#678 -copyctor bug?!

[nitro]

Hello,

Say I have this config setup:

class Base1
{
public:
Base1();
}

class Base2
{
public:
Base2();
protected:
Base2(Base1& other); // copyctor
}

class Derived : public Base1, public Base2
{
public:
Derived();
};

If I try to swig this code with -copyctor enabled, then
SWIG will generate a call to the copy constructor of
Derived. However, Derived does not have a copy
constructor, so this leads to a compilation error.

-Matthias

[Olly Betts]

I checked this with trunk r12766 and it seems it's since been fixed. I had to add ";" after "}" for the first two classes, then wrap the snippet in %inline { [...] }, saved as 1485859.i, then processed with:

./preinst-swig -copyctor -python -c++ -module tmp 1485859.i
g++ -fPIC -I /usr/include/python2.6 -Wall -W -shared -o tmp.so 1485859_wrap.cxx

[William Fulton]

The generated code from the testcase as shown does indeed compile under g++-4.4.3 for swig-1.3.31 and 2.0.4. Looks like there is a mistake in the original code. If

Base2(Base1& other); // copyctor

is replaced with

Base2(Base2& other); // non-const copyctor

then a copy constructor is generated resulting in a compiler error in the wrappers:

example_wrap.cxx:327: error: no matching function for call to ‘Derived::Derived(const Derived&)’
example_wrap.cxx:231: note: candidates are: Derived::Derived()
example_wrap.cxx:229: note: Derived::Derived(Derived&)

[Olly Betts]

Still fails with current git master (7ab24e7e865e0f1281f002fefdaba11b80416ea2), though the error now seems to be slightly different:

test_wrap.cxx: In function ‘PyObject* _wrap_new_Derived__SWIG_1(PyObject*, Py_ssize_t, PyObject**)’:
test_wrap.cxx:3283:35: error: binding reference of type ‘Derived&’ to ‘const Derived’ discards qualifiers
 3283 |   result = (Derived *)new Derived((Derived const &)*arg1);
      |                                   ^~~~~~~~~~~~~~~~~~~~~~
test_wrap.cxx:3100:7: note:   initializing argument 1 of ‘Derived::Derived(Derived&)’
 3100 | class Derived : public Base1, public Base2
      |       ^~~~~~~

[Olly Betts]

Reproduced with 8a24c19d2621e0de6a4429a039dc75d3a761acd4.

[Olly Betts]

Reproduced with 595009cf2ff952d963a663113e82b032c4fa770f.

[Olly Betts]

Reproduced with 5e1a37c6c562b7cb62c62e439c5b6b647e6d6df5 (retested because that commit fixes a copyctor bug).

[William Fulton]

This bug is a problem with non-const copy constructors in an inheritance chain. SWIG always assumed the implicit copy constructor was defined with a const reference parameter. For reference, https://en.cppreference.com/w/cpp/language/copy_constructor outlines nicely when the implicit constructor uses a const or non-const reference parameter.

Fixed in 74e1deef6bd86135ee2a5f007b6347202813d143.