rust/src/test/run-pass/overloaded-deref-count.rs

// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use std::cell::Cell;
use std::ops::{Deref, DerefMut};
use std::vec::Vec;

struct DerefCounter<T> {
    count_imm: Cell<uint>,
    count_mut: uint,
    value: T
}

impl<T> DerefCounter<T> {
    fn new(value: T) -> DerefCounter<T> {
        DerefCounter {
            count_imm: Cell::new(0),
            count_mut: 0,
            value: value
        }
    }

    fn counts(&self) -> (uint, uint) {
        (self.count_imm.get(), self.count_mut)
    }
}

impl<T> Deref<T> for DerefCounter<T> {
    fn deref(&self) -> &T {
        self.count_imm.set(self.count_imm.get() + 1);
        &self.value
    }
}

impl<T> DerefMut<T> for DerefCounter<T> {
    fn deref_mut(&mut self) -> &mut T {
        self.count_mut += 1;
        &mut self.value
    }
}

pub fn main() {
    let mut n = DerefCounter::new(0i);
    let mut v = DerefCounter::new(Vec::new());

    let _ = *n; // Immutable deref + copy a POD.
    assert_eq!(n.counts(), (1, 0));

    let _ = (&*n, &*v); // Immutable deref + borrow.
    assert_eq!(n.counts(), (2, 0)); assert_eq!(v.counts(), (1, 0));

    let _ = (&mut *n, &mut *v); // Mutable deref + mutable borrow.
    assert_eq!(n.counts(), (2, 1)); assert_eq!(v.counts(), (1, 1));

    let mut v2 = Vec::new();
    v2.push(1i);

    *n = 5; *v = v2; // Mutable deref + assignment.
    assert_eq!(n.counts(), (2, 2)); assert_eq!(v.counts(), (1, 2));

    *n -= 3; // Mutable deref + assignment with binary operation.
    assert_eq!(n.counts(), (2, 3));

    // Mutable deref used for calling a method taking &self.
    // N.B. This is required because method lookup hasn't been performed so
    // we don't know whether the called method takes mutable self, before
    // the dereference itself is type-checked (a chicken-and-egg problem).
    (*n).to_string();
    assert_eq!(n.counts(), (2, 4));

    // Mutable deref used for calling a method taking &mut self.
    (*v).push(2);
    assert_eq!(v.counts(), (1, 3));

    // Check the final states.
    assert_eq!(*n, 2);
    assert_eq!((*v).as_slice(), &[1, 2]);
}