Struct core2::io::Cursor

source · [−]
pub struct Cursor<T> { /* private fields */ }
Expand description

A Cursor wraps an in-memory buffer and provides it with a Seek implementation.

Cursors are used with in-memory buffers, anything implementing AsRef<[u8]>, to allow them to implement Read and/or Write, allowing these buffers to be used anywhere you might use a reader or writer that does actual I/O.

The standard library implements some I/O traits on various types which are commonly used as a buffer, like Cursor<[Vec]<u8>> and Cursor<&[u8]>.

Examples

We may want to write bytes to a File in our production code, but use an in-memory buffer in our tests. We can do this with Cursor:

use std::io::prelude::*;
use core2::io::{self, Seek, SeekFrom, Write};
use std::fs::File;

// a library function we've written
fn write_ten_bytes_at_end<W: Write + Seek>(writer: &mut W) -> io::Result<()> {
    writer.seek(SeekFrom::End(-10))?;

    for i in 0..10 {
        writer.write(&[i])?;
    }

    // all went well
    Ok(())
}

// Here's some code that uses this library function.
//
// We might want to use a BufReader here for efficiency, but let's
// keep this example focused.
let mut file = File::create("foo.txt").map_err(|e| io::Error::from(e))?;

write_ten_bytes_at_end(&mut file)?;

// now let's write a test
#[test]
fn test_writes_bytes() {
    // setting up a real File is much slower than an in-memory buffer,
    // let's use a cursor instead
    use core2::io::Cursor;
    let mut buff = Cursor::new(vec![0; 15]);

    write_ten_bytes_at_end(&mut buff).unwrap();

    assert_eq!(&buff.get_ref()[5..15], &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
}

Implementations

Creates a new cursor wrapping the provided underlying in-memory buffer.

Cursor initial position is 0 even if underlying buffer (e.g., [Vec]) is not empty. So writing to cursor starts with overwriting [Vec] content, not with appending to it.

Examples
use core2::io::Cursor;

let buff = Cursor::new(Vec::new());

Consumes this cursor, returning the underlying value.

Examples
use core2::io::Cursor;

let buff = Cursor::new(Vec::new());

let vec = buff.into_inner();

Gets a reference to the underlying value in this cursor.

Examples
use core2::io::Cursor;

let buff = Cursor::new(Vec::new());

let reference = buff.get_ref();

Gets a mutable reference to the underlying value in this cursor.

Care should be taken to avoid modifying the internal I/O state of the underlying value as it may corrupt this cursor’s position.

Examples
use core2::io::Cursor;

let mut buff = Cursor::new(Vec::new());

let reference = buff.get_mut();

Returns the current position of this cursor.

Examples
use core2::io::{Cursor, Seek, SeekFrom};
use std::io::prelude::*;

let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);

assert_eq!(buff.position(), 0);

buff.seek(SeekFrom::Current(2)).unwrap();
assert_eq!(buff.position(), 2);

buff.seek(SeekFrom::Current(-1)).unwrap();
assert_eq!(buff.position(), 1);

Sets the position of this cursor.

Examples
use core2::io::Cursor;

let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);

assert_eq!(buff.position(), 0);

buff.set_position(2);
assert_eq!(buff.position(), 2);

buff.set_position(4);
assert_eq!(buff.position(), 4);

Trait Implementations

Returns the contents of the internal buffer, filling it with more data from the inner reader if it is empty. Read more

Tells this buffer that amt bytes have been consumed from the buffer, so they should no longer be returned in calls to read. Read more

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

Returns the “default value” for a type. Read more

This method tests for self and other values to be equal, and is used by ==. Read more

This method tests for !=.

Pull some bytes from this source into the specified buffer, returning how many bytes were read. Read more

Read the exact number of bytes required to fill buf. Read more

Read all bytes until EOF in this source, placing them into buf. Read more

Determines if this Reader can work with buffers of uninitialized memory. Read more

Creates a “by reference” adaptor for this instance of Read. Read more

Transforms this Read instance to an Iterator over its bytes. Read more

Creates an adaptor which will chain this stream with another. Read more

Creates an adaptor which will read at most limit bytes from it. Read more

Seek to an offset, in bytes, in a stream. Read more

Write a buffer into this writer, returning how many bytes were written. Read more

Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more

Attempts to write an entire buffer into this writer. Read more

Writes a formatted string into this writer, returning any error encountered. Read more

Creates a “by reference” adaptor for this instance of Write. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The resulting type after obtaining ownership.

Creates owned data from borrowed data, usually by cloning. Read more

🔬 This is a nightly-only experimental API. (toowned_clone_into)

Uses borrowed data to replace owned data, usually by cloning. Read more

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.