CODEWITHSUNDEEP
delphidelphi-xematlab-compiler
deColaman
deColaman

Reputation: 223

Delphi XE Create structured array using MCR for use in Matlab Compiled dll

I'm using Delphi XE and the Matlab 2012B compiler on Windows 7.

I'm trying to write several wrapper functions so DLL files created with the Matlab 2012b Compiler can be more easily called from Delphi XE. I found that I should use the _proxy functions when using the MCR, which indeed allowed me to call several functions successfully. I can also pass strings to Matlab without problems by passing them as PAnsiChar.

I'm currently trying to create a StructArray with some field names. As I've already successfully created numeric arrays and matrices, I'm pretty sure the first 2 parameters are OK. I expect the last one is causing the error, but I don't know how to solve this (yet). Looking at the Matlab help and example files I'm doing what should be done. Obviously I'm wrong...

I know that with Matlab r13 we had to pass the fieldnames as an array[0..n] of pAnsiChar instead of an array of pAnsiChar. I tried this here as well to no avail.

Can someone tell me if I have indeed made the correct function mapping to mxCreateStructArray(_730_proxy) and if I'm passing the parameters as expected?

type
  mxArray = pointer;

// mxArray *mxCreateStructArray(mwSize ndim, const mwSize *dims, int nfields, const char **fieldnames);
function MCRdll_CreateStructArray(aDimCount: integer; aDims: pointer; aFieldCount: integer; aFields: PPAnsiChar): mxArray; cdecl; external 'mclmcrrt8_0.dll' name 'mxCreateStructArray_730_proxy';

function MCR_CreateStructArray(aFieldNames: TArray<string>): mxArray;
var
  i: integer;
  lstDims: array of integer;
  lstNames: array of pAnsiChar;
begin
  SetLength(lstNames, Length(aFieldNames));
  for i := 0 to Length(aFieldNames) - 1 do
    lstNames[i] := ToPAnsiChar(aFieldNames[i]); //Creates a new PAnsiChar with the content of aFieldNames[i] 

  SetLength(lstDims, 2);
  lstDims[0] := 1;
  lstDims[1] := Length(aFieldNames);

  //This call raises an "External Exception" from Matlab.
  Result := MCRdll_CreateStructArray(Length(lstDims), @lstDims, Length(lstNames), @lstNames);
end;

Upvotes: 0

Views: 375

Answers (1)

David Heffernan
David Heffernan

Reputation: 613481

The MATLAB C API function is:

mxArray *mxCreateStructArray(mwSize ndim, const mwSize *dims, 
    int nfields, const char **fieldnames);

As I understand it, mwSize is by default the same as int. That translates to Integer in Delphi. The const char** parameter is the address of an array of const C strings. Translate that to Delphi and you have:

function MCRdll_CreateStructArray(ndim: Integer; dims: PInteger; 
    nFields: Integer; fieldnames: PPAnsiChar): mxArray; cdecl; 
    external 'mclmcrrt8_0.dll' name 'mxCreateStructArray_730_proxy';

Now, how to get the parameters. Well, assuming you want a vector, dims is an array of length 2, and ndim is that length. I'd declare that as a static array:

var
  dims: array [0..1] of Integer;

As for the field names, those are variable length. So you need a dynamic array of PAnsiChar. That is:

var
  fieldnames: array of PAnsiChar;

You also need to pass the vector length for your struct array to your function. That makes your function be something like this:

function MCR_CreateStructArray(len: Integer; 
  const aFieldNames: array of AnsiString): mxArray;
var
  i: integer;
  dims: array [0..1] of Integer;
  fieldnames: array of PAnsiChar;
begin
  if Length(aFieldNames)=0 then
  begin
    Result := nil;
    exit;
  end;

  dims[0] := 1;
  dims[1] := len;

  SetLength(fieldnames, Length(aFieldNames));
  for i := 0 to high(fieldnames) do
    fieldnames[i] := PAnsiChar(aFieldNames[i]);

  Result := MCRdll_CreateStructArray(Length(dims), @lstDims[0], 
    Length(fieldnames), @fieldnames[0]);
end;

An alternative to the final parameter is to pass PPAnsiChar(fieldnames). That works because a dynamic array variable is the address of the first element.

So, what was wrong with your version? The biggest mistake you made was to use untyped pointers for the two arrays that you pass to MCRdll_CreateStructArray. This means that the compiler cannot check that you got the indirection correct. And you did not.

First of all in your code you pass @lstDims to the second parameter. Now lstDims is a dynamic array in your code. The implementation of that has lstDims being a pointer to the first element. So, informally, lstDims has type ^Integer. And therefore @lstDims has type ^^Integer. That's one level of indirection too far. And you made the exact same mistake in the final parameter.

One final point. I've change the signature of the function to receive an array of AnsiString. That's the easy way for me to write the code because I don't need to worry about the UTF-16 to ANSI conversion, and can use a simple PAnsiChar cast. You'd probably benefit from this helper:

function ToAnsiStringArray(const arr: array of string): TArray<AnsiString>;
var
  i: Integer;
begin
  SetLength(Result, Length(arr));
  for i := 0 to high(Result) do
    Result[i] := AnsiString(arr[i]);
end;

I've not compiled any of this so there may be some imprecision. I trust you'll not be put off by that.

Upvotes: 1

Related Questions