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5.3.4 When a block is freed with an inappropriate deallocation function

In the following example, a block allocated with new[] has wrongly been deallocated with free:

Mismatched free() / delete / delete []
   at 0x40043249: free (vg_clientfuncs.c:171)
   by 0x4102BB4E: QGArray::~QGArray(void)
                    (tools/qgarray.cpp:149)
   by 0x4C261C41: PptDoc::~PptDoc(void)
                    (include/qmemarray.h:60)
   by 0x4C261F0E: PptXml::~PptXml(void) (pptxml.cc:44)
 Address 0x4BB292A8 is 0 bytes
   inside a block of size 64 alloc'd
   at 0x4004318C: operator new[](unsigned int) 
                    (vg_clientfuncs.c:152)
   by 0x4C21BC15: KLaola::readSBStream(int)
                    const (klaola.cc:314)
   by 0x4C21C155: KLaola::stream(KLaola::OLENode
                    const *) (klaola.cc:416)
   by 0x4C21788F: OLEFilter::convert(QCString
                    const &) (olefilter.cc:272)

In ‘C++’ it's important to deallocate memory in a way compatible with how it was allocated. The deal is:

• If allocated with malloc, calloc, realloc, valloc or memalign, you must deallocate with free.
• If allocated with new[], you must deallocate with delete[].
• If allocated with new, you must deallocate with delete.

The worst thing is that on Linux apparently it doesn't matter if you do mix these up, but the same program may then crash on a different platform, Solaris for example. So it's best to fix it properly. According to the KDE folks “it's amazing how many C++ programmers don't know this”.

The reason behind the requirement is as follows. In some C++ implementations, delete[] must be used for objects allocated by new[] because the compiler stores the size of the array and the pointer-to-member to the destructor of the array's content just before the pointer actually returned. This implies a variable-sized overhead in what's returned by new or new[].