Auto vs. Private Types: Why the Discrepancy in Variable Declaration?

Auto and Private Types: Why the Contradiction?

In the provided code, using auto to declare a variable b that receives the return value of f.Baz() succeeds, while using the explicit type Foo::Bar results in an error. This inconsistency has puzzled many developers.

The rules for auto deduction generally follow those of template type deduction. This means that, just as you can pass objects of private types to template functions, you can also declare variables of those types using auto.

The reason for this lies in the fact that, while the name of a private type is inaccessible, the type itself remains usable. This is evident in the ability to return objects of private types to client code. The compiler can deduce the type of the returned value, even if the name of the type is not exposed.

In the case of auto, the compiler performs type deduction based on the expression it follows. Since f.Baz() returns an object of type Foo::Bar, the compiler assigns auto the type Foo::Bar. However, since the name Foo::Bar is private, the compiler cannot create a variable of that type directly. Instead, it uses an anonymous type that has the same members and properties as Foo::Bar. This allows b to access the i member, even though the explicit type name is not accessible.

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