我可以擁有可以從切片和擁有緩衝區構建的結構嗎？

Question

我會盡量做到儘可能清楚。假設我有：

struct Foo<'a> { 
    buffer: &'a [u8], 
} 

impl<'a> Foo<'a> { 
    fn create_builder() -> FooBuilder { 
     FooBuilder::new() 
    } 

    fn from_slice(slice: &[u8]) -> Foo { 
     Foo { buffer: slice } 
    } 
} 

struct FooBuilder { 
    in_progress_buffer: Vec<u8>, 
} 

impl FooBuilder { 
    fn new() -> FooBuilder { 
     FooBuilder { in_progress_buffer: Vec::new() } 
    } 

    fn push(&mut self, item: u8) { 
     self.in_progress_buffer.push(item); 
    } 

    fn build_foo(self) -> Foo { 
     Foo { buffer: self.in_progress_buffer } 
    } 
} 

fn main() { 
    // Option1: Gradually construct Foo from FooBuilder 
    let mut foo_builder = FooBuilder::new(); 
    foo_builder.push(7); 
    let foo = foo_builder.build_foo(); 

    // Option2: Construct Foo from a slice 
    let v = vec![7]; 
    let foo2 = Foo::from_slice(&v); 
} 

這給出了一個編譯錯誤：

error[E0106]: missing lifetime specifier 
    --> src/main.rs:28:27 
    | 
28 |  fn build_foo(self) -> Foo { 
    |       ^^^ expected lifetime parameter 
    | 
    = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from 
    = help: consider giving it a 'static lifetime 

是這種模式可能嗎？我如何解決編譯錯誤？ 我不知道給什麼一生符，因爲在FooBuilder版本中，FooBuilder擁有緩衝區，我不想強​​迫Foo用戶保持FooBuilder範圍的整個期間Foo用於

Answer 1

您可以使用std::borrow::Cow;作爲docs狀態：

it can enclose and provide immutable access to borrowed data, and clone the data lazily when mutation or ownership is required

use std::borrow::Cow; 

struct Foo<'a> { 
    buffer: Cow<'a, [u8]>, 
} 

impl<'a> Foo<'a> { 
    fn create_builder() -> FooBuilder { 
     FooBuilder::new() 
    } 

    fn from_slice(slice: &[u8]) -> Foo { 
     Foo { buffer: slice.into() } // note .into() 
    } 
} 

struct FooBuilder { 
    in_progress_buffer: Vec<u8>, 
} 

impl<'a> FooBuilder { 
    fn new() -> FooBuilder { 
     FooBuilder { in_progress_buffer: Vec::new() } 
    } 

    fn push(&mut self, item: u8) { 
     self.in_progress_buffer.push(item); 
    } 

    fn build_foo(self) -> Foo<'a> { 
      Foo { buffer: self.in_progress_buffer.into() } // note .into() 
    } 
} 

此外，您將需要foo_builder可變以便能夠對其執行push。