Value returns the value associated with this context for key, or nil if no value is associated with key. Successive calls to Value with the same key returns the same result.

Use context values only for request-scoped data that transits processes and API boundaries, not for passing optional parameters to functions.

A key identifies a specific value in a Context. Functions that wish to store values in Context typically allocate a key in a global variable then use that key as the argument to context.WithValue and Context.Value. A key can be any type that supports equality; packages should define keys as an unexported type to avoid collisions.

Packages that define a Context key should provide type-safe accessors for the values stores using that key:

// Package user defines a User type that's stored in Contexts.
package user

import ""

// User is the type of value stored in the Contexts.
type User struct {...}

// key is an unexported type for keys defined in this package.
// This prevents collisions with keys defined in other packages.
type key int

// userKey is the key for user.User values in Contexts.  It is
// unexported; clients use user.NewContext and user.FromContext
// instead of using this key directly.
var userKey key = 0

// NewContext returns a new Context that carries value u.
func NewContext(ctx context.Context, u *User) context.Context {
	return context.WithValue(ctx, userKey, u)

// FromContext returns the User value stored in ctx, if any.
func FromContext(ctx context.Context) (*User, bool) {
	u, ok := ctx.Value(userKey).(*User)
	return u, ok

Value is referenced in 0 repositories