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[cxx-interop] Add documentation about inferring SWIFT_SHARED_REFERENCE in c++ inheritance #1078

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[cxx-interop] Add documentation about inferring SWIFT_SHARED_REFERENCE in c++ inheritance #1078

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[cxx-interop] Add documentation about inferring SWIFT_SHARED_REFERENC…
fahadnayyar Jun 10, 2025
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Removed Swift class types from the text
fahadnayyar Jul 1, 2025
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fahadnayyar Jul 4, 2025
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added some more info
fahadnayyar Jul 4, 2025
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11 changes: 11 additions & 0 deletions documentation/cxx-interop/index.md
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Expand Up @@ -1255,6 +1255,17 @@ object.doSomething()
// `object` will be released here.
```

### Inference of Shared Reference behaviour in Derived Types
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TODO: ### -> ####


When a C++ type inherits from a `SWIFT_SHARED_REFERENCE` base type, the Swift compiler automatically infers `SWIFT_SHARED_REFERENCE` annotation for the derived type.
The derived type also gets imported as a reference type, and uses the same `retain` and `release` functions as its base class.
This inference works as long as all the annotated base types in the inheritance chain (including multiple or indirect inheritance) have the same `retain` and `release` functions.
If multiple base types have conflicting `retain` or `release` functions, the derived type is imported as a Swift value type, and the compiler emits a warning.


Note that this inference currently applies only to `SWIFT_SHARED_REFERENCE`.
It does not apply to types annotated with `SWIFT_IMMORTAL_REFERENCE` or `SWIFT_UNSAFE_REFERENCE`.

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@j-hui I changed some text based on your suggestions but added my flavor as well. How does it look now?

### Inheritance and Virtual Member Functions

Similar to value types, casting an instance of a derived reference type to a
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