Function With varargin Argument That Can Be Compiled With codegen

Hi Community,

What is the recipe to defining a function with a varargin argument that can be compiled with codegen?

Here is my example that doesn't work:

function test_functionArgument_varargin(varargin) %#codegen
  assert( all(size(varargin)<= [1 , 3])); assert(isa(varargin, 'cell' ));

  number = double(varargin{1});
  string = varargin{2};
 
 disp(number);
 disp(string);
 
end

And this is an example of the function's call output (without compiling the function):

test_functionArgument_varargin(1,"AAAA",55)

     1

AAAA

Executing

codegen test_functionArgument_varargin

produces these errors that have a quite cryptic message at least to me:

Type Function Line Description

1 test_functionArgument_varargin 4 Varargin contains zero elements; element 1 is requested.

2 test_functionArgument_varargin 5 Varargin contains zero elements; element 2 is requested.

3 test_functionArgument_varargin 7 Undef. function or variable 'number'. The first assignment to a local variable determines its class.

4 test_functionArgument_varargin 8 Undefined function or variable 'string'. The first assignment to a local variable determines its class.

5 test_functionArgument_varargin 5 varargin and varargout are not supported here.

Answer 1

[Walter Roberson]

C has no native way of expressing optional arguments. The convention that has grown up in C is to arrange all of the optional parts into a single **void pointer and bundle pointers to values in a vector, and put a null pointer at the end of the vector. The code would cast the individual *void pointers to the appropriate type for each element. It would be hypothetically possible for codegen to go through the trouble of implementing this convention, but it is a bit awkward.

C++'s native way of expressing option arguments is to define multiple functions with the same name but different function signatures, and then at compile time to select the function definition that matches the function signature that the code was actually called with. Generating such code automatically from MATLAB code that uses varargin would not be simple.

[Bob Randall]

Hi Walter,

Thank you for the answer. I wish things were a little bit more akward. I was hoping that codegen would be a sofisticaded tool able to compile most of the matlab scripts we write with just some explicit type casting required. Unfortunately, I found myself writing matlab scripts or rewriting original matlab functions that resemble to C99 code just to be able to make it work with codegen. Even when codgen produces an executable, then it needs to be completely retested to ensure that behaves the same way as the original matlab script.