lifetime

来源： https://stackoverflow.com/questions/61195410/why-cant-i-assign-one-dereference-of-a-reference-of-a-reference-to-another-when

问题 I'd like to implement a custom Iterator like below, but cannot solve reference problem. use itertools::Product; use std::ops::Range; struct Iter2DMut<'a, T: 'a> { data: &'a mut [T], shape: (usize, usize), idx_iter: Product<Range<usize>, Range<usize>>, } impl<'a, T: 'a> Iterator for Iter2DMut<'a, T> { type Item = &'a mut T; fn next(&mut self) -> Option<Self::Item> { if let Some((i, j)) = self.idx_iter.next() { Some(&mut self.data[i + self.shape.0 * j]) } else { None } } } and get the following

问题 I have a struct with a lifetime: struct HasLifetime<'a>( /* ... */ ); There is there is an implementation of the trait Foo : impl<'a, 'b: 'a> Foo for &'a mut HasLifetime<'b> { } I want to implement the following function: fn bar_to_foo<'a, 'b: 'a>(bar: &'a mut Lifetime<'b>) -> impl Foo { bar } This won't compile because the returned impl is only valid for 'a . However, specifying impl Foo + 'a results in: error[E0909]: hidden type for `impl Trait` captures lifetime that does not appear in

问题 I have a struct with a lifetime: struct HasLifetime<'a>( /* ... */ ); There is there is an implementation of the trait Foo : impl<'a, 'b: 'a> Foo for &'a mut HasLifetime<'b> { } I want to implement the following function: fn bar_to_foo<'a, 'b: 'a>(bar: &'a mut Lifetime<'b>) -> impl Foo { bar } This won't compile because the returned impl is only valid for 'a . However, specifying impl Foo + 'a results in: error[E0909]: hidden type for `impl Trait` captures lifetime that does not appear in

问题 I have a struct with a lifetime: struct HasLifetime<'a>( /* ... */ ); There is there is an implementation of the trait Foo : impl<'a, 'b: 'a> Foo for &'a mut HasLifetime<'b> { } I want to implement the following function: fn bar_to_foo<'a, 'b: 'a>(bar: &'a mut Lifetime<'b>) -> impl Foo { bar } This won't compile because the returned impl is only valid for 'a . However, specifying impl Foo + 'a results in: error[E0909]: hidden type for `impl Trait` captures lifetime that does not appear in

问题 In the C++20 standard, it is said that array types are implicit lifetime type . Does it mean that an array to a non implicit lifetime type can be implicitly created? The implicit creation of such an array would not cause creation of the array's elements? Consider this case: //implicit creation of an array of std::string //but not the std::string elements: void * ptr = operator new(sizeof (std::string) * 10); //use launder to get a "pointer to object" (which object?) std::string * sptr = std:

问题 With nightly rust: Playground struct Foo<T, F: Fn(&T, &T) -> T> { value: T, func: F } fn main() { let lambda = |&x, &y| x + y; let foo = Foo { value: 5 as i32, func: lambda }; } Error message: Compiling playground v0.0.1 (/playground) error[E0308]: mismatched types --> src/main.rs:8:15 | 8 | let foo = Foo { | ^^^ one type is more general than the other | = note: expected type `std::ops::FnOnce<(&i32, &i32)>` found type `std::ops::FnOnce<(&i32, &i32)>` Note that the expected type and found

问题 I think this is something obvious I'm missing, but here goes.. use std::io; pub trait Source<'a, T> { fn push(&self, t: T) -> io::Result<()>; fn link(&mut self, sink: &dyn Sink<'a, T>) -> io::Result<()>; } pub trait Sink<'a, T> { fn push(&self, t: T) -> io::Result<()>; fn link(&mut self, source: &dyn Source<T>) -> io::Result<()>; } pub struct SyncSource<'a, T> { sink: Option<&'a dyn Sink<'a, T>>, } impl<'a, T> SyncSource<'a, T> { pub fn new() -> SyncSource<'a, T> { SyncSource { sink: None, }

问题 I was implementing linked lists by following along too many linked lists. When trying to implement iter_mut() , I did it myself and made the following code: type Link<T> = Option<Box<Node<T>>>; pub struct List<T> { head: Link<T>, } struct Node<T> { elem: T, next: Link<T>, } impl<T> List<T> { pub fn iter_mut(&mut self) -> IterMut<T> { IterMut::<T>(&mut self.head) } } pub struct IterMut<'a, T>(&'a mut Link<T>); impl<'a, T> Iterator for IterMut<'a, T> { type Item = &'a mut T; fn next<'b>(&'b mut

问题 I have a PyQt5 application that shows a small list. It allows the user to copy list items. When the user copies a list item, it uses delayed rendering to place a reference to the item onto the clipboard. When the item is pasted from the clipboard, it attempts to toggle the selection and place the next item into the clipboard automatically. Delayed rendering works the first time. However, when I attempt to clear or re-use the clipboard, I get an internal Qt error which prints a message but