4 ms·

The distinction matters, because the multiple inheritance in common lisp (and other linearizing languages) is a lot saner and easier to use than multiple inheri

by tetha 17y ago

The distinction matters, because the multiple inheritance in common lisp (and other linearizing languages) is a lot saner and easier to use than multiple inheritance where the programmer has to define the method lookup sequence, which in term implies that multiple inheritance in itself is not the sole cause of complexity in this context, but rather the implementation of multiple inheritance in the given language (C++).

dkarl 17y ago
CLOS method dispatch does not meet the design criteria of C++. It uses runtime information about inheritance relationships, for example. Also, note that there is also a semantic difference here between C++ (and Java) and most other languages. In C++ multiple inheritance, if classes A and B define method foo, and A and B have no inheritance relationship, then A::foo and B::foo are entirely unrelated methods -- just as if they were named A::foo and B::bar. The fact that they have the same name does not automatically create a relationship between them, as it would in CLOS or in duck-typed languages like Python. That means that a call to foo in a class that inherits from A and B is just as ambiguous as if the programmer had typed "foo, or maybe bar." This unrelatedness is inherent to C++'s type system. It would make no sense to a C++ programmer to treat A::foo and B::foo as being alternative dispatch targets for the same method call. Even if you allow the compiler to gloss over this unrelatedness, I still think it is a matter of taste whether it is simpler to require programmer disambiguation or to have a well-defined algorithm for resolving ambiguous names. C++ usually takes heat for providing powerful more-than-meets-the-eye mechanisms that allow programmers to "hide" program semantics inside language features. Here C++ takes the opposite approach and requires the programmer to explicitly resolve ambiguous names, and it takes heat from a CLOS programmer, who could write an entire operating system using a single family of generic functions all having the same name ;-) Damned if you do, damned if you don't.
- jacquesm 17y ago
  > who could write an entire operating system using a single family of generic functions all having the same name ;-) I sense an upcoming challenge! ;)

by tetha 17y ago

dkarl 17y ago

CLOS method dispatch does not meet the design criteria of C++. It uses runtime information about inheritance relationships, for example. Also, note that there is also a semantic difference here between C++ (and Java) and most other languages. In C++ multiple inheritance, if classes A and B define method foo, and A and B have no inheritance relationship, then A::foo and B::foo are entirely unrelated methods -- just as if they were named A::foo and B::bar. The fact that they have the same name does not automatically create a relationship between them, as it would in CLOS or in duck-typed languages like Python. That means that a call to foo in a class that inherits from A and B is just as ambiguous as if the programmer had typed "foo, or maybe bar." This unrelatedness is inherent to C++'s type system. It would make no sense to a C++ programmer to treat A::foo and B::foo as being alternative dispatch targets for the same method call. Even if you allow the compiler to gloss over this unrelatedness, I still think it is a matter of taste whether it is simpler to require programmer disambiguation or to have a well-defined algorithm for resolving ambiguous names. C++ usually takes heat for providing powerful more-than-meets-the-eye mechanisms that allow programmers to "hide" program semantics inside language features. Here C++ takes the opposite approach and requires the programmer to explicitly resolve ambiguous names, and it takes heat from a CLOS programmer, who could write an entire operating system using a single family of generic functions all having the same name ;-) Damned if you do, damned if you don't.

jacquesm 17y ago
> who could write an entire operating system using a single family of generic functions all having the same name ;-) I sense an upcoming challenge! ;)