Error Codes in COM

To indicate success or failure, COM methods and functions return a value of type HRESULT. An HRESULT is a 32-bit integer. The high-order bit of the HRESULT signals success or failure. Zero (0) indicates success and 1 indicates failure.

This produces the following numeric ranges:

• Success codes: 0x0–0x7FFFFFFF.
• Error codes: 0x80000000–0xFFFFFFFF.

A small number of COM methods do not return an HRESULT value. For example, the AddRef and Release methods return unsigned long values. But every COM method that returns an error code does so by returning an HRESULT value.

To check whether a COM method succeeds, examine the high-order bit of the returned HRESULT. The Windows SDK headers provide two macros that make this easier: the SUCCEEDED macro and the FAILED macro. The SUCCEEDED macro returns TRUE if an HRESULT is a success code and FALSE if it is an error code. The following example checks whether CoInitializeEx succeeds.

HRESULT hr = CoInitializeEx(NULL, COINIT_APARTMENTTHREADED | 
    COINIT_DISABLE_OLE1DDE);

if (SUCCEEDED(hr))
{
    // The function succeeded.
}
else
{
    // Handle the error.
}

Sometimes it is more convenient to test the inverse condition. The FAILED macro does the opposite of SUCCEEDED. It returns TRUE for an error code and FALSE for a success code.

HRESULT hr = CoInitializeEx(NULL, COINIT_APARTMENTTHREADED | 
    COINIT_DISABLE_OLE1DDE);

if (FAILED(hr))
{
    // Handle the error.
}
else
{
    // The function succeeded.
}

Later in this module, we will look at some practical advice for how to structure your code to handle COM errors. (See Error Handling in COM.)

Next

Creating an Object in COM