« Return to Thread: Accepting None as a valid input for the constructor of a C++ class.

Re: Accepting None as a valid input for the constructor of a C++ class.

by wsfulton :: Rate this Message:

Josh Cherry wrote:

>
> On Sun, 5 Jul 2009, Bo Peng wrote:
>
>> I have a C++ class that is wrapped into a Python shadow class. In
>> addition to regular Python input (say, a number), I would like to
>> allow users to pass None from the user interface and tells the C++
>> class no invalid input is given. It is more or less like using -1 as a
>> default parameter when only positive numbers are allowed.
>>
>> However, SWIG does not seem to allow such usage because it will not
>> pass None to the C++ class even if it accepts such an input (e.g. with
>> a constructor that accepts a PyObject * object). Whenever I pass None,
>> I would get an error message such as "Invalid null reference".
>
> My understanding/recollection is that None is acceptable for any pointer
> type (and is converted to a null pointer), but is not acceptable for a
> reference (which makes sense). I'm not clear on the details of what
> you're doing, but that would appear to be the cause of that error message.
> This behavior is dictated by the standard typemaps for pointers and
> references.
>

Indeed and the typemaps showing the difference with the extra check (for
!argp) are in swigtype.swg and the preprocessed versions are shown below:

%typemap(in, noblock=1) SWIGTYPE *(void *argp = 0, int res = 0) {
res = SWIG_ConvertPtr($input, &argp,$descriptor, $disown | 0 );
if (!SWIG_IsOK(res)) {
SWIG_exception_fail(SWIG_ArgError(res), "in method '" "$symname"
"', argument " "$argnum"" of type '" "$type""'");
}
$1 = reinterpret_cast< $ltype >(argp);
}

%typemap(in, noblock=1) SWIGTYPE & (void *argp = 0, int res = 0) {
res = SWIG_ConvertPtr($input, &argp, $descriptor, 0 );
if (!SWIG_IsOK(res)) {
SWIG_exception_fail(SWIG_ArgError(res), "in method '" "$symname"
"', argument " "$argnum"" of type '" "$type""'");
}
if (!argp) { SWIG_exception_fail(SWIG_ValueError, "invalid null
reference " "in method '" "$symname" "', argument " "$argnum"" of type
'" "$type""'"); }
$1 = reinterpret_cast< $ltype >(argp);
}

William

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« Return to Thread: Accepting None as a valid input for the constructor of a C++ class.