C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  ECPP questions          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Dave Guest Posted: Sun Oct 19, 2003 4:02 pm    Post subject: ECPP questions Hello all, I have a couple of questions regarding memory allocation and Herb's first book (page 146): 1. When explaining why operator delete has two parameters, the answer is that "It's just preference". What does this mean? 2. There is a reference to "placement delete" just after the quote mentioned above. What is placement delete??? If you create an object with placement new, you've haven't allocated anything; you've only caused a constructor invocation. Why would I ever want to call delete not having allocated anything? Thanks, Dave [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Aaron Bentley Guest Posted: Sun Oct 19, 2003 9:41 pm    Post subject: Re: ECPP questions Dave wrote: Quote: Hello all, I have a couple of questions regarding memory allocation and Herb's first book (page 146): [snip] 2. There is a reference to "placement delete" just after the quote mentioned above. What is placement delete??? When a constructor throws, the memory allocated with new must be deallocated. For Placement new, there is a corresponding placement delete to deallocate the memory. Otherwise, it will leak. Quote: If you create an object with placement new, you've haven't allocated anything; you've only caused a constructor invocation. Frequently, you *have* allocated something-- for example, you could use placement new to allocate from shared memory. -- Aaron Bentley www.aaronbentley.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Andrea Griffini Guest Posted: Sun Oct 19, 2003 9:46 pm    Post subject: Re: ECPP questions On 19 Oct 2003 12:02:41 -0400, "Dave" <better_cs_now (AT) yahoo (DOT) com> wrote: Quote: What is placement delete??? If you create an object with placement new, you've haven't allocated anything; you've only caused a constructor invocation. Why would I ever want to call delete not having allocated anything? With "placement new" and despite the name the concept is not the one of creating an object at a specific memory address, but to invoke a "special" allocator passing it additional parameters to get the memory in which the object should be built. There is also a standard provided "special" allocator that takes a void* as a parameter that just returns that pointer as the address in which the object should be created; but this is just one of the possible "placement new" allocators: you can have special allocators that accept other parameters instead of a pointer and that do any computation you like to obtain the address in which the object should be built into. When you allocate an object using the standard (non-placement) new, and the object being constructed throws an exception during the execution of the constructor, the compiler will call the standard operator delete to free the memory block that was reserved for the object whose construction was aborted. When an object is instead being allocated using a placement new throws an exception during the constructor it would be clearly a terrible idea to call the standard delete operator (because the memory was *not* coming from the standard new operator), so the C++ standard mandates that if there is a placement delete operator whose signature matches the one of the allocator used then *that* special delete will be called passing the same parameters used to allocate the memory, and if there is no such a compatible placement delete then there will be no attempt to free the block. Of course with the standard provided placement new operator just ignoring the delete operation is perfectly meaningful and generates no leak because the allocator didn't indeed allocate any memory, but in the general case this is not true. HTH Andrea [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Richter Guest Posted: Wed Oct 22, 2003 1:24 am    Post subject: Re: ECPP questions Hi, Quote: 1. When explaining why operator delete has two parameters, the answer is that "It's just preference". What does this mean? The second (size_t) argument of operator delete is the size of the object allocated, similar to the size_t argument to new. Whereas this argument is clearly required for allocating objects, the size_t argument of delete might be not required for most systems, especially those where the operator delete is implemented by means of the free() call of the C library which keeps the size of the memory in the memory itself and doesn't require a size argument. However, I *DO NOT* agree that this is "just a preference". While this is true for most systems, there are memory allocators out in the wilderness which do not keep the memory size of the object automatically, and where a memory release requires the size of the memory along with its address. Unfortunately, I found quite some implementations where this second argument to operator delete was broken. (older versions of g++, and IIRC VS 6.0) Quote: 2. There is a reference to "placement delete" just after the quote mentioned above. What is placement delete??? If you create an object with placement new, you've haven't allocated anything; you've only caused a constructor invocation. Why would I ever want to call delete not having allocated anything? I would more or less call "placement new" a misnomer. "new with arguments" would be much more apropriate. One common usage for this construction is where you want to place an object into an already allocated memory area; in that case the argument is the memory block, and there is indeed rarely the need to delete the memory again thru the operator delete. However, "placement new arguments" also has some benefits besides this. For example, consider a system with main memory and designated graphics memory. If you would like to allocate memory from the pool of graphics memory, an operator new that takes a flag value which memory pool to allocate the memory from would definitely make sense. However, it is a common misunderstanding that for objects allocated by "operator new with arguments" the corresponding "operator delete with arguments" gets called. This is only the case if you allocated the object within a constructor of another object, and (the other's object) construction failed afterwards, requiring to get rid of the already allocated components. Instead, for regular "deletion" of objects, the regular operator delete will be called. It is your job to figure out where the memory came from and what to do about it. See also my answer to your post below. After all, I think that the C++ requires some reconsideration in this field, this is all a bit too messy for me... (-; So long, Thomas [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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