Botan compile error VS2015 - c++

Question

I have a strange situation here. I am trying to use the Botan crypto library with VS2015 (because some other parts of the project use some heavy C++11 code which VS2013 is unable to compile) and I get a pretty long compilation error (see below).
After trying out various things, I came to the conclusion, that even if one of the botan headers is included in the compiled c++ source file, the compiler will throw the error below. Right now I have a single line in the file:
#include <botan/botan.h>
and this is the error I get:
C:\Program Files\Microsoft Visual Studio 14.0\VC\INCLUDE\xmemory0(876): error C2664: 'Botan::secure_allocator<_Newfirst>::secure_allocator(const Botan::secure_allocator<_Newfirst> &)': cannot convert
argument 1 from 'std::_Wrap_alloc<Botan::secure_allocator<Botan::byte>>' to 'const Botan::secure_allocator<_Newfirst> &'
with
[
_Newfirst=std::_Container_proxy
]
C:\Program Files\Microsoft Visual Studio 14.0\VC\INCLUDE\xmemory0(876): note: Reason: cannot convert from 'std::_Wrap_alloc<Botan::secure_allocator<Botan::byte>>' to 'const Botan::secure_allocator<_Ne
wfirst>'
with
[
_Newfirst=std::_Container_proxy
]
C:\Program Files\Microsoft Visual Studio 14.0\VC\INCLUDE\xmemory0(876): note: No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
C:\Program Files\Microsoft Visual Studio 14.0\VC\INCLUDE\vector(587): note: see reference to function template instantiation 'std::_Wrap_alloc<Botan::secure_allocator<_Newfirst>>::_Wrap_alloc<std::_Wr
ap_alloc<Botan::secure_allocator<Botan::byte>>>(_Other &) noexcept' being compiled
with
[
_Newfirst=std::_Container_proxy,
_Other=std::_Wrap_alloc<Botan::secure_allocator<Botan::byte>>
]
C:\Program Files\Microsoft Visual Studio 14.0\VC\INCLUDE\vector(587): note: see reference to function template instantiation 'std::_Wrap_alloc<Botan::secure_allocator<_Newfirst>>::_Wrap_alloc<std::_Wr
ap_alloc<Botan::secure_allocator<Botan::byte>>>(_Other &) noexcept' being compiled
with
[
_Newfirst=std::_Container_proxy,
_Other=std::_Wrap_alloc<Botan::secure_allocator<Botan::byte>>
]
C:\Program Files\Microsoft Visual Studio 14.0\VC\INCLUDE\vector(585): note: while compiling class template member function 'void std::_Vector_alloc<std::_Vec_base_types<_Ty,_Alloc>>::_Free_proxy(void)
'
with
[
_Ty=Botan::byte,
_Alloc=Botan::secure_allocator<Botan::byte>
]
C:\Program Files\Microsoft Visual Studio 14.0\VC\INCLUDE\vector(552): note: see reference to function template instantiation 'void std::_Vector_alloc<std::_Vec_base_types<_Ty,_Alloc>>::_Free_proxy(voi
d)' being compiled
with
[
_Ty=Botan::byte,
_Alloc=Botan::secure_allocator<Botan::byte>
]
C:\Program Files\Microsoft Visual Studio 14.0\VC\INCLUDE\vector(679): note: see reference to class template instantiation 'std::_Vector_alloc<std::_Vec_base_types<_Ty,_Alloc>>' being compiled
with
[
_Ty=Botan::byte,
_Alloc=Botan::secure_allocator<Botan::byte>
]
c:\Botan\include\botan-1.11\botan/rng.h(43): note: see reference to class template instantiation 'std::vector<Botan::byte,Botan::secure_allocator<Botan::byte>>' being compiled
NMAKE : fatal error U1077: 'C:\PROGRA~1\MICROS~3.0\VC\bin\cl.exe' : return code '0x2'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files\Microsoft Visual Studio 14.0\VC\BIN\nmake.exe"' : return code '0x2'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files\Microsoft Visual Studio 14.0\VC\BIN\nmake.exe"' : return code '0x2'
Stop.
Since I was able to compile and run the botan tests, I have the feeling that I missed something, but I have no idea what. Does anyone has any experience with this?
(BTW: The same code compiles nicely with g++ 4.9)

Answer 1

Looking at the sources, it seems that Botan::secure_allocator doesn't provide a template constructor of the form
template<class U> secure_allocator(const secure_allocator<U>& other);
This is required by the Standard. In the current working draft, N4527, the relevant bits are in [17.6.3.5] Table 28 - Allocator requirements; also useful is the example in paragraph 9.
So, we can't blame the standard library implementation that comes with VC 14 for requiring this in order to compile. The error is on Botan's side in my opinion.
A quick fix is to add such a definition to Botan::secure_allocator:
template<class U> secure_allocator(const secure_allocator<U>&) BOTAN_NOEXCEPT { }
Since instantiations of this allocator template don't have any non-static data members, an empty body should be fine. However, I'm not familiar with the library, and we're talking about cryptography here, so, before using this to do any serious stuff, please confirm the change with the library authors.
Another possible workaround:
I've noticed that the code that calls the mixed-type constructor seems to only be enabled when iterator debugging is enabled, which happens by default in Debug mode.
Have you tried compiling in release mode? If my observations are correct, you won't get this error anymore, since the additional machinery for iterator debugging will be disabled.
To get the same behaviour in Debug mode, set the _ITERATOR_DEBUG_LEVEL macro to 0 globally.
Debug iterators can be useful to detect errors (as long as the performance hit doesn't affect you), so I wouldn't use this as a permanent fix, but it could be useful as a temporary workaround if you don't want to modify the Botan header file.
This could also explain why you were able to compile the tests: maybe they're compiled in release mode, or, anyway, with a combination of settings that disables iterator debugging?