traits

问题 This question already has answers here : Why doesn't Rust support trait object upcasting? (3 answers) Closed 2 years ago . Given the code trait Base { } trait Derived : Base { } struct Foo { } impl Base for Foo { } impl Derived for Foo { } fn main() { let b : Box<Derived> = Box::new( Foo { } ); let a : Box<Base> = b; } When I compile as I'm sure you know I get the following error message: error[E0308]: mismatched types --> src/main.rs:14:25 | 14 | let a : Box<Base> = b; | ^ expected trait

问题 This question already has answers here : Why doesn't Rust support trait object upcasting? (3 answers) Closed 2 years ago . Given the code trait Base { } trait Derived : Base { } struct Foo { } impl Base for Foo { } impl Derived for Foo { } fn main() { let b : Box<Derived> = Box::new( Foo { } ); let a : Box<Base> = b; } When I compile as I'm sure you know I get the following error message: error[E0308]: mismatched types --> src/main.rs:14:25 | 14 | let a : Box<Base> = b; | ^ expected trait

问题 This question already has answers here : Why doesn't Rust support trait object upcasting? (3 answers) Closed 2 years ago . Given the code trait Base { } trait Derived : Base { } struct Foo { } impl Base for Foo { } impl Derived for Foo { } fn main() { let b : Box<Derived> = Box::new( Foo { } ); let a : Box<Base> = b; } When I compile as I'm sure you know I get the following error message: error[E0308]: mismatched types --> src/main.rs:14:25 | 14 | let a : Box<Base> = b; | ^ expected trait

问题 I am modifying some code to depend on rand version 0.5. At first, I was worried how I would enable generating random values of my own types with Standard , but I found out this is legal: impl ::rand::distributions::Distribution<MyType> for ::rand::distributions::Standard { // ... } Why is it legal? I thought implementing an external trait for an external type is illegal. 回答1: The entire purpose of these rules (called the orphan rules or coherence rules ) is to avoid having any conflicting

问题 A trait in C++ encapsulates a family of operations that allow an Algorithm or Data Structure to operator with that type with which it is instantiated. char_traits are an example for grouping string - and file-required functions. But not all traits have "trait" in their name, right? numeric_limits comes to mind. Is this a "Trait", too? Even without the name "trait" in it? So, are there other Templates that could/should be considered a "Trait"? Besides the examples I found: allocator_traits how

问题 A trait in C++ encapsulates a family of operations that allow an Algorithm or Data Structure to operator with that type with which it is instantiated. char_traits are an example for grouping string - and file-required functions. But not all traits have "trait" in their name, right? numeric_limits comes to mind. Is this a "Trait", too? Even without the name "trait" in it? So, are there other Templates that could/should be considered a "Trait"? Besides the examples I found: allocator_traits how

问题 A trait in C++ encapsulates a family of operations that allow an Algorithm or Data Structure to operator with that type with which it is instantiated. char_traits are an example for grouping string - and file-required functions. But not all traits have "trait" in their name, right? numeric_limits comes to mind. Is this a "Trait", too? Even without the name "trait" in it? So, are there other Templates that could/should be considered a "Trait"? Besides the examples I found: allocator_traits how

问题 Basicly ,I want to make a function behave differently for a vector(type) parameter and a non-vector type parameter . #include <vector> using namespace std; template <typename type> struct is_vector { static const bool value = false; }; template <typename type> struct is_vector<vector<type>> { static const bool value = true; }; template <typename type> type read() { if (is_vector<type>::value) { type vec(10); vec.front()=1;//left of '.front' must have class/struct/union return vec; } else {

问题 Basicly ,I want to make a function behave differently for a vector(type) parameter and a non-vector type parameter . #include <vector> using namespace std; template <typename type> struct is_vector { static const bool value = false; }; template <typename type> struct is_vector<vector<type>> { static const bool value = true; }; template <typename type> type read() { if (is_vector<type>::value) { type vec(10); vec.front()=1;//left of '.front' must have class/struct/union return vec; } else {

问题 Hi i want to encrypt some field in database when user create or edit data. IF create, the encryption work, but when user edit data, the value that save in database will be normal text not encrypted <?php namespace App\Traits; use Crypt; trait Encryptable { public function toArray() { $array = parent::toArray(); foreach ($array as $key => $attribute) { if (in_array($key, $this->encryptable) && $array[$key]!='') { try { $array[$key] = Crypt::decrypt($array[$key]); } catch (\Exception $e) { } }