[alexchamberlain]

Point 1 needs expanding... Assuming that you only need to access MxStringBuffer via a pointer, you don't need to include it in the header. However, this is somewhat deceptive and doesn't self-document. I don't think there is much advantage to it at all.

Edit: In fact, you must need the declaration to access the internals.

[haberman]

Structures that are kept opaque both enforce encapsulation and give binary/ABI compatibility for free. These are two very large advantages. The documentation of a type should come from the supported operations you can perform on it, not the way it represents its data internally.

[ExpiredLink]

When you put the definition of _MxStringBuffer into the *.c file you can "access the internals". The advantage of forward declaration is encapsulation.