| --- Log opened Thu Sep 24 00:00:40 2020 |
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| 02:03 | | * McMartin sets a small fraction of his soul on fire |
| 02:04 | | * McMartin has now successfully constructed code that makes use of std::enable_if. |
| 02:08 | <~Vornicus> | what does that do |
| 02:09 | <&McMartin> | terrible things |
| 02:10 | <~Vornicus> | well, yes, I mean, it's C++ |
| 02:10 | <&McMartin> | The only answer I can put into words right now is "why would you use it", and the answer is "if you have a class defined in terms of another class, and you want certain methods to exist if and only if a compile-time expression is true" |
| 02:10 | <&McMartin> | There's a suite of other things with names like std::is_default_constructible<T> that are intended to be used in concert with it. |
| 02:15 | <&McMartin> | What it *does* is end up specifying a type that will or will not compile depending on its template arguments, and then rely on the "Substitution Failure Is Not An Error" property of C++ template expansion to essentially selectively delete definitions from different expansions of the same template. |
| 02:16 | <&McMartin> | And I've now built up enough understanding of what's going on to actually make it do a thing I intended it to do. |
| 02:16 | <~Vornicus> | so like -- hang on let me see here |
| 02:16 | <&McMartin> | This is, like, the dark mirror of finally understanding how call/cc works |
| 02:17 | <&McMartin> | Also, "subsitution failure is not an error" is usually abbreviated SFINAE |
| 02:17 | <&McMartin> | https://pbs.twimg.com/media/EeqLiY3XkAA4aK3?format=jpg&name=small |
| 02:20 | <~Vornicus> | okay: I reimplement glsl's basic types, which include a variety of vector types that vary based on the stored type - vec3 is 3 floats, ivec2 is 2 integers, bvec4 is 4 booleans. Most operators exist but are restricted to the vector types whose underlying scalar type they would normally work on; I could use enable_if to specify ivec and vec both as subclasses of this big nasty template and then ivec will have the bit operators enabled |
| 02:21 | <~Vornicus> | which I do grant sounds like a terrible idea |
| 02:23 | <~Vornicus> | or alternatively I could do something with it to come up with the swizzles based on the dimensionality of the thing or something |
| 02:25 | <&McMartin> | You don't need to mess with template metaprogramming for the latter |
| 02:26 | <&McMartin> | You can hand-override specific types, so if the dimensionality were part of the type (as a template argument Vec<int, 4> or whatever) then you could write specific implementations for various values of that integer. |
| 02:26 | <&McMartin> | Or! |
| 02:26 | <&McMartin> | You could just define individual functions for this like a human being from the planet Earth. |
| 02:27 | <&McMartin> | Now, the thing I was doing in *this* case involved selectively making a default constructor appear, which turns out to be more difficult than most usages of it |
| 02:28 | <&McMartin> | But the real thing I was doing was "wrapping one of the fields", and so what I *really* want to do is just say "whatever you're wrapping, take any constructor arguments it does and pass it on to them" |
| 02:28 | <&McMartin> | C++11 added approximately a vigintillion corner cases to this, and there are some spells you can use to manage it, but I haven't mastered them yet. |
| 02:29 | <&McMartin> | But that means the solution involves the use of variadic templates and then also a thing that isn't actually a thing the compiler knows exists, but when reasoning about the compiler, is called a "universal reference" or "forwarding" reference. |
| 02:29 | <&McMartin> | As a special case of the thing that the compiler *does* know about, which are "rvalue references" |
| 02:30 | <&McMartin> | Which the compiler uses to distinguish copy from move operations |
| 02:30 | <&McMartin> | ... and this is why you want to use Rust instead if you actually care about this nonsense, but I seek forbidden knowledge this week, I guess |
| 02:37 | <~Vornicus> | that's a lot of fucking corner cases |
| 02:38 | <&McMartin> | I'm using one of the standard companion works regarding "stuff that was added to C++ in 2011" |
| 02:39 | <&McMartin> | Two chapters summing up to a hundred pages cover "so, you're going to have to actually understand how template expansion interoperates with other things, and what types ultimately end up actually getting deduced" |
| 02:40 | <&McMartin> | This for a feature whose ideal endpoint is supposed to be "this is invisible outside of the standard library, but we're exposing it so you can do similar things" |
| 02:45 | <&McMartin> | Now that I dig into it though, it turns out that the thing I want is a thing that is poorly specified because it ends up usurping the copy operations that the compiler does under the hood while passing arguments to functions and such |
| 02:46 | <&McMartin> | Good times |
| 02:46 | <&McMartin> | Well, that's an even better reason to burn down this whole concept. |
| 02:49 | <~Vornicus> | cute |
| 02:52 | <&McMartin> | https://pastebin.com/8wphZgt1 |
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| --- Log closed Fri Sep 25 00:00:41 2020 |