On Wed, Dec 5, 2012 at 12:42 PM, Gabriel Redner <gredner@gmail.com> wrote:
I have recently run up against a limitation of the serialization library in terms of specializing the 'version' trait. Fortunately, I believe the issue has a simple workaround which I don't believe will break existing code.
In general, one can change the version of a serialized class by specializing the 'version' template class: ============================== template <typename T> struct version { BOOST_STATIC_CONSTANT(unsigned int, value = 0); }; ... template <> struct version<MyType> { BOOST_STATIC_CONSTANT(unsigned int, value = 1); }; ==============================
However, this approach does not work if 'MyType' is of the form (see [1]): ============================== template <typename T> struct Foo { struct MyType { int data1; }; };
template <typename T> struct version<Foo<T>::MyType> // ERROR { BOOST_STATIC_CONSTANT(unsigned int, value = 1); }; ============================== It's not possible for the compiler to deduce the enclosing type, so this doesn't work.
However, if MyType can be identified by a metafunction, then I can use enable_if to select the specialization of 'version' that I want. However, this requires support from 'version' itself: ============================== template <typename T, typename Enable = void> // NOTE new template parameter struct version { BOOST_STATIC_CONSTANT(unsigned int, value = 0); }; ... template <typaneme T> struct version<MyType, typename boost::enable_if<is_mytype<T> >::type> { BOOST_STATIC_CONSTANT(unsigned int, value = 1); }; ==============================
So, my proposal is simply to add this extra template parameter to 'version'. I do not believe other uses of 'version' will be affected, but it would not surprise me if I've overlooked something. Presumably the same modification could be made to implementation_level etc.
A patch (for 'version' only) against trunk is enclosed below. Comments are welcome!
Sounds reasonable to me.
Thanks, -Gabe
[1] http://boost.2283326.n4.nabble.com/serialization-versioning-for-class-nested...
=================================================================== --- version.hpp (revision 81723) +++ version.hpp (working copy) @@ -32,7 +32,7 @@
// default version number is 0. Override with higher version // when class definition changes. -template<class T> +template<class T, class Enable=void> struct version { template<class U> @@ -53,8 +53,8 @@ };
#ifndef BOOST_NO_INCLASS_MEMBER_INITIALIZATION -template<class T> -const int version<T>::value; +template<class T, class Enable> +const int version<T, Enable>::value; #endif
} // namespace serialization ===================================================================
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- Jeff