Suppose you want to be able to detect in C++ whether a type has been defined. For example, maybe you want to use a type if it exists. This can happen if, say, you are a library like React Native for Windows, and you need to be able to run with different versions of the Windows SDK. Or you’re writing a library where the client can customize the behavior by defining another class with a well-known name. Perhaps you’re trying to mimic C# partial classes.

The road to madness.

https://devblogs.microsoft.com/oldnewthing/20190709-00/?p=102671

https://devblogs.microsoft.com/oldnewthing/20190710-00/?p=102678

https://devblogs.microsoft.com/oldnewthing/20190711-00/?p=102682