omg! keep it just 'registry_ = registry;'

what I meant was just:

class ScopedTestingManifestHandlerRegistry {
 public:
 ScopedTestingManifestHandlerRegistry(
 const std::vector<ManifestHandler*>& handlers)
: prev_registry(ManifestHandlerRegistry::GetInstance())
 {
   ManifestHandlerRegistry::SetInstanceForTesting( new ManifestHandlerRegistry(handlers)) 
 }
 ~ScopedTestingManifestHandlerRegistry() {
   ManifestHandlerRegistry::SetInstanceForTesting(prev_registry_);
 }

I hope now it is clear enough