Why does my program crash after I call ReallocMemory?

Question

I'm trying to modify the VirtualTreeView to see data in the tree nodes in the design mode.

The allocating node memory is in the private static method so I can't do anything about it. I'm trying to reallocate the memory to match the new size then.

For the test purposes I'm trying to reallocate the same amount of memory:

ReallocMemory(Node, sizeof(Node^))

But the IDE hangs up in the random iteration throwing a lot of AV. Since my knowledge of memory allocation is pretty lacking I think I'm forgetting something. Could you point me please?

Answer 1

ReallocMemory is a function. It returns the new pointer value; it does not modify its argument. You want to call ReallocMem instead, or else use the result of the function:

ReallocMem(Node, SizeOf(Node^));

or

Node := ReallocMemory(Node, SizeOf(Node^));

When either of those functions cannot resize the block of memory in-place, it allocates new memory, copies the old contents into the new buffer, and then frees the original buffer. If you ignore the ReallocMemory result, then you have discarded the new pointer and retained the old, stale pointer in the Node variable. Continued use of a stale pointer would explain access violations and other unpredictable behavior.

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There are two versions of those functions for C++ compatibility. C++ doesn't have Delphi's "compiler magic," which is what allows the compiler to have a single ReallocMem function that accepts and modifies any pointer type.

The ReallocMemory function looks like the C++ realloc function, but they don't behave quite the same way, which is why it's safe to directly overwrite the input variable with the function's return value. When reallocation fails, the function throws an exception, just like ReallocMem, where as realloc just returns a null pointer.