Can auto_ptr<> be used with void and malloc()/free()
in [Visual C]
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From: Richard on 5 May 2010 11:20 [Please do not mail me a copy of your followup] Ulrich Eckhardt <eckhardt(a)satorlaser.com> spake the secret code <07f7b7-sc.ln1(a)satorlaser.homedns.org> thusly: >> You don't need to override operator new/delete. >> >> Do the simplest thing that could possibly work. Overriding operator >> new/delete is more complicated than is necessary. > >Maybe that _is_ the simplest thing possible. Maybe, but in this case: no, its not. Even a simple smart pointer wrapper around this C data structure is simpler for people to understand and maintain than overriding new/delete. Overriding new/delete is *hard* to do properly. It should be avoided unless absolutely required and in this case, it isn't. -- "The Direct3D Graphics Pipeline" -- DirectX 9 draft available for download <http://legalizeadulthood.wordpress.com/the-direct3d-graphics-pipeline/> Legalize Adulthood! <http://legalizeadulthood.wordpress.com>
From: Sousuke on 17 May 2010 10:08 On May 3, 1:02 pm, "bob" <rob...(a)robert.org> wrote: > Hi, > > I need to allocate a variable size struct using malloc() and then free it > using free(). Is there a way I could wrap a pointer returned from malloc() > in auto_ptr<>? I've used auto_ptr<> with objects of struct/class but only > when they were allocated using new() operator. AFAIK, it is generally not > safe to allocate with malloc() and then free with delete(). > > Example: > > struct { > int x; > int y; > unsigned char data[1]; > > } structA; > > structA* pa = (structA*) malloc(100); > > I'd like to wrap pa pointer in auto_ptr<> so I do not have to worry about > freeing it when I leave a given block. > > I think I could do the following: > > unsigned char* p = new unsigned char[100]; > structA* pa = (structA*) p; > _auto_ptr<unsigned char> ptrA(p); > [...] > > Any thoughts? Sorry to drop in late, but here's something no one suggested: use shared_ptr, which supports custom deallocation. (auto_ptr implements single-owner ownership, whereas shared_ptr implements shared ownership, so I think shared_ptr can be used as a replacement for auto_ptr if you want.) #include <stdlib.h> #include <memory> using namespace std::tr1; // Your C-compatible struct struct PacketData { int x; int y; unsigned char data[1]; }; struct PacketDataDeleter { void operator()(PacketData* pd) { free(pd); } }; // Suppose this is the function that reads from the // network and assembles a PacketData PacketData* ReadPacketData(); // Example usage void DoSomething() { shared_ptr<PacketData> pd(ReadPacketData(), PacketDataDeleter()); // use pd }
From: Ulrich Eckhardt on 18 May 2010 03:01 Sousuke wrote: > struct PacketDataDeleter > { > void operator()(PacketData* pd) > { > free(pd); > } > }; [...] > shared_ptr<PacketData> pd(ReadPacketData(), PacketDataDeleter()); Can't you pass "&free" as deleter to the constructor? Anyway, replacing exclusive ownership with shared ownership can work, but doesn't have to. What I often have is exclusive ownership of mutable data and shared ownership of constant data. This works even in the context of multiple threads and actually is a big help there. Having shared mutable data would mandate having a mutex for it, which complicates things. Uli -- C++ FAQ: http://parashift.com/c++-faq-lite Sator Laser GmbH Geschäftsführer: Thorsten Föcking, Amtsgericht Hamburg HR B62 932
From: Sousuke on 18 May 2010 07:48
On May 18, 2:01 am, Ulrich Eckhardt <eckha...(a)satorlaser.com> wrote: > Sousuke wrote: > > struct PacketDataDeleter > > { > > void operator()(PacketData* pd) > > { > > free(pd); > > } > > }; > [...] > > shared_ptr<PacketData> pd(ReadPacketData(), PacketDataDeleter()); > > Can't you pass "&free" as deleter to the constructor? Yes... The Boost documentation left me thinking of that parameter as a function object, but you can indeed pass "free" directly. |