Go 语言规范Selectors
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For a primary expression x that is not a package name, the selector expression
x.f
denotes the field or method f of the value x (or sometimes *x; see below). The identifier f is called the (field or method) selector; it must not be the blank identifier. The type of the selector expression is the type of f. If x is a package name, see the section on qualified identifiers.
A selector f may denote a field or method f of a type T, or it may refer to a field or method f of a nested embedded field of T. The number of embedded fields traversed to reach f is called its depth in T. The depth of a field or method f declared in T is zero. The depth of a field or method f declared in an embedded field A in T is the depth of f in A plus one.
The following rules apply to selectors:
- For a value
xof typeTor*TwhereTis not a pointer or interface type,x.fdenotes the field or method at the shallowest depth inTwhere there is such anf. If there is not exactly onefwith shallowest depth, the selector expression is illegal. - For a value
xof typeIwhereIis an interface type,x.fdenotes the actual method with namefof the dynamic value ofx. If there is no method with namefin the method set ofI, the selector expression is illegal. - As an exception, if the type of
xis a defined pointer type and(*x).fis a valid selector expression denoting a field (but not a method),x.fis shorthand for(*x).f. - In all other cases,
x.fis illegal. - If
xis of pointer type and has the valuenilandx.fdenotes a struct field, assigning to or evaluatingx.fcauses a run-time panic. - If
xis of interface type and has the valuenil, calling or evaluating the methodx.fcauses a run-time panic.
For example, given the declarations:
type T0 struct {
x int
}
func (*T0) M0()
type T1 struct {
y int
}
func (T1) M1()
type T2 struct {
z int
T1
*T0
}
func (*T2) M2()
type Q *T2
var t T2 // with t.T0 != nil
var p *T2 // with p != nil and (*p).T0 != nil
var q Q = pone may write:
t.z // t.z
t.y // t.T1.y
t.x // (*t.T0).x
p.z // (*p).z
p.y // (*p).T1.y
p.x // (*(*p).T0).x
q.x // (*(*q).T0).x (*q).x is a valid field selector
p.M0() // ((*p).T0).M0() M0 expects *T0 receiver
p.M1() // ((*p).T1).M1() M1 expects T1 receiver
p.M2() // p.M2() M2 expects *T2 receiver
t.M2() // (&t).M2() M2 expects *T2 receiver, see section on Callsbut the following is invalid:
q.M0() // (*q).M0 is valid but not a field selector本文中的所有译文仅用于学习和交流目的，转载请务必注明文章译者、出处、和本文链接
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