In some of the IDL I work with I have noticed that there are 2 conventions for marking return values in methods - [in, out]
and [out, retval]
.
It appears that [in, out]
is used when there are multiple return values, for example:
HRESULT MyMethod(
[in] long InputParam,
[in, out] long* OutputParam1,
[in, out] long* OutputParam2
);
It appears that [out, retval]
is used when there is only a single return value, for example:
HRESULT MyMethod2(
[in] long InputParam,
[out, retval] long* OutputParam1
);
Is this a COM IDL convention or just a convention in the code I am working with?
Is there a functional difference in the code that will be generated from the 2 notations, or are they completely interchangeable?
[in, out]
means that a valid value is passed when the method is called and a valid value is there (where the pointer points) when the method returns success. [out]
means that the value pointed to can be whatever when the method is called but it will be valid when the method returns success. Both [out]
and [in, out]
parameters must be pointers - their values are unchanged and valid and the validity requirements only apply to the variables they point to.
[out, retval]
is a syntactic sugar to indicate that when creating a Native COM Support wrapper this very parameter should be converted to a return value. For example
HRESULT MyMethod( [out] long* OutParam1, [out, retval] long* OutParam2 );
becomes
long IWrappedInterface::MyMethod( long* OutParam1 );
If you don't mark it [retval]
the wrapper will contain a method with the original signature:
HRESULT IWrappedInterface::MyMethod( long* OutParam1, long* OutParam2 );
Only the last one [out]
parameter can be marked as [out, retval]
. [in, out]
parameters can't be marked as [retval]
.
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