Resolve Error: Passing by Reference Not Supported for Some Properties

Issue

When you generate C and C++ code from MATLAB^® code, you can use the functions coder.ref, coder.wref, and coder.rref with coder.ceval to pass values by reference to an external C or C++ function. However, code generation does not support passing by reference for:

• A property with a get method or a set method.

• A property that has property validation.

• A System object™ property with an attribute, such as Logical or PositiveInteger, that constrains or modifies the property value.

When you pass one these properties by reference, code generation fails with this error:

Code generation does not support passing property 'myprop' to an external function by reference because this property has an associated set method, get method, property validation, or, for System object properties, an attribute that constrains or modifies the property value.

Possible Solutions

To resolve this error, save the property value in a temporary variable. Then, pass the temporary variable by reference to the external function. After the external function call, assign the temporary variable to the property.

For example, consider the C function addone, which is declared in the file addone.h and defined in the file addone.c.

 addone.c

#include <stdio.h>
#include <stdlib.h>
#include "addone.h"

void addone(double *p)
{
    *p = *p + 1;
}

 addone.h

void addone(double *p);

The MATLAB function useMySimpleClass_error class calls the C function addone by using coder.ceval and passes the input argument by reference to addone by using coder.ref. The input argument to addone is a property of the MATLAB class MySimpleClass. Code generation fails for useMySimpleClass_error because the property prop of the class MySimpleClass uses the validation function mustBePositive.

function out = useMySimpleClass_error(x) %#codegen
myObj = MySimpleClass;
myObj.prop = x;
if ~coder.target("MATLAB")
    coder.updateBuildInfo("addSourceFiles","addone.c");
    coder.ceval("-headerfile","addone.h","addone",coder.ref(myObj.prop));
end
out = myObj.prop;
end

classdef MySimpleClass
    properties
      prop {mustBePositive}
   end
end

To resolve this error, assign myObj.prop to a temporary variable and use the temporary variable in the coder.ceval call. After the coder.ceval call, assign the temporary variable to myObj.prop. For example, code generation for useMySimpleClass succeeds.

function out = useMySimpleClass(x) %#codegen
myObj = MySimpleClass;
myObj.prop = x;
tempVar = myObj.prop;
if ~coder.target("MATLAB")
    coder.updateBuildInfo("addSourceFiles","addone.c");
    coder.ceval("-headerfile","addone.h","addone",coder.ref(tempVar));
end
myObj.prop = tempVar;
out = myObj.prop;
end