Commit 071c91bf authored by Robert Griesemer's avatar Robert Griesemer

First cut at incorporating anonymous fields of pointer types into the

spec.

I have deliberately removed the wording about receivers where it was
before because I think it needs to be more precise. There is a TODO.

DELTA=90  (54 added, 0 deleted, 36 changed)
OCL=17597
CL=17716
parent 8d3a3dd9
......@@ -4,7 +4,7 @@ The Go Programming Language Specification (DRAFT)
Robert Griesemer, Rob Pike, Ken Thompson
----
(October 20, 2008)
(October 23, 2008)
This document is a semi-formal specification of the Go systems
......@@ -843,8 +843,8 @@ Types may be ``complete'' or ''incomplete''. Basic, pointer, function and
interface types are always complete (although their components, such
as the base type of a pointer type, may be incomplete). All other types are
complete when they are fully declared. Incomplete types are subject to
usage restrictions; for instance a variable type cannot be an incomplete
type.
usage restrictions; for instance the type of a variable must be complete
where the variable is declared.
CompleteType = Type .
......@@ -1118,20 +1118,29 @@ types (§Types).
f *();
}
A struct may contain ``anonymous fields'', which are declared with
a type name but no explicit field identifier. Instead, the unqualified type
name acts as the field identifier. Anonymous fields must not be interface types.
A struct may contain ``anonymous fields'', which are declared with a type
but no explicit field identifier. An anonymous field type must be specified as
a type name "T", or as a pointer to a type name ``*T'', and T itself may not be
a pointer or interface type. The unqualified type acts as the field identifier.
// A struct with two anonymous fields of type T1 and P.T2
// A struct with four anonymous fields of type T1, *T2, P.T3 and *P.T4
struct {
T1; // the field name is T1
P.T2; // the field name is the unqualified type name T2
*T2; // the field name is T2
P.T3; // the field name is the unqualified type name T3
*P.T4; // the field name is the unqualified type name T4
x, y int;
}
The unqualified type name of an anonymous field must not conflict with the
field identifier (or unqualified type name for an anonymous field) of any
other field within the struct.
other field within the struct. The following declaration is illegal:
struct {
T; // conflicts with anonymous field *T and *P.T
*T; // conflicts with anonymous field T and *P.T
*P.T; // conflicts with anonymous field T and *T
}
Fields and methods (§Method declarations) of an anonymous field become directly
accessible as fields and methods of the struct without the need to provide the
......@@ -1570,32 +1579,77 @@ A primary expression of the form
x.f
denotes the field or method f of the value denoted by x (or of *x if
x is of pointer type). The identifier f is called the ``selector''.
The following rules apply:
x is of pointer type). The identifier f is called the (field or method)
``selector''.
A selector f may denote a field f declared in a type T, or it may refer
to a field f declared in a nested anonymous field of T. Analogously,
f may denote a method f of T, or it may refer to a method f of the type
of a nested anonymous field of T. The number of anonymous fields traversed
to get to the field or method is called its ``depth'' in T.
More precisely, the depth of a field or method f declared in T is zero.
The depth of a field or method f declared anywhere inside
an anonymous field A declared in T is the depth of f in A plus one.
The following rules apply to selectors:
1) For a value x of type T or *T where T is not an interface type,
x.f denotes the field or method at the shallowest depth in T where there
is such an f. The type of x.f is the type of the field or method f.
If there is not exactly one f with shallowest depth, the selector
expression is illegal.
2) For a variable x of type I or *I where I is an interface type,
x.f denotes the actual method with name f of the value assigned
to x if there is such a method. The type of x.f is the type
of the method f. If no value or nil was assigned to x, x.f is illegal.
3) In all other cases, x.f is illegal.
For x of type S or *S where S is a struct type (§Struct types):
Thus, selectors automatically dereference pointers as necessary. For instance,
for an x of type *T where T declares an f, x.f is a shortcut for (*x).f.
Furthermore, for an x of type T containing an anonymous field A declared as *A
inside T, and where A contains a field f, x.f is a shortcut for (*x.A).f
(assuming that the selector is legal in the first place).
The following examples illustrate selector use in more detail. Given the
declarations:
type T0 struct {
x int;
}
func (recv *T0) M0()
type T1 struct {
y int;
}
func (recv T1) M1()
type T2 struct {
z int;
T1;
*T0;
}
1) If f is declared as a (named or anonymous) field of S then x.f denotes
that field.
func (recv *T2) M2()
2) If f is declared (or forward-declared) as a method of S textually
before x.f then x.f denotes that method and x becomes the receiver
of f.
var p *T2; // with p != nil and p.T1 != nil
3) Otherwise, if there is single anononymous field A of S such that
x.A.f denotes a valid field according to 1) or 2), then x.f is
a shortcut for x.A.f, and x.A becomes the receiver of f.
If there is none or more then one anonymous field of S satisfying
this criterion, x.f is illegal.
we can write:
p.z // (*p).z
p.y // ((*p).T1).y
p.x // (*(*p).T0).x
For x of type I or *I where I is an interface type (§Interface types):
p.M2 // (*p).M2
p.M1 // ((*p).T1).M1
p.M0 // ((*p).T0).M0
- If f is a method declared in I then x.f denotes the actual method with
name f of the value assigned to the variable x and x becomes the receiver
of f. If no value or nil was assigned to x, x.f is illegal.
Otherwise, x.f is illegal.
TODO: Specify what happens to receivers.
Indexes
......
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