StructToData.swift

struct CharacteristicValue {
    let format: Int8
    let exponent: Int8 = -1
    let unit: UInt16
    let namespace: Int8
    let descriptionInfo: Int16
}

var value = CharacteristicValue(format: 14, unit: 0x272F, namespace: 1, descriptionInfo: 0)

//This works
let oldData = NSData(bytes: &value, length: sizeof(CharacteristicValue.self))

//This gives me "Ambiguous Use of 'init'" on the `UnsafePointer` part.
let newData = Data(bytes: UnsafePointer<UInt8>(&value), count: sizeof(CharacteristicValue.self))

I would use strideof instead of sizeof
Let me explain why:
Lets say you have a struct:

struct S1{
    let a:Int
    let b:UInt8
}

sizeof(S1) == 9
strideof(S1)==16

so using sizeof will give you a smaller data which is good. However if you would like to concatenate structs. You have a memory alignment problem (https://en.wikipedia.org/wiki/Data_structure_alignment).

From my current experiences armv7 is not very good at resolving non aligned memory access (and it showed for me only in release mode -> optimisation level -O). arm64 seems much more stable.

So in your case if you only want to serialse single data structure / struct. It might work fine, but beware of potential memory alignment issues.

@mzaks: Good point. I briefly considered strideof but for some reason assumed that it would be wrong to use it for a single struct (I guess because I implicitly assumed the padding bytes at the end could cause a problem). But that's not the case. /cc @Gernot