ownership

问题 Even after reading the chapters about reference ownership and borrowing, I cannot understand some things in the following code, effectively stopping me from calling more than one method from clap::App ! extern crate clap; use clap::App; fn main() { let mut app = App::new("name me").args_from_usage("<input_file> 'Sets the input file to use'"); let matches = app.get_matches(); app.print_help(); println!( "Using input file: {}", matches.value_of("input_file").unwrap() ); } Compiling this code

问题 Even after reading the chapters about reference ownership and borrowing, I cannot understand some things in the following code, effectively stopping me from calling more than one method from clap::App ! extern crate clap; use clap::App; fn main() { let mut app = App::new("name me").args_from_usage("<input_file> 'Sets the input file to use'"); let matches = app.get_matches(); app.print_help(); println!( "Using input file: {}", matches.value_of("input_file").unwrap() ); } Compiling this code

问题 Let's say I have a library which has a Document class. An instance of Document can own several instances of Field . Field has multiple subclasses (for example IntegerField and StringField ), and even the API user can subclass it and supply subclass instances to Document (let's say the user is allowed to develop a custom type of data to store in a field). I'd like to expose the Field instances owned by Document through an API in such a way that users can interact with them, but without

问题 Let's say I have a library which has a Document class. An instance of Document can own several instances of Field . Field has multiple subclasses (for example IntegerField and StringField ), and even the API user can subclass it and supply subclass instances to Document (let's say the user is allowed to develop a custom type of data to store in a field). I'd like to expose the Field instances owned by Document through an API in such a way that users can interact with them, but without

问题 It is my understanding that when a variable whose type implements Drop goes out of scope, a call to the fn drop(&mut self) function is inserted, and passed a newly-created mutable reference to the variable going out of scope. However, how is that possible in cases where the variable was immutably bound, and it would be illegal to borrow it mutably? Here's an example of what I'm talking about: fn main() { let x = vec![1, 2, 3]; let y = &mut x; } Which produces the following error: cannot

问题 I'm in a totally new situation. I have the access on root on a reseller account. One of the clients for that reseller has a file, he can't modify. Is a file installed with a plugin in wordpress. That much I understood. He is not the owner of the file. I have to change the owner of that file. I have shell acces and cron acces but I was not able to use it to solve the problem. The solution I come out till now, that doesn't work is adding a new cron job(copy and paste from a forum) #!/bin/bash

问题 When using HashMap's get method, I get an Option<&T> , I've encountered it again this time with Option<&String> . I'd like to get an owned value Option<String> . Is this possible without me writing map(|x| x.to_owned()) ? I'm just wondering if there's a way to write a blanket implementation for any of the utility traits to achieve that? 回答1: Option comes with utility methods for various transformations, which are listed in its documentation. For any T that implements Clone (which String does)

问题 I want to know - are there any guidelines about syntax of C++ (non-)member functions, that allows me to understand (without comments, if possible) the ownership policy of its arguments and return value. By ownership I mean, that the owner is responsible for the destruction of the owned object. I distinguish the following rules about arguments: take ownership don't take ownership share and about return value: release ('return by value' is in this group) don't release share For example, passing

问题 I've just written a small Rust program which calculates Fibonacci numbers and memoizes the calculation. It works, but I'm a little confused about why, especially the recursive call. (It also probably isn't idiomatic.) Here's the program: use std::collections::HashMap; fn main() { let n = 42; // hardcoded for simplicity let mut cache = HashMap::new(); let answer = fib(n, &mut cache); println!("fib of {} is {}", n, answer); } fn fib(n: i32, cache: &mut HashMap<i32,i32>) -> i32 { if cache

问题 This question already has an answer here : Cannot move out of borrowed content when unwrapping (1 answer) Closed last year . I want to iterate over a vector using .iter_mut() and .map() : fn calculate_distances(planes : &mut Vec<Aeroplane>, x: f64, y: f64) { fn calculate_distance(x1: &f64, y1: &f64, x2: &f64, y2: &f64) -> f6 { ... } planes.iter_mut().map(|a| if a.position.is_some() { let pos: &Position = &a.position.unwrap(); a.distance = Some(calculate_distance(&x, &y, &pos.latitude, &pos