Memory Allocation in COM

Sometimes a method allocates a memory buffer on the heap and returns the address of the buffer to the caller. COM defines a pair of functions for allocating and freeing memory on the heap.

• The CoTaskMemAlloc function allocates a block of memory.
• The CoTaskMemFree function frees a block of memory that was allocated with CoTaskMemAlloc.

We saw an example of this pattern in the Open dialog box example:

PWSTR pszFilePath;
hr = pItem->GetDisplayName(SIGDN_FILESYSPATH, &pszFilePath);
if (SUCCEEDED(hr))
{
    // ...
    CoTaskMemFree(pszFilePath);
}

The GetDisplayName method allocates memory for a string. Internally, the method calls CoTaskMemAlloc to allocate the string. When the method returns, pszFilePath points to the memory location of the new buffer. The caller is responsible for calling CoTaskMemFree to free the memory.

Why does COM define its own memory allocation functions? One reason is to provide an abstraction layer over the heap allocator. Otherwise, some methods might call malloc while others called new. Then your program would need to call free in some cases and delete in others, and keeping track of it all would quickly become impossible. The COM allocation functions create a uniform approach.

Another consideration is the fact that COM is a binary standard, so it is not tied to a particular programming language. Therefore, COM cannot rely on any language-specific form of memory allocation.

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COM Coding Practices