Richard Peters wrote:
Hello,
I'm looking for a way to serialize a variant. I see two possibly difficult things that I don't know how to accomplish: 1) In order to be able to retrieve a variant from an archive, the type of the value held by the variant must be stored. 2) When retrieving the data from an archive, once the type is read, a value of that type must be extracted from the archive and assigned to the variant. Has anybody tried this before, is this as difficult as it looks? Any help would be greatly appreciated.
I would expect the external representation of a variant<T1, ..., Tn> to be: int index; Ti ti; where index denotes the type and is returned by varant<>::which(). Assuming a variant<int, string> for simplicity, the save pseudocode should be something like: int index = v.which(); ar << v; switch( v ) { case 0: ar << get<int>( v ); break; case 1: ar << get<string>( v ); break; } You can rewrite it generically in terms of apply_visitor: struct variant_saver { typedef void result_type; template<class A, class T> void operator( A & a, T const & t ) const { a << t; } }; apply_visitor( bind( variant_saver(), ref(ar), _1 ), v ); Disclaimer: I haven't compiled any of this. ;-) Loading should be the reverse operation: int index; ar >> index; switch( index ) { case 0: { int t; ar << t; v = t; break; } case 1: { string t; ar << t; v = t; break; } } This, of course, belongs in variant.hpp. Don't let our shared_ptr debates fool you. :-)