Module :: clone_dyn

Derive to clone dyn structures.

By default, Rust does not support cloning for trait objects due to the Clone trait requiring compile-time knowledge of the type's size. The clone_dyn crate addresses this limitation through procedural macros, allowing for cloning collections of trait objects. The crate's purpose is straightforward: it allows for easy cloning of dyn< Trait > with minimal effort and complexity, accomplished by applying the derive attribute to the trait.

Alternative

There are few alternatives dyn-clone, dyn-clonable. Unlike other options, this solution is more concise and demands less effort to use, all without compromising the quality of the outcome.

Basic use-case

Demonstrates the usage of clone_dyn to enable cloning for trait objects.

By default, Rust does not support cloning for trait objects due to the Clone trait requiring compile-time knowledge of the type's size. The clone_dyn crate addresses this limitation through procedural macros, allowing for cloning collections of trait objects.

Overview

This example shows how to use the clone_dyn crate to enable cloning for trait objects, specifically for iterators. It defines a custom trait, IterTrait, that encapsulates an iterator with specific characteristics and demonstrates how to use CloneDyn to overcome the object safety constraints of the Clone trait.

The IterTrait Trait

The IterTrait trait is designed to represent iterators that yield references to items (&'a T). These iterators must also implement the ExactSizeIterator and DoubleEndedIterator traits. Additionally, the iterator must implement the CloneDyn trait, which allows cloning of trait objects.

The trait is implemented for any type that meets the specified requirements.

Cloning Trait Objects

Rust's type system does not allow trait objects to implement the Clone trait directly due to object safety constraints. Specifically, the Clone trait requires knowledge of the concrete type at compile time, which is not available for trait objects.

The CloneDyn trait from the clone_dyn crate provides a workaround for this limitation by allowing trait objects to be cloned. Procedural macros generates the necessary code for cloning trait objects, making it possible to clone collections of trait objects.

The example demonstrates how to implement Clone for boxed IterTrait trait objects.

get_iter Function

The get_iter function returns a boxed iterator that implements the IterTrait trait. If the input is Some, it returns an iterator over the vector. If the input is None, it returns an empty iterator.

It's not possible to use impl Iterator here because the code returns iterators of two different types:

• std::slice::Iter when the input is Some.
• std::iter::Empty when the input is None.

To handle this, the function returns a trait object ( Box< dyn IterTrait > ). However, Rust's Clone trait cannot be implemented for trait objects due to object safety constraints. The CloneDyn trait addresses this problem by enabling cloning of trait objects.

use_iter Function

The use_iter function demonstrates the use of the CloneDyn trait by cloning the iterator. It then iterates over the cloned iterator and prints each element.

Main Function

The main function demonstrates the overall usage by creating a vector, obtaining an iterator, and using the iterator to print elements.

# #[ cfg( not( feature = "enabled" ) ) ]
# fn main() {}
# #[ cfg( feature = "enabled" ) ]
# fn main()
# {

  use clone_dyn::{ clone_dyn, CloneDyn };

  /// Trait that encapsulates an iterator with specific characteristics, tailored for your needs.
  // Uncomment to see what macro expand into
  // #[ clone_dyn( debug ) ]
  #[ clone_dyn ]
  pub trait IterTrait< 'a, T >
  where
    T : 'a,
    Self : Iterator< Item = T > + ExactSizeIterator< Item = T > + DoubleEndedIterator,
    // Self : CloneDyn,
    // There’s no need to explicitly define this bound because the macro will handle it for you.
  {
  }

  impl< 'a, T, I > IterTrait< 'a, T > for I
  where
    T : 'a,
    Self : Iterator< Item = T > + ExactSizeIterator< Item = T > + DoubleEndedIterator,
    Self : CloneDyn,
  {
  }

  ///
  /// Function to get an iterator over a vector of integers.
  ///
  /// This function returns a boxed iterator that implements the `IterTrait` trait.
  /// If the input is `Some`, it returns an iterator over the vector.
  /// If the input is `None`, it returns an empty iterator.
  ///
  /// Rust's type system does not allow trait objects to implement the `Clone` trait directly due to object safety constraints.
  /// Specifically, the `Clone` trait requires knowledge of the concrete type at compile time, which is not available for trait objects.
  ///
  /// In this example, we need to return an iterator that can be cloned. Since we are returning a trait object ( `Box< dyn IterTrait >` ),
  /// we cannot directly implement `Clone` for this trait object. This is where the `CloneDyn` trait from the `clone_dyn` crate comes in handy.
  ///
  /// The `CloneDyn` trait provides a workaround for this limitation by allowing trait objects to be cloned.
  /// It uses procedural macros to generate the necessary code for cloning trait objects, making it possible to clone collections of trait objects.
  ///
  /// It's not possible to use `impl Iterator` here because the code returns iterators of two different types:
  /// - `std::slice::Iter` when the input is `Some`.
  /// - `std::iter::Empty` when the input is `None`.
  ///
  /// To handle this, the function returns a trait object (`Box<dyn IterTrait>`).
  /// However, Rust's `Clone` trait cannot be implemented for trait objects due to object safety constraints.
  /// The `CloneDyn` trait addresses this problem by enabling cloning of trait objects.

  pub fn get_iter< 'a >( src : Option< &'a Vec< i32 > > ) -> Box< dyn IterTrait< 'a, &'a i32 > + 'a >
  {
    match &src
    {
      Some( src ) => Box::new( src.iter() ),
      _ => Box::new( core::iter::empty() ),
    }
  }

  /// Function to use an iterator and print its elements.
  ///
  /// This function demonstrates the use of the `CloneDyn` trait by cloning the iterator.
  /// It then iterates over the cloned iterator and prints each element.
  pub fn use_iter< 'a >( iter : Box< dyn IterTrait< 'a, &'a i32 > + 'a > )
  {
    // Clone would not be available if CloneDyn is not implemented for the iterator.
    // And being an object-safe trait, it can't implement Clone.
    // Nevertheless, thanks to CloneDyn, the object is clonable.
    //
    // This line demonstrates cloning the iterator and iterating over the cloned iterator.
    // Without `CloneDyn`, you would need to collect the iterator into a container, allocating memory on the heap.
    iter.clone().for_each( | e | println!( "{e}" ) );

    // Iterate over the original iterator and print each element.
    iter.for_each( | e | println!( "{e}" ) );
  }

  // Create a vector of integers.
  let data = vec![ 1, 2, 3 ];
  // Get an iterator over the vector.
  let iter = get_iter( Some( &data ) );
  // Use the iterator to print its elements.
  use_iter( iter );

# }

#[ allow( non_local_definitions ) ]
impl< 'c, T > Clone for Box< dyn IterTrait< 'c, T > + 'c >
{
  #[ inline ]
  fn clone( &self ) -> Self
  {
    clone_dyn::clone_into_box( &**self )
  }
}

#[ allow( non_local_definitions ) ]
impl< 'c, T > Clone for Box< dyn IterTrait< 'c, T > + Send + 'c >
{
  #[ inline ]
  fn clone( &self ) -> Self
  {
    clone_dyn::clone_into_box( &**self )
  }
}

#[ allow( non_local_definitions ) ]
impl< 'c, T > Clone for Box< dyn IterTrait< 'c, T > + Sync + 'c >
{
  #[ inline ]
  fn clone( &self ) -> Self
  {
    clone_dyn::clone_into_box( &**self )
  }
}

#[ allow( non_local_definitions ) ]
impl< 'c, T > Clone for Box< dyn IterTrait< 'c, T > + Send + Sync + 'c >
{
  #[ inline ]
  fn clone( &self ) -> Self
  {
    clone_dyn::clone_into_box( &**self )
  }
}

Try out cargo run --example clone_dyn_trivial.
See code.

To add to your project

cargo add clone_dyn

Try out from the repository

git clone https://github.com/Wandalen/wTools
cd wTools
cd examples/clone_dyn_trivial
cargo run