Introduction to EON SDK - Construction IT research at VTT

User Guide
Techniques
Techniques
Smart pointers
All COM objects encapsulate an integer value that keeps track of
object usage. Each time a client creates a COM object the AddRef
function of that object is called. This increment the object’s
reference counter by one and prevents the destruction of that object
when it goes out of scope. This way separate parts of the client or
even different clients may use the same object that will persist as
long as its reference counter is larger than zero. When an object’s
Release function is called its reference counter is decrement by one,
if this result in a zero value the object deletes it self.
Each time a new pointer is introduced AddRef should be called,
otherwise the pointer may become invalid. And before the pointer goes
out of scope the programmer has to call Release otherwise there will
be a memory leak. Trouble occurs when a programmer has several
pointers to several different COM objects, e.g.:
#include
#include "shlobj.h" // Shell COM components
#include "objbase.h"
int main(void)
{
HRESULT hr = CoInitialize(NULL);
if (FAILED(hr))
{
CoUninitialize();
return -1;
}
IUnknown* pUnk1;
// CoCreateInstance calls AddRef
hr = CoCreateInstance
(
CLSID_ShellLink,
NULL,
CLSCTX_INPROC_SERVER,
IID_IUnknown,
(void **) &pUnk1
);
if (FAILED(hr))
{
CoUninitialize();
return -1;
}
IUnknown* pUnk2 = pUnk1;
ULONG lRef = pUnk2->AddRef();
printf("Reference counter = %d\n", lRef);
lRef = pUnk2->Release();
printf("Reference counter = %d\n", lRef);
lRef = pUnk1->Release();
printf("Reference counter = %d\n", lRef);
}
CoUninitialize();
return 0;
Obviously it is quite easy to forget ether AddRef and Release on pUnk2
or Release on pUnk1 since CoCreateInstance did AddRef for us. To avoid
this behavior we could encapsulate all COM pointers in a template
class that calls AddRef when it is assigned to an object and Release
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EON SDK 2.53.0