traits

I'm having trouble understanding how values of boxed traits come into existence. Consider the following code: trait Fooer { fn foo(&self); } impl Fooer for i32 { fn foo(&self) { println!("Fooer on i32!"); } } fn main() { let a = Box::new(32); // works, creates a Box<i32> let b = Box::<i32>::new(32); // works, creates a Box<i32> let c = Box::<Fooer>::new(32); // doesn't work let d: Box<Fooer> = Box::new(32); // works, creates a Box<Fooer> let e: Box<Fooer> = Box::<i32>::new(32); // works, creates a Box<Fooer> } Obviously, variant a and b work, trivially. However, variant c does not, probably

I'm trying to read some lines from a file, skipping the first few and printing the rest, but I keep getting errors about used value after move: use std::fs::File; use std::io::{self, BufRead, BufReader, Read}; use std::path::Path; fn skip_and_print_file(skip: &usize, path: &Path) { let mut skip: usize = *skip; if let Ok(file) = File::open(path) { let mut buffer = BufReader::new(file); for (index, line) in buffer.lines().enumerate() { if index >= skip { break; } } print_to_stdout(&mut buffer); } } fn print_to_stdout(mut input: &mut Read) { let mut stdout = io::stdout(); io::copy(&mut input,

I am trying to initialise an array of structs in Rust: enum Direction { North, East, South, West, } struct RoadPoint { direction: Direction, index: i32, } // Initialise the array, but failed. let data = [RoadPoint { direction: Direction::East, index: 1 }; 4]; When I try to compile, the compiler complains that the Copy trait is not implemented: error[E0277]: the trait bound `main::RoadPoint: std::marker::Copy` is not satisfied --> src/main.rs:15:16 | 15 | let data = [RoadPoint { direction: Direction::East, index: 1 }; 4]; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std:

Simple question, is it possible to dynamically add traits to a php class in runtime without using eval? As Glavic said, you can't without using eval() or reflection hacks (and I'm not even sure about that). But it's very unlikely you really need to. You can achieve a lot with dynamic class composition (composing a class with some functionality you want into another class). That's simply a matter of putting a reference to the class with the desired functionality into a variable in the hosting class. class SomeClassWithNeededFunctionality {} class

I find some confusing use of trait in some unittesting code, such as: trait MyTrait { val t1 = ... //some expression val t2 = ... //some expression } And then instantiate the trait using new and meanwhile some expressions wrapped by curly braces followed the instantiation. test("it is a test") { new MyTrait { // do something with t1 and t2 } } I am confused by this strange syntax. My question is: why use follow trait instantiation by curly braces? what is the purpose of trait instantiation in this case and other cases might also be helpful? Steve Buzzard You are not instantiating the traits:

Is it possible to declare a variable var_b of the same type as another variable, var_a ? For example: template <class T> void foo(T t) { auto var_a = bar(t); //make var_b of the same type as var_a } F_1 bar(T_1 t) { } F_2 bar(T_2 t) { } Sure, use decltype : auto var_a = bar(t); decltype(var_a) b; You can add cv-qualifiers and references to decltype specifiers as if it were any other type: const decltype(var_a)* b; decltype(var_a) var_b; And a Lorem Ipsum to reach the required minimum of 30 characters per answer. Despite the nice answer of @TartanLlama, this is another way one can use decltype