allocator.zig

pub struct AllocatorVtable {
    allocFn: fn (self: &Allocator, n: usize) -> %[]u8,
    reallocFn: fn (self: &Allocator, old_mem: []u8, new_size: usize) -> %[]u8,
    freeFn: fn (self: &Allocator, mem: []u8),
}
pub struct Allocator {
    vtable: &AllocatorVtable,
}


struct SpaceSavingAllocator {
    @attribute("first_element") parent: Allocator,
    state: SomeStateStuff,

    fn init(self: &Self) {
        self.parent = Allocator{
            .vtable = &space_saving_allocator_vtable,
        };
    }
    fn alloc(almostSelf: &Allocator, n: usize) -> %[]u8 {
        const self = (&Self)(almostSelf);
        ...
    }
    fn realloc(almostSelf: &Allocator, old_mem: []u8, new_size: usize) -> %[]u8 {
        const self = (&Self)(almostSelf);
        ...
    }
    fn free(almostSelf: &Allocator, mem: []u8) {
        const self = (&Self)(almostSelf);
        ...
    }
}
// a constant global vtable shared among all SpaceSavingAllocator instances
// saves space and provides security by preventing these pointers from changing.
const space_saving_allocator_vtable = AllocatorVtable{
    .allocFn = SpaceSavingAllocator.alloc,
    .reallocFn = SpaceSavingAllocator.realloc,
    .freeFn = SpaceSavingAllocator.free,
};


struct CacheFriendlyAllocator {
    @attribute("first_element") parent: Allocator,
    // embed the vtable in the object
    vtable: AllocatorVtable,
    state: SomeStateStuff,

    fn init(self: &Self) {
        self.parent = Allocator{
            // point to ourself
            .vtable = &self.vtable,
        };
        self.vtable = AllocatorVtable{
            .allocFn = CacheFriendlyAllocator.alloc,
            .reallocFn = CacheFriendlyAllocator.realloc,
            .freeFn = CacheFriendlyAllocator.free,
        };
    }

    fn alloc(almostSelf: &Allocator, n: usize) -> %[]u8 {
        const self = (&Self)(almostSelf);
        ...
    }
    fn realloc(almostSelf: &Allocator, old_mem: []u8, new_size: usize) -> %[]u8 {
        const self = (&Self)(almostSelf);
        ...
    }
    fn free(almostSelf: &Allocator, mem: []u8) {
        const self = (&Self)(almostSelf);
        ...
    }
}


fn testAllocator(allocator: &Allocator) {
    var blob = allocator.allocFn(10);
    blob = allocator.reallocFn(blob, 20);
    allocator.freeFn(blob);
}
@attribute("test")
fn testMain() {
    {
        var allocatorImpl: SpaceSavingAllocator = undefined;
        allocatorImpl.init();
        testAllocator(&allocatorImpl.parent);
    }
    {
        var allocatorImpl: CacheFriendlyAllocator = undefined;
        allocatorImpl.init();
        testAllocator(&allocatorImpl.parent);
    }
}