如何在 rust 特征中声明这样的 function

Question

for example. need a global object and determine the object at runtime

class A{
};

A* a = nullptr;


// B inherit from A
void Import(B* b){
    a = b;
}

I have tried using the Dyn but must Determine the type

static mut instance: &dyn plugin::BasePlugin;
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^help: provide a definition for the static: `= <expr>;`

Answer 1

You should not use static mut . It's an unsafe-by-design construct meant for low-level code, not normal users who just want a global variable.

Rust assumes single-threaded programs don't exist, so every global variable must be thread-safe. Therefore, to mutate it you will have to use a Mutex or some other thread-safety alternative like ArcSwap . You will need lazy_static to initialize these types.

You won't be able to use just any temporary scope-limited reference in a global variable. & is not a general-purpose way to refer to objects in Rust. Consider using Box<dyn BasePlugin> or Arc<dyn BasePlugin> instead. These allow holding objects by reference without scoped lifetime limitations.

You could use a &'static reference, but in practice that would require leaking heap memory. If you plan to set the variable once, it would be okay. If you plan to set it more than once, that's not elegant, and may become problematic.

And finally, consider not using global variables at all. Create objects you need stored in local variables (even as early as in main() ), and pass them down as arguments to functions that need them. You could make a struct App and put data you need as fields of that struct.