GoLang: Reading and casting bytes into struct fields

Question

I am reading from an io.Reader into a Struct, field by field.

// structFields returns a sequence of reflect.Value
for field := range structFields {
    switch field.Kind() {
    case reflect.String:
        // Omitted
    case reflect.Uint8:
        value := make([]byte, 2)
        reader.Read(value)

        var num uint8
        err := binary.Read(bytes.NewBuffer(value[:]), binary.LittleEndian, &num)
        if err != nil { return err }
        field.SetUint(int64(num))
    // Case statements for each of the other uint and int types omitted
    }
}

Unfortunately the block for reflect.Uint8 needs to be repeated for each of the Uint and Int data types since I need to create the var num correctly in each case.

Is there a way I can simplify this switch statement?

Although this does not solve this exact problem, you could replace all this logic with a simple call to binary.Read() which reads in all the fields of a struct in one go. — fuz
– fuz, Commented Jul 22, 2014 at 22:22

user418324 · Accepted Answer · 2014-07-23 00:12:45Z

2

Instead of using var num uint8 and field.SetUint(int64(num)) just pass a pointer to the struct field to binary.Read:

ptr := field.Addr().Interface()
err := binary.Read(bytes.NewBuffer(value[:]), binary.LittleEndian, ptr)

And make the case statement say:

case reflect.Uint8, reflect.Int, reflect.Uint, ...:

Then you need to deal with differently-sized numbers. Fortunately you can just pass your reader directly to binary.Read and it'll take care of it:

err := binary.Read(reader, binary.LittleEndian, ptr)

Finally, as FUZxxl says, you can just pass a pointer to the entire struct to binary.Read and it'll do all this for you.

answered Jul 23, 2014 at 0:12

user418324

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2 Comments

OneOfOne Over a year ago

@FUZxxl's solution wouldn't work unless all fields are sized ints, floats or complexes, it wouldn't work with any kind of a slice or a string.

tgrosinger Over a year ago

Speaking of string fields, any recommendations on how to read those? I am trying to use a tag on the struct field, but I can not get to tags from a reflect.Value

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