How to define a copyable struct containing a String? [duplicate]

Question

I have a struct like that

#[derive(Copy, Clone)] enum Command {     Quit,     Error { msg: String }, } 

The compiler complains that it cannot generate a copy constructor for Error.

I need to make the struct copyable to pass over a channel to another thread.

error: the trait `Copy` may not be implemented for this type; variant `Error` does not implement `Copy` [E0205] #[derive(Copy, Clone)] ^~~~~~~~~~~~~~~~~~~~~~ note: in this expansion of #[derive_Copy] (defined in src/main.rs) 

It compiles if msg is an i32. It looks strange that such basic type as String is not copyable.

Answer 1

For me looks strange that such basic type as string is not copyable.

Rust is explicit first.

One of the oft heard complains from C programmers (and most notably Linus Torvald) with regard to C++ is that there is too much implicit copying in C++, which hides memory allocation. Coupled with implicit conversions, they can really creep in the most unexpected places.

Rust instead is designed with an intent to expose the complexity of the underlying operations. It does implicitly perform some conversions (borrowing, from &T to &Trait), but those are cheap (and generally constant time).

This explicitness shows in the two traits you have here:

• Clone is about indicating how to create a new instance, and must be called explicitly. Most types (but not all) can be copied using it.

• Copy is a specific compiler trait which indicates that the developer wishes to activate implicit copying for the type; it is only available if a shallow copy is equivalent to a deep copy, which ensures that no memory allocation will occur as part of those implicit copies

So, to ensure your remark:

• String is copyable, use .clone()
• String is not implicitly copyable, because that would cause non-obvious memory allocations to occur