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0 pointscycloptic5y ago0 comments

Can you explain how this is a problem in modern C++? I was under the impression that all the STL containers (string, vector, list, map, etc.) worked the same and have an allocator parameter. Are there other areas where these are missing? Or is the issue that STL implementations almost always default to an allocator that uses malloc? I'm not trying to dog on Zig here (it's a nice little language) but this just doesn't seem to be something that only Zig can do.

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11 comments · 3 top-level

mwkaufma5y ago· 4 in thread

Many core modern C++ types don't permit customizing the allocator. E.g. std::function

mwkaufma5y ago

Furthermore, C++ dependencies commonly instantiate types like std::vector with the default allocator internally, rather than exposing it to the host application.

cyclopticOP5y ago

Thank you for the examples. I'm not sure std::function is a good comparison. After some research it seems this used to be in the spec, but it was removed because nobody supported it correctly and it seems it was too difficult to do it in a type-safe manner anyway: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p030...

The other thing is that Zig doesn't seem to have any real plans to support C++-style closures right now. If they ever find a type-safe way to do it while supporting custom allocators, then that would be interesting, but at the moment I wouldn't say it's any better than C++ in this regard.

I actually have seen some C/C++ libraries that do allow changing the default allocator although it's usually only low-level libraries that bother to do this.

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tlb5y ago

However, most std::functions have a small built-in buffer for captured variables, like 4 pointers worth. If you limit yourself to only capturing that many, there's no allocation.

mwkaufma5y ago

I'm not saying that the default allocation strategy isn't good (in the general case), just that it's not customizable (for special needs).

voldacar5y ago· 3 in thread

It isn't really a matter of can do/cannot do. It's more about the default patterns promoted by the idiomatic way of writing code. Yeah you could write C++ code that constantly passes around allocators while also using STL heavily, but it will be verbose, unnatural, and ugly.

As well as having nicer syntax in general and real metaprogramming instead of the brain damage that is templates, zig promotes this kind of allocator-aware programming style in a way that's clean and idiomatic

cyclopticOP5y ago

I'm not sure what you mean that it's verbose, unnatural and ugly. To me it looks the same.

In C++, you have to pass around an allocator to your templates. You can typedef this away if you want.

In Zig, you have to pass around an allocator as a function argument or a struct member. You can comptime this away if you want.

Is there some fundamental way that I missed that Zig changes this? If your actual complaint is that C++ templates are bad and you're saying Zig comptime is better, that's different than having woes about allocators.

fsociety5y ago

The difference is you have to pass an allocator to the standard library functions in Zig. That’s why it is idiomatic compared to C++.

voldacar5y ago

have fun trying to globally override new and delete in C++

(hint: there is no way to do this)

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logicchains5y ago· 1 in thread

A concrete example is std::stable_sort. As far as I'm aware there's no way to pass it a custom allocator/buffer to avoid it allocating memory.

cyclopticOP5y ago

Thank you for the example.

j / k navigate · click thread line to collapse

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11 comments · 3 top-level

mwkaufma5y ago· 4 in thread

Many core modern C++ types don't permit customizing the allocator. E.g. std::function

mwkaufma5y ago

Furthermore, C++ dependencies commonly instantiate types like std::vector with the default allocator internally, rather than exposing it to the host application.

cyclopticOP5y ago

I actually have seen some C/C++ libraries that do allow changing the default allocator although it's usually only low-level libraries that bother to do this.

1 more reply

tlb5y ago

However, most std::functions have a small built-in buffer for captured variables, like 4 pointers worth. If you limit yourself to only capturing that many, there's no allocation.

mwkaufma5y ago

I'm not saying that the default allocation strategy isn't good (in the general case), just that it's not customizable (for special needs).

voldacar5y ago· 3 in thread

cyclopticOP5y ago

I'm not sure what you mean that it's verbose, unnatural and ugly. To me it looks the same.

In C++, you have to pass around an allocator to your templates. You can typedef this away if you want.

In Zig, you have to pass around an allocator as a function argument or a struct member. You can comptime this away if you want.

fsociety5y ago

The difference is you have to pass an allocator to the standard library functions in Zig. That’s why it is idiomatic compared to C++.

voldacar5y ago

have fun trying to globally override new and delete in C++

(hint: there is no way to do this)

1 more reply

logicchains5y ago· 1 in thread

A concrete example is std::stable_sort. As far as I'm aware there's no way to pass it a custom allocator/buffer to avoid it allocating memory.

cyclopticOP5y ago

Thank you for the example.

j / k navigate · click thread line to collapse