why cannot I assign a newly allocated byte array to a variable with []byte as type?

Question

I am trying to declare a variable of type array with unspecified size []byte and then filling it with an allocated array of size 256 as follows:

var buf []byte
buf = new([256]byte)

Unfortunately, it does not work. The compile error returned is the following:

cannot use *new([256]byte) (type [256]byte) as type []byte in assignment

Any idea?

Answer 1

You want to assign an array to a slice, which won't work (see "Go Slices: usage and internals")

To make a slice instead, use:

var buf []byte
buf = make([]byte, 256)

The difference:

An array variable denotes the entire array; it is not a pointer to the first array element (as would be the case in C).
This means that when you assign or pass around an array value you will make a copy of its contents. (To avoid the copy you could pass a pointer to the array, but then that's a pointer to an array, not an array.)
One way to think about arrays is as a sort of struct but with indexed rather than named fields: a fixed-size composite value.

Versus:

A slice is a descriptor of an array segment.
It consists of a pointer to the array, the length of the segment, and its capacity (the maximum length of the segment).

---

If you want to create an array (an assign it to an array), that would have been:

var buf [256]byte
buf = [256]byte{}

Answer 2

You can also use the newly allocated array as a slice by slicing it like this:

var slice []byte = buf[:]

This creates slice as a slice that is backed by the array buf.