Returns a pointer to an archive that represents the same data that the current archive covers, but that can be cached and reused later In the case of standard archives, this function will just return a pointer to itself. If the archive is actually a temporary proxy to another archive, and has a shorter lifecycle than the source archive, it should return either a pointer to the underlying archive, or if the data becomes inaccessible when the proxy object disappears (as is the case with text format archives) then nullptr