Map of types in Golang?

Question

I am writing a parser for audio/video stream which is known to be comprised of frames of several certain types. Each frame type has its own subheader format, so I define a struct type for each to use them for variables passed as the 3rd argument to binary.Read:

type TypeVideoIHeader struct {
    MType       byte // type of media e.g. H.264
    FPS         byte
    Width       byte
    Height      byte
    DateTime    int32
    Length      int32
}

type TypeVideoPHeader struct {
    Length      int32
}

type TypeAudioHeader struct {
    MType       byte
    SampleRate  byte
    Length      int16
}

Which type a frame is is defined by a certain byte in its header, so I put those into constants:

type FrameType byte

const (
    VideoI  FrameType = 0xFC
    VideoP  FrameType = 0xFD
    Audio   FrameType = 0xFA
)

Now, before I can call binary.Read I have to create a variable of the right type (one of the 3 structs above) which the function will fill with values from the stream.

How do I initialise a variable of the right type just by a FrameType variable? Is there a concise and elegant solution?

Say if this kind of trick was possible:

TMap := map[FrameType]type{
    VideoI: TypeVideoIHeader,
    VideoP: TypeVideoPHeader,
    Audio: TypeAudioHeader,
}

var videoISubHeader TMap[VideoI]

— that would be sort of what I am after.

user_965900 · Answer

TMap := map[FrameType]func() interface{} {
    VideoI: func() interface{} { return &TypeVideoIHeader{} },
    VideoP: func() interface{} { return &TypeVideoPHeader{} },
    Audio:  func() interface{} { return &TypeAudioHeader{} },
}

var videoISubHeader = TMap[VideoI]()

if err := binary.Read(r, order, videoISubHeader); err != nil {
    panic(err)
}

Note that binary.Read expects the 3rd argument to be a pointer, else it won't be able to read the binary data into the passed in variable, which is why the functions use &.

Map of types in Golang?

1 Answers