Re: Is it ever unsafe to realloc?

From:
fmatthew5876 <fmatthew5876@gmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Thu, 12 Jul 2012 13:08:54 -0700 (PDT)
Message-ID:
<6067508c-7b5e-46b9-a2b9-20e85052bc31@googlegroups.com>
On Thursday, July 12, 2012 3:19:36 PM UTC-4, Seungbeom Kim wrote:

On 2012-07-11 18:40, fmatthew5876 wrote:

Suppose I have a pool of memory (say a dynamic array) that I
allocate using malloc(), cleanup using free(), and use placement new
and explicit destructor calls to handle constructors/destructors.

If I want to resize this memory pool (invalidating any iterators/
pointers to objects in the pool) can I safely call realloc() to do it
no matter what kind of C++ objects are being stored? Are there some
classes of C++ types where I have to allocate a new pool using malloc()
and use move/copy constructors? If so what are the cases where realloc
is not safe?


realloc() potentially allocates a new block and bit-copies (as with
memcpy()) the old contents into the new block. This is safe for POD
objects, but not in general: the copy (or move) constructor should
handle the relocation.


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By why do copy/move constructors have to always be called? Can some
objects maintain internal pointers to parts of themselves in vtables?
For instance with virtual functions and/or virtual inheritance?

For instance when would this be unsafe?

class A {
//stuff
};

A* ptr = (A*)malloc(sizeof(A)*2);
new (ptr) A();
new (ptr+1) A();

ptr = (A*) realloc(ptr, sizeof(A) * 3);
new (ptr+2) A();

ptr[0].~A();
ptr[1].~A();
ptr[2].~A();
free(ptr);

or this?

A* ptr1 = (A*)malloc(sizeof(A));
A* ptr2 = (A*)malloc(sizeof(A));

new (ptr1) A();
memcpy(ptr2, ptr1, sizeof(A));

ptr2.~A():
free(ptr1);
free(ptr2);

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