问题
there are quite a few faces for the new operator in c++, but I'm interested in placement new.
Suppose you allocate memory at a specific memory location
int memoryPool[poolSize*sizeof(int)];
int* p = new (mem) int; //allocates memory inside the memoryPool buffer
delete p; //segmentation fault
How can I correctly deallocate memory in this case? What if instead of built-in type int I would use some class called myClass?
myClass memoryPool[poolSize*sizeof(myClass )];
myClass * p = new (mem) myClass ; //allocates memory inside the memoryPool buffer
delete p; //segmentation fault
Thanks for your help.
回答1:
In the first case, there's no point in using placement new, since int doesn't have a constructor.
In the second case, it's either pointless (if myClass is trivial) or wrong, since there are already objects in the array.
You use placement new to initialise an object in a block of memory, which must be suitably aligned, and mustn't already contain a (non-trivial) object.
char memory[enough_bytes]; // WARNING: may not be properly aligned.
myClass * c = new (memory) myClass;
Once you've finished with it, you need to destroy the object by calling its destructor:
c->~myClass();
This separates the object's lifetime from that of its memory. You might also have to release the memory at some point, depending on how you allocated it; in this case, it's an automatic array, so it's automatically released when it goes out of scope.
回答2:
In your case there is no need to deallocate it, your int array will be deallocated once you return from your function. You should only call explicitly your destructor:
p->~myclass();
to keep you buffer correctly aligned use std::aligned_storage, look in here for example:
http://www.cplusplus.com/reference/type_traits/aligned_storage/
来源：https://stackoverflow.com/questions/21551972/how-do-you-delete-an-object-allocated-with-placement-new