX function, MATLAB uses a copy-on-write strategy. This means that the data is not physically copied until you try to modify it. When you access the data in a read-only fashion, you are looking at the original data. However, once you modify the data, MATLAB will create a copy of the data to preserve the original data's integrity.

In the example you've provided, you're modifying the elements of the array with this loop:

for (auto& elem : largeArray) {
    if (elem < 0) {
        elem = 0;
    }
}

Hope this clarifies the doubt!

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