What is the simplest way to expand a slice to its capacity?


I have a program that uses a buffer pool to reduce allocations in a few performance-sensitive sections of the code.

Something like this: play link

// some file or any data source
var r io.Reader = bytes.NewReader([]byte{1,2,3})

// initialize slice to max expected capacity
dat := make([]byte, 20)

// read some data into it. Trim to length.
n, err := r.Read(dat)
dat = dat[:n]

// now I want to reuse it: 
for len(dat) < cap(dat) {
        dat = append(dat, 0) 

// add it to free list for reuse later
// bufferPool.Put(dat)

I always allocate fixed length slices, which are guaranteed to be larger than the maximum size needed. I need to reduce size to the actual data length to use the buffer, but I also need it to be the maximum size again to read into it the next time I need it.

The only way I know of to expand a slice is with append, so that is what I am using. The loop feels super dirty though. And potentially inefficient. My benchmarks show it isn’t horrible, but I feel like there has to be a better way.

I know only a bit about the internal representation of slices, but if I could only somehow override the length value without actually adding data, it would be really nice. I don’t really need to zero it out or anything.

Is there a better way to accomplish this?


“Extending” a slice to its capacity is simply a slice expression, and specify the capacity as the high index. The high index does not need to be less than the length. The restriction is:

For arrays or strings, the indices are in range if 0 <= low <= high <= len(a), otherwise they are out of range. For slices, the upper index bound is the slice capacity cap(a) rather than the length.


b := make([]byte, 10, 20)
fmt.Println(len(b), cap(b), b)

b = b[:cap(b)]
fmt.Println(len(b), cap(b), b)

Output (try it on the Go Playground):

10 20 [0 0 0 0 0 0 0 0 0 0]
20 20 [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]

Answered By – icza

Answer Checked By – Candace Johnson (GoLangFix Volunteer)

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