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[spec/struct] Improve introduction/overview #4334

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3 changes: 2 additions & 1 deletion spec/class.dd
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Original file line number Diff line number Diff line change
Expand Up @@ -128,7 +128,8 @@ class B : A { } // B inherits from A

$(H3 $(LNAME2 fields, Fields))

$(P Class members are always accessed with the `.` operator.
$(P Non-static member variables are called fields.
Members of a class instance are accessed with the `.` operator.
)

$(P Members of a base class can be accessed by prepending the name of
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57 changes: 39 additions & 18 deletions spec/struct.dd
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Expand Up @@ -4,14 +4,14 @@ $(SPEC_S Structs and Unions,

$(HEADERNAV_TOC)

$(H2 $(LNAME2 intro, Introduction))
$(H2 $(LNAME2 intro, Overview))

$(P Whereas $(DDLINK spec/class, Classes, classes) are reference types,
structs and unions are value types.
Structs are simple aggregations of data and their
associated operations on that data.
)

$(H3 $(LNAME2 structs, Structs))

$(GRAMMAR
$(GNAME StructDeclaration):
$(D struct) $(GLINK_LEX Identifier) $(D ;)
Expand All @@ -23,21 +23,16 @@ $(GNAME AnonStructDeclaration):
$(D struct) $(GLINK AggregateBody)
)
$(GRAMMAR
$(GNAME UnionDeclaration):
$(D union) $(GLINK_LEX Identifier) $(D ;)
$(D union) $(GLINK_LEX Identifier) $(GLINK AggregateBody)
$(GLINK2 template, UnionTemplateDeclaration)
$(I AnonUnionDeclaration)

$(GNAME AnonUnionDeclaration):
$(D union) $(GLINK AggregateBody)
)
$(GRAMMAR
$(GNAME AggregateBody):
$(D {) $(GLINK2 module, DeclDefs)$(OPT) $(D })
)

$(P The following example declares a struct type with a single integer field:)
$(P Structs are simple aggregations of data and their
associated operations on that data.)

$(P The non-static data members of a struct are called $(I fields).
Members of a struct instance are accessed with the `.` operator.
The following example declares a struct type with a single integer field:)

$(SPEC_RUNNABLE_EXAMPLE_RUN
---
Expand All @@ -48,7 +43,7 @@ struct S

void main()
{
S a;
S a; // declare struct instance
a.i = 3;

S b = a; // copy a
Expand All @@ -58,6 +53,10 @@ void main()
}
---
)
$(P Assigning (or initializing) a struct instance from an
$(DDSUBLINK spec/expression, .define-lvalue, lvalue) will copy the original struct.)

$(H3 $(LNAME2 storage, Storage))

$(P For local variables, a struct/union instance is allocated on the stack
by default. To allocate on the heap, use $(DDSUBLINK spec/expression, new_expressions,
Expand All @@ -66,13 +65,35 @@ void main()
$(PANEL
A $(LNAME2 struct-pointer, pointer to a struct) or union is automatically
dereferenced when using the `.` operator to access members.

$(SPEC_RUNNABLE_EXAMPLE_RUN
---
struct S { int i; }

S* p = new S;
assert(p.i == 0); // `p.i` is the same as `(*p).i`
S* q = p;

p.i = 2; // `p.i` is the same as `(*p).i`
assert(q.i == 2); // q points to the same struct instance as p
---
)
$(NOTE There is no `->` operator as in C.)
)

$(P **See also:** $(RELATIVE_LINK2 struct_layout, Struct Layout).)

$(H3 $(LNAME2 unions, Unions))

$(GRAMMAR
$(GNAME UnionDeclaration):
$(D union) $(GLINK_LEX Identifier) $(D ;)
$(D union) $(GLINK_LEX Identifier) $(GLINK AggregateBody)
$(GLINK2 template, UnionTemplateDeclaration)
$(I AnonUnionDeclaration)

$(GNAME AnonUnionDeclaration):
$(D union) $(GLINK AggregateBody)
)
$(P A struct can contain multiple fields which are stored sequentially.
Conversely, multiple fields in a union use overlapping storage.)

Expand Down Expand Up @@ -169,8 +190,8 @@ $(H3 $(LNAME2 recursive-types, Recursive Structs and Unions))

$(H2 $(LNAME2 struct_layout, Struct Layout))

$(P The non-static data members of a struct are called $(I fields). Fields are laid
out in lexical order. Fields are aligned according to the $(DDSUBLINK spec/attribute, align, Align Attribute)
$(P Fields are laid out in lexical order.
Fields are aligned according to the $(DDSUBLINK spec/attribute, align, Align Attribute)
in effect.
Unnamed padding is inserted between fields to align fields. There is no padding between
the first field and the start of the object.
Expand Down