It may be worth reminding people that this is not specific to DebugOnly but general to all C++ classes: In C++, there is no such thing as a class with size 0. So expecting DebugOnly<T> to be of size 0 is not misunderstanding DebugOnly, it is misunderstanding C++. The only way to have empty classes behave as if they had size 0, is to inherit from them instead of having them as the types of members.That's called the Empty Base Class Optimization. http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Empty_Base_Optimization
Since DebugOnly<T> incurs a size overhead in non-debug builds, maybe we should officially consider it bad practice to have any DebugOnly<T> class members. Having to guard them in #ifdef DEBUG takes away much of the point of DebugOnly<T>, doesn't it? Benoit 2014-07-15 21:21 GMT-04:00 Nicholas Nethercote <n.netherc...@gmail.com>: > Hi, > > The comment at the top of mfbt/DebugOnly.h includes this text: > > * Note that DebugOnly instances still take up one byte of space, plus > padding, > * when used as members of structs. > > I'm in the process of making js::HashTable (a very common class) > smaller by converting some DebugOnly fields to instead be guarded by > |#ifdef DEBUG| (bug 1038601). > > Below is a list of remaining DebugOnly members that I found using > grep. People who are familiar with them should inspect them to see if > they belong to classes that are commonly instantiated, and thus if > some space savings could be made. > > Thanks. > > Nick > > > uriloader/exthandler/ExternalHelperAppParent.h: DebugOnly<bool> mDiverted; > layout/style/CSSVariableResolver.h: DebugOnly<bool> mResolved; > layout/base/DisplayListClipState.h: DebugOnly<bool> mClipUsed; > layout/base/DisplayListClipState.h: DebugOnly<bool> mRestored; > layout/base/DisplayListClipState.h: DebugOnly<bool> mExtraClipUsed; > gfx/layers/Layers.h: DebugOnly<uint32_t> mDebugColorIndex; > ipc/glue/FileDescriptor.h: mutable DebugOnly<bool> > mHandleCreatedByOtherProcessWasUsed; > ipc/glue/MessageChannel.cpp: DebugOnly<bool> mMoved; > ipc/glue/BackgroundImpl.cpp: DebugOnly<bool> mActorDestroyed; > content/media/MediaDecoderStateMachine.h: DebugOnly<bool> > mInRunningStateMachine; > dom/indexedDB/ipc/IndexedDBParent.h: DebugOnly<RequestType> mRequestType; > dom/indexedDB/ipc/IndexedDBParent.h: DebugOnly<RequestType> mRequestType; > dom/indexedDB/ipc/IndexedDBParent.h: DebugOnly<RequestType> mRequestType; > dom/indexedDB/ipc/IndexedDBChild.h: DebugOnly<RequestType> mRequestType; > dom/indexedDB/ipc/IndexedDBChild.h: DebugOnly<RequestType> mRequestType; > dom/indexedDB/ipc/IndexedDBChild.h: DebugOnly<RequestType> mRequestType; > _______________________________________________ > dev-platform mailing list > dev-platform@lists.mozilla.org > https://lists.mozilla.org/listinfo/dev-platform > _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
