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It's more like, "I don't see why a tastier dinner has to be more expensive". It's a tradeoff situation, and while I might be willing to buy a great $50+ meal fr

It's more like, "I don't see why a tastier dinner has to be more expensive". It's a tradeoff situation, and while I might be willing to buy a great $50+ meal from time to time instead of just a $20 one, I don't have any inclination to pay $1000 even if the meal is a tiny little bit better than the just-great one. It's a matter of tradeoffs. Library design is a balancing act. > We need generics because users have types that they need libraries to work with. This is right where I become sceptical. Most libraries shouldn't care for the user's types at all. They should expose their own types so you can work the library - not the other way around. Please provide an actual use case where the library has to "know" about the user's types (and please don't mention std::sort. It's as common an example as is "Dog :: Animal" examples to argue for class inheritance, and is just as irrelevant). > Your own example, of an intrusively-linked list, illustrates this: without generics, you could not write a linked-list library component that would be usable for that. As I mentioned, linked list can tangentially benefit from a very very thing generics layer on top of a fixed data structure implementation. The layer does nothing more than "instanciate" the types, but there is no code generated. But then again, the added convenience / safety is minimal, I once wrote a C++ implementation that I was quite happy with, and never used it. > This is why C programs are crammed with so many custom one-off hash tables There are probably not huge issues with a design like "HashTable ht = { .hash = &foo_key_hash, .equals = &foo_key_equals }" (if quick results are what you're after), but yes it is nice to have a few lines of type-safe wrap generated over some generic container interface. An alternative reason why you'll find a good amount of custom hashtables in C code bases is, I suppose, that there are a lot of different ways to implement hash tables. Also writing a little code here might not be so bad. I've had 1 use case for a "hash" table in 2021 (glyph hash table for a new iteration of my font rendering code) and I've gotten away without even implementing it - the for-loop I've written has never shown up in any performance profile. Considering that container data structures are probably the highest profile application for generics, I'm still not convinced that generics are needed in a systems programming language...

ncmncm 5y ago
You miss the point. You can get more money for the dinner only if you deliver more. Your $20 meal delivers more than your $2 meal. If you can only deliver the $2 meal, you aren't getting $20 for it. I have already cited two separate examples, which you have studiously ignored. Look online and find them in their thousands. It is hard to find even one C++ library that is not made better by its ability to integrate with users' choices of types.
- jstimpfle 5y ago
  Updated to not ignore your examples.
tsimionescu 5y ago
> This is right where I become sceptical. Most libraries shouldn't care for the user's types at all. They should expose their own types so you can work the library - not the other way around. This is an utterly bizarre statement. Should people convert collections of Xs from library 1 to other kinds of collections of Ys to work with library 2? A very basic use case that shows up all the time everywhere, especially in systems programming, is that I have an array of items and I want to pass it to some library. But, to work with an array, you need to know the size of elements of the array, so that arr[x] knows how to compute the address of x. There are exactly 3 ways to achieve this: 1. The library only accepts arrays of some specific type (e.g. int[]). If you have some other type, you have to find some way of converting to the type known in the library - in O(n) time. 2. The library expects arrays of some fixed-size type that can hold any value (e.g. void*[]). This typically adds huge overhead, since now the elements of the array are not together in memory, they are scattered all over the place; and the library must fetch them from memory before doing anything with them. 3. Generics - the library accepts T[], and can easily do sizeof(T) to know how to access arr[x]. Even qsort is actually an example of C's convoluted support for generic arrays: you're explicitly passing the array base and its type (the number of bytes) to qsort. C's extremely basic type system anyway essentially identifies types with byte sizes in practice, so it's not obvious that qsort works like this (and of course, it's less safe, since instead of passing an int, you have to pass void* + sizeof(int), potentially getting them mixed up).
- jstimpfle 5y ago
  > A very basic use case that shows up all the time everywhere, especially in systems programming, is that I have an array of items and I want to pass it to some library. My thinking is this. Either a library function cares about what type of data you pass to it because it is designed to do some operation on it - then it will accept only a specific type, like "void func(Foo *foos, int count)". Or, the function does not care about the type of data but just wants to send it somewhere else. In this case, you can pass a void-pointer + size to it. Interfaces like sorting functions can be seen as exceptions under the umbrella term "container library". Like the qsort() example, usually all you need is a few metrics for the data type - size, maybe alignment - and at most a few "method calls" - equality, simple get/set operations - and that's it. For all I can say these situations are not very common in systems programming, at least not for what I do. This stuff tends to come up in application level programming. It's definitely very common in my Python scripts. When it does come up in systems occasionally, a vtable struct is a good way to deal with the situation generically. In the qsort() case, the function pointer can be passed directly. > 1. The library only accepts arrays of some specific type (e.g. int[]). If you have some other type, you have to find some way of converting to the type known in the library - in O(n) time. Considering how computers work, I don't think it is much of a constraint to assume that arrays are of the form "Foo *foos". Coincidentally C _does_ have builtin "generics" for arrays of things, and real machines do have assembly instructions to quickly compute addresses based on hardcoded element size. > (and of course, it's less safe, since instead of passing an int, you have to pass void* + sizeof(int), potentially getting them mixed up). There is little real chance of getting them mixed up. If you actually run the code you will notice.

ncmncm 5y ago

You miss the point. You can get more money for the dinner only if you deliver more. Your $20 meal delivers more than your $2 meal. If you can only deliver the $2 meal, you aren't getting $20 for it. I have already cited two separate examples, which you have studiously ignored. Look online and find them in their thousands. It is hard to find even one C++ library that is not made better by its ability to integrate with users' choices of types.

jstimpfle 5y ago
Updated to not ignore your examples.

tsimionescu 5y ago

> This is right where I become sceptical. Most libraries shouldn't care for the user's types at all. They should expose their own types so you can work the library - not the other way around. This is an utterly bizarre statement. Should people convert collections of Xs from library 1 to other kinds of collections of Ys to work with library 2? A very basic use case that shows up all the time everywhere, especially in systems programming, is that I have an array of items and I want to pass it to some library. But, to work with an array, you need to know the size of elements of the array, so that arr[x] knows how to compute the address of x. There are exactly 3 ways to achieve this: 1. The library only accepts arrays of some specific type (e.g. int[]). If you have some other type, you have to find some way of converting to the type known in the library - in O(n) time. 2. The library expects arrays of some fixed-size type that can hold any value (e.g. void*[]). This typically adds huge overhead, since now the elements of the array are not together in memory, they are scattered all over the place; and the library must fetch them from memory before doing anything with them. 3. Generics - the library accepts T[], and can easily do sizeof(T) to know how to access arr[x]. Even qsort is actually an example of C's convoluted support for generic arrays: you're explicitly passing the array base and its type (the number of bytes) to qsort. C's extremely basic type system anyway essentially identifies types with byte sizes in practice, so it's not obvious that qsort works like this (and of course, it's less safe, since instead of passing an int, you have to pass void* + sizeof(int), potentially getting them mixed up).

jstimpfle 5y ago
> A very basic use case that shows up all the time everywhere, especially in systems programming, is that I have an array of items and I want to pass it to some library. My thinking is this. Either a library function cares about what type of data you pass to it because it is designed to do some operation on it - then it will accept only a specific type, like "void func(Foo *foos, int count)". Or, the function does not care about the type of data but just wants to send it somewhere else. In this case, you can pass a void-pointer + size to it. Interfaces like sorting functions can be seen as exceptions under the umbrella term "container library". Like the qsort() example, usually all you need is a few metrics for the data type - size, maybe alignment - and at most a few "method calls" - equality, simple get/set operations - and that's it. For all I can say these situations are not very common in systems programming, at least not for what I do. This stuff tends to come up in application level programming. It's definitely very common in my Python scripts. When it does come up in systems occasionally, a vtable struct is a good way to deal with the situation generically. In the qsort() case, the function pointer can be passed directly. > 1. The library only accepts arrays of some specific type (e.g. int[]). If you have some other type, you have to find some way of converting to the type known in the library - in O(n) time. Considering how computers work, I don't think it is much of a constraint to assume that arrays are of the form "Foo *foos". Coincidentally C _does_ have builtin "generics" for arrays of things, and real machines do have assembly instructions to quickly compute addresses based on hardcoded element size. > (and of course, it's less safe, since instead of passing an int, you have to pass void* + sizeof(int), potentially getting them mixed up). There is little real chance of getting them mixed up. If you actually run the code you will notice.