src/secure_allocator.hpp: define missing 'rebind' type

[trofi]

gcc-13 added an assert to standard headers to make sure custom allocators have intended implementation of rebind type instead of inherited rebind. gcc change:
https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=64c986b49558a7

Without the fix build fails on this week's gcc-13 as:

[ 92%] Building CXX object tests/CMakeFiles/test_security_curve.dir/test_security_curve.cpp.o
In file included from /<<NIX>>/gcc-13.0.0/include/c++/13.0.0/ext/alloc_traits.h:34,
                 from /<<NIX>>/gcc-13.0.0/include/c++/13.0.0/bits/stl_uninitialized.h:64,
                 from /<<NIX>>/gcc-13.0.0/include/c++/13.0.0/memory:69,
                 from tests/../src/secure_allocator.hpp:42,
                 from tests/../src/curve_client_tools.hpp:49,
                 from tests/test_security_curve.cpp:53:
/<<NIX>>/gcc-13.0.0/include/c++/13.0.0/bits/alloc_traits.h: In instantiation of 'struct std::__allocator_traits_base::__rebind<zmq::secure_allocator_t<unsigned char>, unsigned char, void>':
/<<NIX>>/gcc-13.0.0/include/c++/13.0.0/bits/alloc_traits.h:94:11:   required by substitution of 'template<class _Alloc, class _Up> using std::__alloc_rebind = typename std::__allocator_traits_base::__rebind<_Alloc, _Up>::type [with _Alloc = zmq::secure_allocator_t<unsigned char>; _Up = unsigned char]'
/<<NIX>>/gcc-13.0.0/include/c++/13.0.0/bits/alloc_traits.h:228:8:   required by substitution of 'template<class _Alloc> template<class _Tp> using std::allocator_traits< <template-parameter-1-1> >::rebind_alloc = std::__alloc_rebind<_Alloc, _Tp> [with _Tp = unsigned char; _Alloc = zmq::secure_allocator_t<unsigned char>]'
/<<NIX>>/gcc-13.0.0/include/c++/13.0.0/ext/alloc_traits.h:126:65:   required from 'struct __gnu_cxx::__alloc_traits<zmq::secure_allocator_t<unsigned char>, unsigned char>::rebind<unsigned char>'
/<<NIX>>/gcc-13.0.0/include/c++/13.0.0/bits/stl_vector.h:88:21:   required from 'struct std::_Vector_base<unsigned char, zmq::secure_allocator_t<unsigned char> >'
/<<NIX>>/gcc-13.0.0/include/c++/13.0.0/bits/stl_vector.h:423:11:   required from 'class std::vector<unsigned char, zmq::secure_allocator_t<unsigned char> >'
tests/../src/curve_client_tools.hpp:64:76:   required from here
/<<NIX>>/gcc-13.0.0/include/c++/13.0.0/bits/alloc_traits.h:70:31: error: static assertion failed: allocator_traits<A>::rebind_alloc<A::value_type> must be A
   70 |                         _Tp>::value,
      |                               ^~~~~

The change adds trivial rebind definition with expected return type.

[bluca]

Build fails on windows:

2>C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\xutility(419) : error C2664: 'zmq::secure_allocator_t<T>::secure_allocator_t(const zmq::secure_allocator_t<T> &) throw()' : cannot convert parameter 1 from 'zmq::secure_allocator_t<T>' to 'const zmq::secure_allocator_t<T> &'
2>        with
2>        [
2>            T=std::_Aux_cont
2>        ]
2>        and
2>        [
2>            T=uint8_t
2>        ]
2>        and
2>        [
2>            T=std::_Aux_cont
2>        ]
2>        Reason: cannot convert from 'zmq::secure_allocator_t<T>' to 'const zmq::secure_allocator_t<T>'
2>        with
2>        [
2>            T=uint8_t
2>        ]
2>        and
2>        [
2>            T=std::_Aux_cont
2>        ]
2>        No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
2>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\xutility(417) : while compiling class template member function 'std::_Container_base_aux_alloc_real<_Alloc>::_Container_base_aux_alloc_real(_Alloc)'
2>        with
2>        [
2>            _Alloc=zmq::secure_allocator_t<uint8_t>
2>        ]
2>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\vector(421) : see reference to class template instantiation 'std::_Container_base_aux_alloc_real<_Alloc>' being compiled
2>        with
2>        [
2>            _Alloc=zmq::secure_allocator_t<uint8_t>
2>        ]
2>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\vector(439) : see reference to class template instantiation 'std::_Vector_val<_Ty,_Alloc>' being compiled
2>        with
2>        [
2>            _Ty=uint8_t,
2>            _Alloc=zmq::secure_allocator_t<uint8_t>
2>        ]
2>        c:\projects\libzmq\src\curve_client_tools.hpp(65) : see reference to class template instantiation 'std::vector<_Ty,_Ax>' being compiled
2>        with
2>        [
2>            _Ty=uint8_t,
2>            _Ax=zmq::secure_allocator_t<uint8_t>
2>        ]
2>ws_decoder.cpp
2>ws_connecter.cpp

[trofi]

Thank you! Let's mark as draft. I'll need some time to understand why Reason: cannot convert from 'zmq::secure_allocator_t<T>' to 'const zmq::secure_allocator_t<T>' became special.

[jwakely]

Suggested change

	template <class U> struct rebind
	secure_allocator_t() { }
	template<class U> secure_allocator_t(const secure_allocator_t<U>&) { }
	template <class U> struct rebind

The error is because you need a converting constructor that can be used to construct Alloc<U> from Alloc<T> and vice versa. This adds it.

The other requirements for an allocator type are value_type, allocate and deallocate, and operator== which can both come from the std::allocator base class.

[jwakely]

became special

It didn't.

To rebind an allocator you need to be able to determine the rebound type, and construct an instance of the rebound type from the original type. With the missing rebind the rebound type is std::allocator<U> and that can be constructed from a secure_allocator_t<T> (via slicing). But that's broken.

With the new rebind member the rebound type is correct: secure_allocator_t<U>. But you can't construct that from secure_allocator_t<T>. So you need to add a converting constructor, and then you also need a user-declared default constructor.

[trofi]

Thank you! Taken default constructor and conversion constructor definitions from the non-no-op variant as well. I think it is equivalent to the definition you suggested.

[jwakely]

Looks OK to me

[kloczek]

Is it possible to release new version with that fix? 🤔