// Create a cid from a marshaled string
c, err := cid.Decode("bafzbeigai3eoy2ccc7ybwjfz5r3rdxqrinwi4rwytly24tdbh6yk7zslrm")
if err != nil {...}
fmt.Println("Got CID: ", c)

import (
  cid "github.com/ipfs/go-cid"
  mc "github.com/multiformats/go-multicodec"
  mh "github.com/multiformats/go-multihash"
)
// Create a cid manually by specifying the 'prefix' parameters
pref := cid.Prefix{
	Version: 1,
	Codec: uint64(mc.Raw),
	MhType: mh.SHA2_256,
	MhLength: -1, // default length
}
// And then feed it some data
c, err := pref.Sum([]byte("Hello World!"))
if err != nil {...}
fmt.Println("Created CID: ", c)

// To test if two cid's are equivalent, be sure to use the 'Equals' method:
if c1.Equals(c2) {
	fmt.Println("These two refer to the same exact data!")
}

// To check if some data matches a given cid, 
// Get your CIDs prefix, and use that to sum the data in question:
other, err := c.Prefix().Sum(mydata)
if err != nil {...}
if !c.Equals(other) {
	fmt.Println("This data is different.")
}