Rust Ownership / Borrowing in struct constructors

user_15447211 0
Viewed 285

I'm trying to learn Rust as someone who is somewhat familiar with C++.

I think i grasped the concept of ownership and borrowing pretty well but there is something that confuses me in struct constructors.

struct A{
    b: u32
}
impl A{
    fn constructor(a:u32)->A{
        let obj= A{b:a};
        return obj;
    }
}
fn main(){
    let c= A::constructor(3);
}

In the constructor of A we are allocating an A object in the stack memory of the constructor function and returning the ownership of the object. However, after we return the object, isn't the stack memory of the constructor function going to be de-allocated since it is going to be popped of the stack, leaving us with a dangling pointer c.

I know that passing the ownership outside the the constructor will prevent it from de-allocating when scope of the constructor ends but isn't de-allocation of the stack memory when the function finishes independent from the ownership process?

I would really appreciate any help. I like to continue learning rust with a solid understanding of its memory managment.

ownership rust stack-memory scope c++
1 Answers

In Rust, transferring ownership often translates to "destructive bitwise copies".

When you move a struct from one let binding to another, you're performing a bitwise copy and marking the old binding as "uninitialized".

When you return a struct from a function to its caller, you're performing a bitwise copy to the calling stack frame, and you don't have to worry about uninitializing anything because the scope is gone.

In practice, much of this copying and moving and destroying is optimized away, but you can write your programs as if it isn't. So in your case, Foo::constructor creates a stack frame with enough space for a Foo in it. When main calls Foo::constructor, its stack frame has enough space for a Foo, and when it returns, the bits are copied over.

I'll stress that in practice, something this size would almost certainly get inlined, this is just the mental model. If you're worried about performance, check out a crate called criterion for benchmarking.

user_91867830
Related