Can auto_ptr<> be used with void and malloc()/free()
in [Visual C]
|
From: bob on 3 May 2010 14:02 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? Thanks.
From: John H. on 3 May 2010 16:27 bob wrote: > I need to allocate a variable size struct using malloc() and then free it > using free(). Why? That seems atypical. > Is there a way I could wrap a pointer returned from malloc() > in auto_ptr<>? Perhaps. See below. > I've used auto_ptr<> with objects of struct/class but only > when they were allocated using new() operator. This is good. auto_ptr was designed to work with new and delete (scalar versions). > AFAIK, it is generally not > safe to allocate with malloc() and then free with delete(). That is correct. > Example: > > struct { > int x; > int y; > unsigned char data[1]; > } structA; > > structA* pa = (structA*) malloc(100); Whenever you fine yourself reaching for malloc in C++, you should question whether that is really what you should be doing. It looks like you are trying to treat your structure as something that has an x, y, and then some data which is explicitly one char big, but really might be more (perhaps 92 bytes in your case). Rather than doing obscure things with memory, I think you are better of using a different data structure. Consider making data an std::vector. That way it can have a dynamic size, and the cleanup is done for you. You don't have to worry about auto_ptr. > 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. Well in general that can be a fine goal, but in your situation I am not sure. > I think I could do the following: > > unsigned char* p = new unsigned char[100]; > structA* pa = (structA*) p; > _auto_ptr<unsigned char> ptrA(p); > [...] So this is a different problem than the first one mentioned, where you needed malloc and free? I guess you are trying to accomplish the same thing with the data structure, but using new/delete instead. Perhaps it is slightly better than the malloc/free version, but it still seems to be missing the mark. > Any thoughts? I think you want to rethink things and I would need to know more about what you are trying to do to give you good advice. But if you really want to use auto_ptr with malloc/free you might consider something like the following. I am not sure if it works correctly or safely. #include <memory> #include <cstdlib> struct structA { int x; int y; unsigned char data[1]; void operator delete(void * this_ptr); private: void * operator new(unsigned int); }; void structA::operator delete(void * this_ptr) { std::cout << "delete" << std::endl; free(this_ptr); } int main() { void * memory = malloc(100); structA * fake_structA = static_cast<structA *>(memory); std::auto_ptr<structA> autoptr(fake_structA); return 0; }
From: Richard on 3 May 2010 17:27 [Please do not mail me a copy of your followup] "bob" <robert(a)robert.org> spake the secret code <Oid8bru6KHA.3880(a)TK2MSFTNGP04.phx.gbl> thusly: >I need to allocate a variable size struct using malloc() and then free it >using free(). [...] There are better ways to do this in C++. The only time you need to use malloc/free is when you are interoperating with a C (not C++) library. Personally, when I need to talk to C code, I hide it behind an abstraction that handles all the interoperability concerns for me. Part of that is using c'tors and d'tors that handle malloc/free business. -- "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: John H. on 3 May 2010 17:58 bob wrote: > I think I could do the following: > > unsigned char* p = new unsigned char[100]; > structA* pa = (structA*) p; > _auto_ptr<unsigned char> ptrA(p); > [...] That might not work very well, as you allocated with new[] and auto_ptr will deallocate with scalar delete.
From: Ulrich Eckhardt on 4 May 2010 02:57
bob wrote: > 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<>? No, auto_ptr uses delete, which you must not use with a pointer returned from malloc. What I'm not sure about is whether you could override operators new/delete for your struct and redirect these accordingly to do The Right Thing(tm). I'd try asking this in comp.lang.c++.moderated. > structA* pa = (structA*) malloc(100); Use static_cast. If everything fails, you can also write an auto_ptr-like class yourself. Or take this one: http://lists.boost.org/Archives/boost/2007/11/130127.php Cheers! Uli -- C++ FAQ: http://parashift.com/c++-faq-lite Sator Laser GmbH Geschäftsführer: Thorsten Föcking, Amtsgericht Hamburg HR B62 932 |