#1164 Error when chaining methods in constructor

[Renato Garcia]

Hi,

I have a wrapped c++ class Foo:

class Foo
{
public:
Foo();

~Foo();

Foo& operator()(int add);

void show();

private:
int value;
};

When using code like:

import foo
f = foo.Foo()(4)

The Foo destructor will be called in the second line, and a call:

f(5)

will result in a broke.

Note that the following code will work as expected:

import foo
f = foo.Foo()
f(5)
f(4)(13)(7)
f.show()

I have tested this code (the project is attached) with swig 2.0.3, python 2.7.1, gcc 4.6.0

Regards,

[Renato Garcia]

Project to reproduce the error

[szager]

That is some unusual syntax; I don't think there's an exact equivalent in C++. The closest would be:

Foo f = Foo()(4);

... which is different, in that two Foo objects are constructed, rather than one.

Anyways, handling that syntax correctly relies on knowing with certainty that Foo::operator() always returns itself, i.e., it always does "return *this;", and that is information which can't be deduced from the method signature. So, supporting this would probably require a feature enhancement to SWIG; something like:

%feature("returnthis") Foo::operator()(int add);

Without any enhancements to SWIG, you can work around this with a typemap:

%typemap(out) Foo& {
$result = arg1;
Py_INCREF(arg1);
}

[Olly Betts]

Testing the example from the tarball with the commands in the initial report shows this still happens (though I need to also call f.show() to make it crash:

Python 3.9.10 (main, Feb 22 2022, 13:54:07) 
[GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import foo
>>> f = foo.Foo()(4)
FOO DESTRUCTION
>>> f(5)
<foo.Foo; proxy of <Swig Object of type 'Foo *' at 0x7f7584026d80> >
>>> f.show()
malloc(): unaligned tcache chunk detected
Aborted

SWIG could perhaps in cases where the method returns a reference or pointer to the class type generate a check if it is returning a pointer to itself, though explicit annotation is probably a better answer.

The suggested workaround doesn't seem to help - I adjusted it to this to get it to even compile:

%typemap(out) Foo& {
    $result = (PyObject *)$1;
    Py_INCREF($result);
}