问题描述
有没有办法提取枚举中的元素数量?
Is there a way to extract the number of elements in an enum?
简单示例(使用虚构的 number_of_elements
方法):
Simple example (with imaginary number_of_elements
method):
enum FooBar { A = 0, B, C, };
println!("Number of items: {}", FooBar.number_of_elements());
// "Number of items: 3"
在 C 中,我通常会...
In C I'd normally do...
enum FooBar { A = 0, B, C, };
#define FOOBAR_NUMBER_OF_ITEMS (C + 1)
然而,与此等效的 Rust 不起作用:
However the Rust equivalent to this doesn't work:
enum FooBar { A = 0, B, C, };
const FOOBAR_NUMBER_OF_ITEMS: usize = (C as usize) + 1;
// Raises an error:
// unimplemented constant expression: enum variants
在枚举中包含最后一项非常不方便,因为如果没有考虑到所有成员,匹配的枚举会出错.
Including the last item in the enum is very inconvenient because matching enums will error if all members aren't accounted for.
enum FooBar { A = 0, B, C, FOOBAR_NUMBER_OF_ITEMS, };
有没有办法将枚举中的项目数作为常量值?
Is there a way to get the number of items in an enum as a constant value?
注意:尽管这与问题没有直接关系,但我想要此功能的原因是我正在使用 builder-pattern 构建一系列仅运行一次才有意义的操作.出于这个原因,我可以使用一个固定大小的数组,即枚举的大小.
Note: even though this isn't directly related to the question, the reason I was wanting this feature is I'm using the builder-pattern to construct a series of actions which only make sense to run once. For this reason I can use a fixed size array the size of the enum.
推荐答案
您可以使用 程序宏:
extern crate proc_macro;
extern crate syn;
#[macro_use]
extern crate quote;
use proc_macro::TokenStream;
#[proc_macro_derive(EnumVariantCount)]
pub fn derive_enum_variant_count(input: TokenStream) -> TokenStream {
let syn_item: syn::DeriveInput = syn::parse(input).unwrap();
let len = match syn_item.data {
syn::Data::Enum(enum_item) => enum_item.variants.len(),
_ => panic!("EnumVariantCount only works on Enums"),
};
let expanded = quote! {
const LENGTH: usize = #len;
};
expanded.into()
}
留给读者练习,以确保可以在同一模块中多次使用此派生宏.
It is left as an excercise to the reader to ensure that this derive macro can be used multiple times within the same module.
要使用宏,只需将 #[derive(EnumVariantCount)]
附加到您的枚举.现在应该有一个名为 LENGTH
的全局常量.
To use the macro, just attach #[derive(EnumVariantCount)]
to your enum. There should now be a global constant named LENGTH
.
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