shap@polya.Stanford.EDU (Jonathan S. Shapiro) (10/05/88)
Sorry about changeing the title, but my machine went to lunch when I tried to followup. Someone mad the absurd assertion that the public part of a class could be generated from the private part. Not only is this not true in C++; it is not true in Ada, Modula-2, or any other language he said it was true in. The issue of data hiding in C++ is this: The language is interested in providing an abstraction for compilation without giving up performance. The fact that this has unfortunate header dependencies is considered less important. There are two good reasons *not* to make the privaate part pointed-to from the standpoint of language design: 1. It adds tremendous cost. Private members in practice tend to be private because they are per-object state. As such, the object tends to make heavy use of them, so the indirection cost is very significant. 2. When the size of private/protected components is unknown, the implementation of inheritance, especially multiple inheritance, becomes significantly more difficult. From the standpoint of customer environment, there are two compatibility concerns that *compel* making the private stuff part of the class: 1. C++ was designed to be C-linkable. Changing the structure model would have broken this. 2. C++ was designed to be implementable on *existing* technology in the interest of portability and speed of propagation. A library mechanism could not be assumed, which certainly precludes this sort of separation - at least if one is concerned with consistency. Jon