#Call a std::tuple::get<T> = value for each (T value) in Ts... values

79 messages · Page 1 of 1 (latest)

golden jungle
# 
#include <array>
#include <tuple>
#include <stack>
#include <stdexcept>

template <size_t N, typename... Ts>
class StructOfArrays
{
  private:
    std::stack<size_t> free_handles_;
    std::tuple<std::array<Ts, N>...> arrays_;

  public:
    StructOfArrays()
    {
        for (std::size_t i = 0; i < N; i++)
        {
            free_handles_.push(N - 1 - i); // Push in reverse order
        }
    }

    template <typename T>
    T& GetValue(size_t handle) const
    {
        return std::get<std::array<T, N>>(arrays_)[handle];
    }
 
    size_t Insert(const Ts&... values)
    {
        if (free_handles_.empty())
        {
            throw std::runtime_error("StructOfArrays is full!");
        }
        size_t handle = free_handles_.top();
        free_handles_.pop();

        std::get<std::array<T, N>>...(arrays_)[handle] = Ts... values; // this line is the issue here that I need help with
    }

    template <typename T>
    void Remove(size_t handle)
    {
        // .. something
        free_handles_.push(handle);
    }

};
fickle rootBOT
#

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agile zealot
#

what are you trying to do here

golden jungle
#

lets see, I'm trying do my implementation of the StructOfArrays data structure

#

and

agile zealot
#

why

golden jungle
#

physics engine

agile zealot
#

no i know

#

im asking why you're making a struct for this / helper

golden jungle
#

I'm so close!

#

huh?

agile zealot
#

this is one of those things that ime just make things harder to work with / reason about over enough time

#

like, you're getting rid of array semantics here for example

#

there is no such thing as inserting into an array in C++

#

the value just exists

golden jungle
#

right, im not inserting into an array, im inserting into the StructOfArrays.. it handles that for us and give us back the index

agile zealot
#

yes

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and that's arguably not array

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that's a vector

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but it's besides the point, what i'm trying to get across is that, this is not an abstraction that is "worth it" to do

golden jungle
#

why is that

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dont i get to squash all my data of the same type together to save memory, and then it takes advantage of SIMD caching or something something CPU things

agile zealot
#

SoA vs AoS is a "weird" concept to reason about

#

because we get into cpu caching and prefetching / memory access patterns

#

and what you're doing here could be hurting it, im not too sure what code it compiles to

golden jungle
#

well my current way is just unordered maps with UUIDs

agile zealot
#

like, it's not much harder to just write

struct Objects {
  std::array<Position, 256> positions;
///  ...
};```
#

and that's going to be infinitely more clearer about intent

agile zealot
golden jungle
#

whats the profiler

agile zealot
#

well it seems like you're doing this for speed / memory savings ^^

#

I would hope you're not just guessing

golden jungle
#

totally guessing,

agile zealot
#

you shouldn't do that :p

golden jungle
#

someone said it was faster! Andrew Kelley's talk was really convincing

agile zealot
#

dont listen to other people when it comes to performance saying X is faster than Y

#

i don't care if god himself is saying it

golden jungle
#

but thats what youre saying

#

!

agile zealot
#

there is never a single X > Y that exists

#

im not talking about performance ^^

#

i'm talking about code maintiability / expressiveness (was, technically, but i digress)

golden jungle
#

but isnt this a well established desgin architecture for HPC like physics enginse

agile zealot
#

you know what's well established in HPC?

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no one knows anything and no one can guess how a system is going to interact

#

so you use a profiler to figure out what is slow and what isn't

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is there some level of common knowledge of "this is probably faster" yes, but it's not useful for you to act that way

golden jungle
#

meow meow alright.... I promise not to use it if you show me how to fix it, cause im just interested in how to unpack it properly at this point

agile zealot
#

there is no single truth of what is faster and isn't lel

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except one thing

golden jungle
#

something about folding? right

agile zealot
#

which is, more work takes more time than doing less work

#

but that's it

golden jungle
#

yes sir

agile zealot
#

my blog post is still in progress 😭

image.png
golden jungle
#

😮

agile zealot
#

ugh i need to take the time to properly finish this later

#

anyways

#

one second

#

https://godbolt.org/z/4Y86hhMfG @golden jungle

Compiler Explorer - C++

template <size_t N, typename... Ts>
class StructOfArrays
{
private:
std::stack<size_t> freehandles;
std::tuple<std::array<Ts, N>...> arrays_;

template &lt;std::size_t... Is&gt;
void insert_impl(size_t handle, const Ts&amp;... values, std::index_sequence&lt;Is...&gt;)
{
    ((std::get&lt;Is&gt;(...
golden jungle
#

its a build failed no?

agile zealot
#

you should read the error ^^

golden jungle
#

undefined reference to `main'

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? that one

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im just a junior

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trying to learn things

agile zealot
#

well, try to reason about the error

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what type of error is it for example

#

how would you fix it?

golden jungle
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you are just missing a main function no?

agile zealot
#

you tell me

#

can't give you all the answers :p

golden jungle
#

I fixed it ?

agile zealot
#

did you fix it

golden jungle
#

added a main function

#

!solved

fickle rootBOT
#

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magic cloak
#

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