Visual Studio 6 vs 2005 allocators

(If you didn’t sleep enough, don’t try to understand the following)

If one day you’re in Visual Studio 2005, creating an object that comes from a DLL compiled with Visual Studio 6, you might run into some allocators issues. Normally things go smoothly because your object is allocated and deallocated with the VC2005 allocator if you create it yourself, or by the VC6 allocator if you’re using specific creation functions that come with your DLL.

What can become very tricky is when this object has a virtual destructor, which means that your object was allocated with the 2005 allocator, but is deallocated with the VC6 allocator. This is not good, and will probably result in a crash nearly impossible to solve unless the above-mentionned DLL is re-built for 2005. Which may not always be possible if you the owner of the DLL doesn’t want to do it himself or give his source code.

Here’s a solution (I’m really happy we have code gurus here..) : allocate the memory for this object using the VC6 allocator. How do you do that? Easy (aha)! Simply load the VC6 runtime dll (msvcrt), find the pointer to the allocator function, and call it !

vc6handle = LoadLibrary(TEXT(”msvcrt”));

typedef void* (*vc6_malloc) (size_t _Size);
vc6_malloc vc6funcptr = (vc6_malloc) GetProcAddress(vc6handle,”malloc”);
TheClass* toto = (TheClass*) vc6funcptr(sizeof(*TheClass));
toto = new (toto) TheClass;

FreeLibrary(vc6handle);

You’ll notice that the new function takes the buffer that was allocated as a parameter. This is not the regular C++ allocator, but a custom one that can take a previously allocated buffer.

I won’t even explained how they did understand the problem …

Any question? =)

Tags: allocator, vc6, visual studio

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