[adrian_b]

While the C language has a lot of bad implicit casts that should have never been allowed, mainly those involving unsigned types, and which have been inherited by its derivatives, implicit casts as a programming language feature are extremely useful when used in the right way.

Implicit casts are the only reason for the existence of the object-oriented programming languages, where any object can be implicitly cast to any type from which it inherits, so it can be passed as an argument to any function that expects an argument of that type, including member functions.

The whole purpose of inheritance is to allow the programmer to use implicit casts. Otherwise, one would just declare a structure member of the class from which one would inherit in the OOP style and a virtual function table pointer, and one could write an identical program with the OOP program, but in a much more verbose way.

(In the C language, not only the implicit mixed signed-unsigned casts are bad, but also any implicit unsigned-unsigned casts are bad, because there are 2 interpretations of "unsigned" frequently used in programs, as either non-negative numbers or as modular numbers, and the direction of the casts that do not lose information is reversed for the 2 interpretations, i.e. for non-negative numbers it is safe to cast only to a wider type, but for modular numbers it is safe to cast only to a narrower type. Moreover, there are also other interpretations of "unsigned", i.e. as binary polynomials or as binary polynomial residues, which cannot be inter-converted with numbers. For all these 4 interpretations, there are distinct machine instructions in the instruction sets of popular CPUs, e.g. in the x86-64 and Aarch64 ISAs, which may be used in C programs through compiler intrinsics. Even worse is that the latest C standards specify that the overflow behavior of "unsigned" is that of modular numbers, while the implicit casts of "unsigned" are those of non-negative numbers. This inconsistency guarantees the existence of perfectly legal C programs, without any undefined behavior, but which nonetheless compute incorrect "unsigned" values, regardless which interpretation was intended for "unsigned".)

[alexvitkov]

> Otherwise, one would just declare a structure member of the class from which one would inherit in the OOP style and a virtual function table pointer, and one could write an identical program with the OOP program, but in a much more verbose way.

No, you don't have to do that. Once you start thinking about memory and manually managing it, it you'll figure out there's simpler, better ways to structure your program, rather than having a deep class hierarchy with a gazillion heap-allocated objects, each with distinct lifetime, all pointing at each other.

Here's a trivial example. Say you're writing a JSON parser - if you approach it with an OOP mindset, you would probably make a JSONValue class, maybe subclass it with JSONNumber/String/Object/Array. You would walk over the input string and heap allocate JSONValues as you go. The problems with this are:

    1. Each allocation can be very slow as it can enter the kernel
    2. Each allocation is a possible failure point, so the number of failure points scales linearly with input size.
    3. When you free the structure, you must walk over the entire tree and free each obejct one by one.
    4. The output of this function is suboptimal as the memory allocator can return values that are far away in memory.

There's an alternate approach that solves all these problems. If you're thinking about the lifetimes of your data, you would notice that this entire data structure is used and discarded at once, so you allocate a single big buffer for all the nodes. You keep a pointer to the head of that buffer, and when you need a new node, you stick it in there and advance the pointer by its size. When you're done you return the first node, which also happens to be the start of the buffer.

Now you have a single point of failure - the buffer allocation, your program is way faster, you only need to free one thing when you're done, and your values are tightly packed in memory, so whatever is using its output will be faster as well. You've spent just a little time thinking about memory and now you have a vastly superior program in every single aspect, and you're happy.

[usrnm]

Memory arenas are a nice concept but I wouldn't say they're necessarily an improvement in every possible situation. They increase complexity, make reasoning about the code and lifetimes harder and can lead to very nasty memory bugs. Definitely something to use with caution and not just blindly by default.

[alexvitkov]

Reasoning about the lifetimes of objects in an arena is as simple as it gets - there's only one lifetime, and pointers between everything allocated on the arena are perfectly safe. The complexity of figuring out what's going on, with with respect to the number of objects and links between is O(1).

There's no universal "God pattern" that you can throw at every problem. I used arenas as an example as I didn't want to write a zero-substance "OOP bad" post, but my point wasn't that instead of always using OOP+inheritance you should always use an arena, it was that if you think about your memory, more often than not there's a vastly superior layout than a bunch of heap objects glued together by prayers and smart pointers.

[usrnm]

That's all nice and fun until you want to pass stuff around and some objects might outlive the arena. Do you keep the whole arena around, do you copy, do you forget to do anything at all and spend a few days debugging weird memory bugs in prod?

[AlotOfReading]

"Non-negative" unsigneds can be validly cast to smaller types. That's why saturating_cast() exists. There are modular numbers where casting to a smaller value is likewise unsafe at a logical level. Your LCRNG won't give you the right period when downcast, even if the modulus value is unchanged.

[knome]

inheritance isn't required for object oriented programming. the primary facet of oop is hiding implementation details behind functions that manipulate that data.

adding values to a dict via add() and removing them via remove() should not expose to the caller if the underlying implementation is an array of hash indexed linked lists or what. the implementation can be changed safely.

inheritance is orthogonal to object orientation. or rather, inheritance requires oop, but oop does not require inheritance.

golang lacks inheritance while remaining oop, for instance, instead using interfaces that allows any type implicitly defining the specified interface to be used the.

[adrian_b]

"Hiding implementation details" means the same as "hiding the actual data type of an object", which means the same as "performing an implicit cast whenever the object is passed as an argument to a function".

Using different words does not necessarily designate different things. Most things that are promoted at a certain time by fashions, like OOP, abuse terminology by giving new names to old things in the attempt of appearing more revolutionary than they really are.

Most classic works about OOP define OOP by the use of inheritance and of virtual functions a.k.a. dynamic polymorphism. Both features have been introduced by SIMULA 67 and popularized by Smalltalk, the grandparents of all OOP languages.

When these 2 features are removed, what remains from OOP are the so-called abstract data types, like in CLU or Alphard, where you have data types that are defined by the list of functions that can process values of that type, but without inheritance and with only static polymorphism (a.k.a. overloading).

The example given by you for hiding an implementation is not OOP, but it is just the plain use of modules, like in the early versions of Ada, Mesa or Modula, which did not have any OOP features, but they had modules, which can export types or functions whose implementations are hidden.

Because all 3 programming language concepts, modules, abstract data types and OOP have as an important goal preventing the access to implementation details, there is some overlap between them, but they are nonetheless distinct enough so that they should not be confused.

Modules are the most general mechanism for hiding implementation details, so they should have been included in any programming language, but the authors of most OOP languages, especially in the past, have believed that the hiding provided by granting access to private structure a.k.a. class members only to member functions is good enough for this purpose. However this leads sometimes to awkward programs where some classes are defined only for the purpose of hiding things, for which real modules would have been more convenient, so many more recent versions of OOP languages have added modules in some form or another.

[knome]

I'll readily admit the languages were marketed that way, but would argue inheritance was a functional, but poor, imitation of dynamic message dispatch. Interfaces, structural typing, or even simply swapping out object types in a language with dynamic types does better for enabling function-based message passing than inheritance does, as they avoid the myriad pitfalls and limitations associated with the technique.

Dynamic dispatch can be accomplished in any language with a function type by using a structure full of functions to dispatch the incoming invocations, as Linux does in C to implement its file systems.

[igouy]

1994 "Object-Oriented Programming in Oberon-2" seems to cover what you discuss.

https://ssw.jku.at/Research/Books/Oberon2.pdf

[uecker]

I am actually ok with the conversions and C and think they are quite convenient. Unsigned in C is modular. I am not sure what you mean by the "latest C standards specify". This did not change. I also do not understand what you mean by the "implicit cast of unsigned are those of non-negative numbers". This seems wrong. If you convert to a larger unsigned type, the value is unchanged and if you convert to a smaller, it is reduced modulo.

[adrian_b]

In older C standards, the overflow of unsigned numbers was undefined.

In recent C standards, it has been defined that unsigned numbers behave with respect to the arithmetic operations as modular numbers, which never overflow.

The implicit casts of C unsigned numbers are from narrower to wider types, e.g. from "unsigned short" to "unsigned" or from "unsigned" to "unsigned long".

These implicit casts are correct for non-negative numbers, because all values that can be represented as e.g. "unsigned short" are included among those represented by "unsigned" and they are preserved by the implicit casts.

However, these implicit casts are incorrect for modular numbers, because they attempt to compute the inverse of a non-invertible function.

For instance, if you have an "unsigned char" that is a modular number with the value "3", it is incorrect to convert it to an "unsigned short" modular number with the value "3", because the same "unsigned char" "3" corresponds also to 255 other "unsigned short" values, i.e. to 259, 515, 781, 1027 and so on.

If you have some very weird reason when you want to convert a number modulo 256 to a number modulo 65536 by choosing a certain number among those with the same residue modulo 256, then you must do this explicitly, because it is not an information-preserving conversion.

If on the other hand you interpret a C "unsigned" as a non-negative number, then the implicit casts are OK, but you must add everywhere explicit checks for unsigned overflow around the arithmetic operations, otherwise you will obtain erroneous results.

[uecker]

The C89 standard has "A computation involving unsigned operands can never overflou. because a result that cannot be represented b! the resulting unsigned integer type is reduced modulo the number that is one greater thnn the largest value that can be represented by the resulting unsipned integer type" (OCR errors) You can finde a copy here: https://web.archive.org/web/20200909074736if_/https://www.pd...

Mathematically, there is no clearly defined way how one would have to map from one residue system in modular arithmetic to the next, so there is no "correct" or "incorrect" way. Mapping to the smallest integer in the equivalency class makes a lot of sense though, as it maps corresponding integers to itself when going to a larger type and and the reverse operation is then the inverse, and this is exactly what C does.