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[英]How can I have static dispatch for a linked list containing different types all implementing a trait?
[英]It is possible to store all types implementing a trait in a list and iterate over that list?
我為許多結構( A
, B
, C
等)實現了一個特征:
pub trait ApplicableFor: Debug + Default {
unsafe fn is_applicable_for(from: *mut u8) -> bool
where
Self: Sized;
}
我需要一個方法來找到哪個結構為此方法調用返回true。 我有代碼:
unsafe fn check_applicable<T: ApplicableFor>(from: *mut u8) -> bool {
T::is_applicable_for(from, to)
}
unsafe fn find_applicable(from: *mut u8) -> ApplicableFor {
if check_applicable::<A>(from) {
A::default()
} else if check_applicable::<B>(from) {
B::default()
} else if check_applicable::<C>(from) {
C::default()
} else {
panic!("Couldn't find appicable");
}
}
在實際的代碼中,我有大約20個結構,因此我想將它們存儲在某個地方,並使用如下代碼來提高可讀性:
unsafe fn find_applicable(from: *mut u8) -> ApplicableFor {
for T in list {
if check_applicable::<T>(from) {
T::default()
}
}
panic!("Couldn't find appicable");
}
我該怎么做或如何更好地重寫呢?
不,Rust不直接提供您需要的那種元編程功能。 即,類型不是存在的具體事物,也不可以放入集合中。
相反,您需要代碼生成。
從ApplicableFor
的簡化版本開始,我們可以編寫一個非常結構化的find_applicable
版本:
trait ApplicableFor {
fn is_applicable_for(from: u8) -> bool;
}
fn find_applicable(from: u8) {
if <A>::is_applicable_for(from) {
println!("Using {}", stringify!(A));
return;
}
if <B>::is_applicable_for(from) {
println!("Using {}", stringify!(B));
return;
}
if <C>::is_applicable_for(from) {
println!("Using {}", stringify!(C));
return;
}
panic!("Couldn't find any applicable types");
}
一旦建立了結構,就可以使用宏對其進行抽象:
fn find_applicable(from: u8) {
macro_rules! find_one {
($ty:ty) => {
if <$ty>::is_applicable_for(from) {
println!("Using {}", stringify!($ty));
return;
}
}
}
find_one!(A);
find_one!(B);
find_one!(C);
panic!("Couldn't find any applicable types");
}
如果我們想重復這種“為這種類型的列表做些事情”的概念怎么辦? 另一個宏:
macro_rules! each_type {
($one_type_macro:tt) => {
$one_type_macro!(A);
$one_type_macro!(B);
$one_type_macro!(C);
};
}
fn find_applicable(from: u8) {
macro_rules! find_one {
($ty:ty) => {
if <$ty>::is_applicable_for(from) {
println!("Using {}", stringify!($ty));
return;
}
}
}
each_type!(find_one);
panic!("Couldn't find any applicable types");
}
對each_type!
的實現來說太each_type!
? 創建一個宏,該宏創建另一個將與另一個宏一起調用的宏:
macro_rules! gen_each_type {
($($ty:ty),*) => {
macro_rules! each_type {
($one_type_macro:tt) => {
$($one_type_macro!($ty);)*
};
}
};
}
gen_each_type![A, B, C];
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