Overloaded methods in COM have wrong vtable order #523
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raphlinus
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Nov 18, 2017
Puts overloaded COM methods in reverse order to follow MSVC ABI convention. Also changes the naming convention to use an underscore and a 1-based number for all overrides but the first, from the current convention which is not consistent. Tested with animations using the DirectComposition interfaces, which are most of the changes. The main other cluster is font management in DirectWrite. Fixes retep998#523.
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Apparently the COM-compatible ABI for overloaded methods is to group overloads together but put them in reverse order. I observed this in my use of DirectComposition APIs, but it seems to be pretty poorly documented. The best I could find is gcc bug 53398.
Note that Microsoft has published guidance discouraging overloads in COM-visible interfaces but seems to be ignoring this more recently. I believe this renders C bindings inoperable, but that's a separate question.
Also note the previous link suggests a naming scheme of
MethodNameMethodName_2MethodName_3. I personally prefer this over the current scheme ofMN1MN2, especially for methods that end in a digit, for exampleIDCompositionArithmeticCompositeEffect::SetCoefficient1. Also note that the 1-based numbering is not consistently applied, for example the overloaded methods in dwrite_3.rs use aMNMN1MN2convention.I'll prepare a patch that applies the
MNMN_2MN_3convention because I think it's best, but am open to other alternatives. It seems unlikely that we'll be breaking many existing users.The text was updated successfully, but these errors were encountered: