Usage of CoTaskMemAlloc?

Question

When is it appropriate to use CoTaskMemAlloc? Can someone give an example?

Answer 1

Use CoTaskMemAlloc when returning a char* from a native C++ library to .NET as a string.

C#

[DllImport("test.dll", CharSet=CharSet.Ansi)]
extern static string Foo();

C

char* Foo()
{
    std::string response("response");
    int len = response.length() + 1;
    char* buff = (char*) CoTaskMemAlloc(len);
    strcpy_s(buff, len, response.c_str());
    return buff;
}

Since .NET uses CoTaskMemFree, you have to allocate the string like this, you can't allocate it on the stack or the heap using malloc / new.

Answer 2

Gosh, I had to think for a while for this one -- I've done a fair amount of small-scale COM programming with ATL and rarely have had to use it.

There is one situation though that comes to mind: Windows Shell extensions. If you are dealing with a set of filesystem objects you might have to deal with PIDLs (pointer to an ID list). These are bizarre little filesystem object abstractions and they need to be explicitly allocated/deallocated using a COM-aware allocator such as CoTaskMemAlloc. There is also an alternative, the IMalloc interface pointer obtained from SHGetMalloc (deprecated) or CoGetMalloc -- it's just an abstraction layer to use, so that your code isn't tied to a specific memory allocator and can use any appropriate one.

The point of using CoTaskMemAlloc or IMalloc rather than malloc() is that the memory allocation/deallocation needs to be something that is "COM-aware" so that its allocation and deallocation are performed consistently at run-time, even if the allocation and deallocation are done by completely unrelated code (e.g. Windows allocates memory, transfers it to your C++ code which later deallocates, or your C++ code allocates, transfers it to someone else's VB code which later deallocates). Neither malloc() nor new are capable of interoperating with the system's run-time heap so you can't use them to allocate memory to transfer to other COM objects, or to receive memory from other COM objects and deallocate.