Cannot implicit convert to a retain_ptr of a base-class

[nordlow]

The following code examples illustrates a current severe limitation with retain_ptr

#include <iostream>
#include <memory>
#include "sg14/memory.hpp"

using namespace std;

class Base
{
public:
    Base()
    {
    }
    mutable unsigned int _rc = 1;
};

class Sub : public Base
{
public:
    Sub(int x_)
        : Base(), x(x_)
    {
        cout << __PRETTY_FUNCTION__ << ":" << endl;
    }
    ~Sub() { cout << __PRETTY_FUNCTION__ << ":" << endl; }
public:
    int x;
};

void test_shared()
{
    using SharedBase = std::shared_ptr<const Base>;
    using SharedSub = std::shared_ptr<const Sub>;
    SharedBase base;
    SharedSub sub;
    base = sub;                 // compiles!
}

struct Base_traits final
{
    static void increment(const Base* x) noexcept
    {
        x->_rc += 1;
        cout << __PRETTY_FUNCTION__ << ": rc=" << x->_rc << endl;
    }
    static void decrement(const Base* x) noexcept
    {
        x->_rc -= 1;
        cout << __PRETTY_FUNCTION__ << ": rc=" << x->_rc << endl;
        if (x->_rc == 0)
        {
            delete x;
        }
    }
};

struct Sub_traits final
{
    static void increment(Sub* x) noexcept
    {
        x->_rc += 1;
        cout << __PRETTY_FUNCTION__ << ": rc=" << x->_rc << endl;
    }
    static void decrement(Sub* x) noexcept
    {
        x->_rc -= 1;
        cout << __PRETTY_FUNCTION__ << ": rc=" << x->_rc << endl;
        if (x->_rc == 0)
        {
            delete x;
        }
    }
};

using RetainBase = sg14::retain_ptr<const Base, Base_traits>;
using RetainSub = sg14::retain_ptr<const Sub, Base_traits>;

void test_retain()
{
    cout << "sizeof(Sub): " << sizeof(Sub) << endl;
    cout << "sizeof(RetainSub): " << sizeof(RetainSub) << endl;
    auto sub = RetainSub(new Sub(42));
    auto base = RetainBase(new Base());
    base = sub; // this errors
}

int main(__attribute__((unused)) int argc,
         __attribute__((unused)) const char* argv[],
         __attribute__((unused)) const char* envp[])
{
    test_shared();
    test_retain();
    return 0;
}

not present in std::shared_ptr. Namely the lack of implicit casting from a retain_ptr of an inherited class (Sub), to a retain_ptr of a base-class (Base) of Sub. Which result in the following compiler error

retain_ptr_test.cpp: In function ‘void test_retain()’:
retain_ptr_test.cpp:83:12: error: no match for ‘operator=’ in ‘base = sub’ (operand types are ‘sg14::retain_ptr<const Base, Base_traits>’ and ‘sg14::retain_ptr<const Sub, Base_traits>’)
In file included from retain_ptr_test.cpp:3:
sg14/memory.hpp:200:17: note: candidate: ‘sg14::retain_ptr<T, R>& sg14::retain_ptr<T, R>::operator=(const sg14::retain_ptr<T, R>&) [with T = const Base; R = Base_traits]’
sg14/memory.hpp:200:17: note:   no known conversion for argument 1 from ‘sg14::retain_ptr<const Sub, Base_traits>’ to ‘const sg14::retain_ptr<const Base, Base_traits>&’
sg14/memory.hpp:205:17: note: candidate: ‘sg14::retain_ptr<T, R>& sg14::retain_ptr<T, R>::operator=(sg14::retain_ptr<T, R>&&) [with T = const Base; R = Base_traits]’
sg14/memory.hpp:205:17: note:   no known conversion for argument 1 from ‘sg14::retain_ptr<const Sub, Base_traits>’ to ‘sg14::retain_ptr<const Base, Base_traits>&&’
sg14/memory.hpp:210:17: note: candidate: ‘sg14::retain_ptr<T, R>& sg14::retain_ptr<T, R>::operator=(std::nullptr_t) [with T = const Base; R = Base_traits; std::nullptr_t = std::nullptr_t]’
sg14/memory.hpp:210:17: note:   no known conversion for argument 1 from ‘sg14::retain_ptr<const Sub, Base_traits>’ to ‘std::nullptr_t’

when compiled with gcc-8 and -std=c++17.

How can I change sg14/memory.hpp to allow this assignment to be allowed?

My two current proposals are

    template <class U>
    using enable_if_sub = std::enable_if_t<std::is_base_of_v<T, U>>;

    template <class S, class = enable_if_sub<S>>
    retain_ptr& operator = (retain_ptr<S, R> &&that) {
        if (*this) { traits_type::decrement(this->get()); } // drop
        ptr = that.detach();
        return *this;
    }

or

    template <class S, class = enable_if_sub<S>>
    retain_ptr& operator = (retain_ptr<S, R> &&that) {
        *this = *reinterpret_cast<retain_ptr<T, R> *>(&that);
        return *this;
    }

or

    template <class S, class = enable_if_sub<S>>
    retain_ptr& operator = (retain_ptr<S, R> &&that) {
        reinterpret_cast<retain_ptr<T, R> *>(&that)->swap(*this);
        return *this;
    }

but all three make AddressSanitizer complain as

=================================================================
==4647==ERROR: AddressSanitizer: new-delete-type-mismatch on 0x602000000010 in thread T0:
  object passed to delete has wrong type:
  size of the allocated type:   8 bytes;
  size of the deallocated type: 4 bytes.
    #0 0x563de6218b28 in operator delete(void*, unsigned long) (/home/per/.emacs.d/auto-builds/gcc-8/AddressSanitized-Debug/home/per/Work/justcxx/retain_ptr_test+0xd7b28)
    #1 0x563de6254df4 in Base_traits::decrement(Base const*) /home/per/Work/justcxx/retain_ptr_test.cpp:52
    #2 0x563de6254f2b in sg14::retain_ptr<Base const, Base_traits>::~retain_ptr() sg14/memory.hpp:197
    #3 0x563de62547fc in test_retain() /home/per/Work/justcxx/retain_ptr_test.cpp:83
    #4 0x563de62548cf in main /home/per/Work/justcxx/retain_ptr_test.cpp:92
    #5 0x7f21d42c1b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #6 0x563de6149569 in _start (/home/per/.emacs.d/auto-builds/gcc-8/AddressSanitized-Debug/home/per/Work/justcxx/retain_ptr_test+0x8569)

0x602000000010 is located 0 bytes inside of 8-byte region [0x602000000010,0x602000000018)
allocated by thread T0 here:
    #0 0x563de6217570 in operator new(unsigned long) (/home/per/.emacs.d/auto-builds/gcc-8/AddressSanitized-Debug/home/per/Work/justcxx/retain_ptr_test+0xd6570)
    #1 0x563de625478a in test_retain() /home/per/Work/justcxx/retain_ptr_test.cpp:82
    #2 0x563de62548cf in main /home/per/Work/justcxx/retain_ptr_test.cpp:92
    #3 0x7f21d42c1b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)

SUMMARY: AddressSanitizer: new-delete-type-mismatch (/home/per/.emacs.d/auto-builds/gcc-8/AddressSanitized-Debug/home/per/Work/justcxx/retain_ptr_test+0xd7b28) in operator delete(void*, unsigned long)
==4647==HINT: if you don't care about these errors you may set ASAN_OPTIONS=new_delete_type_mismatch=0
==4647==ABORTING

. What am I doing wrong?

Update:

I think I found a solution

https://github.com/nordlow/justcxx/blob/master/sg14/memory.hpp

in use here

https://github.com/nordlow/justcxx/blob/master/retain_ptr_test.cpp

without any complaints from AddressSantitizer.