Trait Pread

pub trait Pread<Ctx: Copy, E> {
    // Required method
    fn gread_with<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>(
        &'a self,
        offset: &mut usize,
        ctx: Ctx,
    ) -> Result<N, E>;

    // Provided methods
    fn pread<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>(
        &'a self,
        offset: usize,
    ) -> Result<N, E>
       where Ctx: Default { ... }
    fn pread_with<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>(
        &'a self,
        offset: usize,
        ctx: Ctx,
    ) -> Result<N, E> { ... }
    fn gread<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>(
        &'a self,
        offset: &mut usize,
    ) -> Result<N, E>
       where Ctx: Default { ... }
    fn gread_inout<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>(
        &'a self,
        offset: &mut usize,
        inout: &mut [N],
    ) -> Result<(), E>
       where Ctx: Default { ... }
    fn gread_inout_with<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>(
        &'a self,
        offset: &mut usize,
        inout: &mut [N],
        ctx: Ctx,
    ) -> Result<(), E> { ... }
}

A very generic, contextual pread interface in Rust.

Like Pwrite — but for reading!

Implementing Pread on a data store allows you to then read almost arbitrarily complex types efficiently.

To this end the Pread trait works in conjuction with the TryFromCtx; The TryFromCtx trait implemented on a type defines how to convert data to an object of that type, the Pread trait implemented on a data store defines how to extract said data from that store.

It should be noted though that in this context, data does not necessarily mean &[u8] — Pread and TryFromCtx are generic over what ‘data’ means and could be implemented instead over chunks of memory or any other indexable type — but scroll does come with a set of powerful blanket implementations for data being a continous block of byte-addressable memory.

Pread provides two main groups of functions: pread and gread.

pread is the basic function that simply extracts a given type from a given data store - either using a provided Context in the case of pread_with or with the default context for the given type in the case of pread

gread does in addition to that update the offset it’s currently at, allowing for a cursored read — gread_inout expands on that and reads a number of continous types from the data store. gread again comes with _with variants to allow using a specific context.

Since pread and friends are very generic functions their types are rather complex, but very much understandable; TryFromCtx is generic over Ctx (described here), Output and Error. The Error type is hopefully self-explanatory, however the Output type is rather important; it defines what Pread extracts from the data store and has to match up with what TryFromCtx expects as input to convert into the resulting type. scroll defaults to &[u8] here.

Unless you need to implement your own data store — that is either can’t convert to &[u8] or have a data that does not expose a &[u8] — you will probably want to implement TryFromCtx on your Rust types to be extracted.

Required Methods

fn gread_with<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>( &'a self, offset: &mut usize, ctx: Ctx, ) -> Result<N, E>

Reads a value from self at offset with the given ctx, and updates the offset.

Example

use scroll::Pread;
let offset = &mut 0;
let bytes: [u8; 2] = [0xde, 0xad];
let dead: u16 = bytes.gread_with(offset, scroll::BE).unwrap();
assert_eq!(dead, 0xdeadu16);
assert_eq!(*offset, 2);

Provided Methods

fn pread<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>( &'a self, offset: usize, ) -> Result<N, E>

where Ctx: Default,

Reads a value from self at offset with a default Ctx. For the primitive numeric values, this will read at the machine’s endianness.

Example

use scroll::Pread;
let bytes = [0x7fu8; 0x01];
let byte = bytes.pread::<u8>(0).unwrap();

fn pread_with<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>( &'a self, offset: usize, ctx: Ctx, ) -> Result<N, E>

Reads a value from self at offset with the given ctx

Example

use scroll::Pread;
let bytes: [u8; 2] = [0xde, 0xad];
let dead: u16 = bytes.pread_with(0, scroll::BE).unwrap();
assert_eq!(dead, 0xdeadu16);

fn gread<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>( &'a self, offset: &mut usize, ) -> Result<N, E>

where Ctx: Default,

Reads a value from self at offset with a default Ctx. For the primitive numeric values, this will read at the machine’s endianness. Updates the offset

Example

use scroll::Pread;
let offset = &mut 0;
let bytes = [0x7fu8; 0x01];
let byte = bytes.gread::<u8>(offset).unwrap();
assert_eq!(*offset, 1);

fn gread_inout<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>( &'a self, offset: &mut usize, inout: &mut [N], ) -> Result<(), E>

where Ctx: Default,

Tries to write inout.len() Ns into inout from Self starting at offset, using the default context for N, and updates the offset.

Example

use scroll::Pread;
let mut bytes: Vec<u8> = vec![0, 0];
let offset = &mut 0;
let bytes_from: [u8; 2] = [0x48, 0x49];
bytes_from.gread_inout(offset, &mut bytes).unwrap();
assert_eq!(&bytes, &bytes_from);
assert_eq!(*offset, 2);

fn gread_inout_with<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>( &'a self, offset: &mut usize, inout: &mut [N], ctx: Ctx, ) -> Result<(), E>

Tries to write inout.len() Ns into inout from Self starting at offset, using the context ctx

Example

use scroll::{ctx, LE, Pread};
let mut bytes: Vec<u8> = vec![0, 0];
let offset = &mut 0;
let bytes_from: [u8; 2] = [0x48, 0x49];
bytes_from.gread_inout_with(offset, &mut bytes, LE).unwrap();
assert_eq!(&bytes, &bytes_from);
assert_eq!(*offset, 2);

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementations on Foreign Types

impl<Ctx: Copy, E: From<Error>> Pread<Ctx, E> for [u8]

fn gread_with<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>( &'a self, offset: &mut usize, ctx: Ctx, ) -> Result<N, E>

Implementors