# Allocators
[](https://travis-ci.org/rphmeier/allocators) [](https://crates.io/crates/allocators)
## [Documentation](https://rphmeier.github.io/allocators/allocators/)
This crate provides a different memory allocators, as well as an
`Allocator` trait for creating other custom allocators. A main goal of allocators is composability. For this reason, it also provides some composable primitives to be used as building blocks for chained allocators. This crate leans heavily on unsafe/unstable code at the moment, and should be considered very experimental.
# Why?
For Rust to fulfill its description as a systems programming language, users need to have more fine-grained control over the way memory is allocated in their programs. This crate is a proof-of-concept that these mechanisms can be implemented in Rust and provide a safe interface to their users.
# Allocator Traits
## Allocator
This is the core trait for allocators to implement. All a type has to do is implement two unsafe functions: `allocate_raw` and `deallocate_raw`. This will likely require `reallocate_raw` in the future.
## BlockOwner
Allocators that implement this can say definitively whether they own a block.
# Allocator Types
## Scoped Allocator
This is useful for reusing a block of memory for temporary allocations in a tight loop. Scopes can be nested and values allocated in a scope cannot be moved outside it.
```rust
#![feature(placement_in_syntax)]
use allocators::{Allocator, Scoped};
#[derive(Debug)]
struct Bomb(u8);
impl Drop for Bomb {
fn drop(&mut self) {
println!("Boom! {}", self.0);
}
}
// new scoped allocator with a kilobyte of memory.
let alloc = Scoped::new(1024).unwrap();
// allocate_val makes the value on the stack first.
for i in 0..100 { bombs.push(inner.allocate(Bomb(i)).unwrap())}
// watch the bombs go off!
});
// Allocators also have placement-in syntax.
let my_int = in alloc.make_place().unwrap() { 23 };
println!("My int: {}", *my_int);
```
## Free List Allocator
This allocator maintains a list of free blocks of a given size.
```rust
use allocators::{Allocator, FreeList};
// create a FreeList allocator with 64 blocks of 1024 bytes.
let alloc = FreeList::new(1024, 64).unwrap();
for _ in 0..10 {
// allocate every block
let mut v = Vec::new();
for i in 0u8..64 {
v.push(alloc.allocate([i; 1024]).unwrap());
}
// no more blocks :(.
assert!(alloc.allocate([0; 1024]).is_err());
// all the blocks get pushed back onto the freelist at the end here,
// memory gets reused efficiently in the next iteration.
}
```
This allocator can yield very good performance for situations like the above, where each block's space is being fully used.
# Composable Primitives
These are very underdeveloped at the moment, and lack a fluent API as well. They are definitely a back-burner feature at the moment, since the idea of composable allocators hasn't really proved its value yet.
## Null Allocator
This will probably get a new name since "Null" has some misleading connotations.
It fails to allocate any request made to it, and panics when deallocated with.
## Fallback Allocator
This composes two `BlockOwners`: a main allocator and a fallback. If the main allocator fails to allocate, it turns to the fallback.
## Proxy Allocator
This wraps any allocator and something which implements the `ProxyLogger` trait, which provides functions to log allocation, deallocation, and reallocation in any arbitrary way. It has practical applications in debug builds for measuring how an allocator is being utilized.