Scott Knight
2 min read



The Go programming language does not support the concept of inheritance in the traditional object oriented sense. It does allow for type embedding which provides a powerful method for composition. When getting started with Go these patterns are not always obvious. This post provides a short example of how type embedding is used in one of the Go standard library structs.

The macho package provides support for reading and manipulating Mach-O binary files. The Segment struct provides a good example of type embedding.

// A Segment represents a Mach-O 32-bit or 64-bit load segment command.
type Segment struct {

	// Embed ReaderAt for ReadAt method.
	// Do not embed SectionReader directly
	// to avoid having Read and Seek.
	// If a client wants Read and Seek it must use
	// Open() to avoid fighting over the seek offset
	// with other clients.
	sr *io.SectionReader

The comment in the struct indicates that they’re not embedding the SectionReader directly. This might be confusing at first since a SectionReader is obviouslly embedded in the Segement struct. The ReaderAt interface is embedded without the inclusion of an alias. This means the methods of ReaderAt are promoted to the Segement struct. In turn, the Segment struct can itself be treated directly as a ReaderAt instance. The SectionReader however is embeded with an alias name. More importantly, the alias name is all lower case, which means it can’t be accessed outside of the package. This provides a private SectionReader instance that can be used inside of the implementation of the Segment struct but no methods are promoted. When using this class outside of the package there’s no indication of the SectionReader at all.

This short struct does a great job demonstrating not only type embedding but also method promotion and visibility. To read more about type embedding, head over to the Effective Go entry on embedding.