c-tree.texi

理解和实践操作系统的一本好书

@c ---------------------------------------------------------------------
@c Scopes
@c ---------------------------------------------------------------------

@node Scopes
@section Scopes
@cindex namespace, class, scope

The root of the entire intermediate representation is the variable
@code{global_namespace}.  This is the namespace specified with @code{::}
in C++ source code.  All other namespaces, types, variables, functions,
and so forth can be found starting with this namespace.

Besides namespaces, the other high-level scoping construct in C++ is the
class.  (Throughout this manual the term @dfn{class} is used to mean the
types referred to in the ANSI/ISO C++ Standard as classes; these include
types defined with the @code{class}, @code{struct}, and @code{union}
keywords.)

@menu
* Namespaces::          Member functions, types, etc.
* Classes::             Members, bases, friends, etc.
@end menu

@c ---------------------------------------------------------------------
@c Namespaces
@c ---------------------------------------------------------------------

@node Namespaces
@subsection Namespaces
@cindex namespace
@tindex NAMESPACE_DECL

A namespace is represented by a @code{NAMESPACE_DECL} node.

However, except for the fact that it is distinguished as the root of the
representation, the global namespace is no different from any other
namespace.  Thus, in what follows, we describe namespaces generally,
rather than the global namespace in particular.

The following macros and functions can be used on a @code{NAMESPACE_DECL}:

@ftable @code
@item DECL_NAME
This macro is used to obtain the @code{IDENTIFIER_NODE} corresponding to
the unqualified name of the name of the namespace (@pxref{Identifiers}).
The name of the global namespace is @samp{::}, even though in C++ the
global namespace is unnamed.  However, you should use comparison with
@code{global_namespace}, rather than @code{DECL_NAME} to determine
whether or not a namespace is the global one.  An unnamed namespace
will have a @code{DECL_NAME} equal to @code{anonymous_namespace_name}.
Within a single translation unit, all unnamed namespaces will have the
same name.

@item DECL_CONTEXT
This macro returns the enclosing namespace.  The @code{DECL_CONTEXT} for
the @code{global_namespace} is @code{NULL_TREE}.

@item DECL_NAMESPACE_ALIAS
If this declaration is for a namespace alias, then
@code{DECL_NAMESPACE_ALIAS} is the namespace for which this one is an
alias.

Do not attempt to use @code{cp_namespace_decls} for a namespace which is
an alias.  Instead, follow @code{DECL_NAMESPACE_ALIAS} links until you
reach an ordinary, non-alias, namespace, and call
@code{cp_namespace_decls} there.

@item DECL_NAMESPACE_STD_P
This predicate holds if the namespace is the special @code{::std}
namespace.

@item cp_namespace_decls
This function will return the declarations contained in the namespace,
including types, overloaded functions, other namespaces, and so forth.
If there are no declarations, this function will return
@code{NULL_TREE}.  The declarations are connected through their
@code{TREE_CHAIN} fields.

Although most entries on this list will be declarations,
@code{TREE_LIST} nodes may also appear.  In this case, the
@code{TREE_VALUE} will be an @code{OVERLOAD}.  The value of the
@code{TREE_PURPOSE} is unspecified; back ends should ignore this value.
As with the other kinds of declarations returned by
@code{cp_namespace_decls}, the @code{TREE_CHAIN} will point to the next
declaration in this list.

For more information on the kinds of declarations that can occur on this
list, @xref{Declarations}.  Some declarations will not appear on this
list.  In particular, no @code{FIELD_DECL}, @code{LABEL_DECL}, or
@code{PARM_DECL} nodes will appear here.

This function cannot be used with namespaces that have
@code{DECL_NAMESPACE_ALIAS} set.

@end ftable

@c ---------------------------------------------------------------------
@c Classes
@c ---------------------------------------------------------------------

@node Classes
@subsection Classes
@cindex class
@tindex RECORD_TYPE
@tindex UNION_TYPE
@findex CLASSTYPE_DECLARED_CLASS
@findex TYPE_BINFO
@findex BINFO_TYPE
@findex TYPE_FIELDS
@findex TYPE_VFIELD
@findex TYPE_METHODS

A class type is represented by either a @code{RECORD_TYPE} or a
@code{UNION_TYPE}.  A class declared with the @code{union} tag is
represented by a @code{UNION_TYPE}, while classes declared with either
the @code{struct} or the @code{class} tag are represented by
@code{RECORD_TYPE}s.  You can use the @code{CLASSTYPE_DECLARED_CLASS}
macro to discern whether or not a particular type is a @code{class} as
opposed to a @code{struct}.  This macro will be true only for classes
declared with the @code{class} tag.

Almost all non-function members are available on the @code{TYPE_FIELDS}
list.  Given one member, the next can be found by following the
@code{TREE_CHAIN}.  You should not depend in any way on the order in
which fields appear on this list.  All nodes on this list will be
@samp{DECL} nodes.  A @code{FIELD_DECL} is used to represent a non-static
data member, a @code{VAR_DECL} is used to represent a static data
member, and a @code{TYPE_DECL} is used to represent a type.  Note that
the @code{CONST_DECL} for an enumeration constant will appear on this
list, if the enumeration type was declared in the class.  (Of course,
the @code{TYPE_DECL} for the enumeration type will appear here as well.)
There are no entries for base classes on this list.  In particular,
there is no @code{FIELD_DECL} for the ``base-class portion'' of an
object.

The @code{TYPE_VFIELD} is a compiler-generated field used to point to
virtual function tables.  It may or may not appear on the
@code{TYPE_FIELDS} list.  However, back ends should handle the
@code{TYPE_VFIELD} just like all the entries on the @code{TYPE_FIELDS}
list.

The function members are available on the @code{TYPE_METHODS} list.
Again, subsequent members are found by following the @code{TREE_CHAIN}
field.  If a function is overloaded, each of the overloaded functions
appears; no @code{OVERLOAD} nodes appear on the @code{TYPE_METHODS}
list.  Implicitly declared functions (including default constructors,
copy constructors, assignment operators, and destructors) will appear on
this list as well.

Every class has an associated @dfn{binfo}, which can be obtained with
@code{TYPE_BINFO}.  Binfos are used to represent base-classes.  The
binfo given by @code{TYPE_BINFO} is the degenerate case, whereby every
class is considered to be its own base-class.  The base binfos for a
particular binfo are held in a vector, whose length is obtained with
@code{BINFO_N_BASE_BINFOS}.  The base binfos themselves are obtained
with @code{BINFO_BASE_BINFO} and @code{BINFO_BASE_ITERATE}.  To add a
new binfo, use @code{BINFO_BASE_APPEND}.  The vector of base binfos can
be obtained with @code{BINFO_BASE_BINFOS}, but normally you do not need
to use that.  The class type associated with a binfo is given by
@code{BINFO_TYPE}.  It is not always the case that @code{BINFO_TYPE
(TYPE_BINFO (x))}, because of typedefs and qualified types.  Neither is
it the case that @code{TYPE_BINFO (BINFO_TYPE (y))} is the same binfo as
@code{y}.  The reason is that if @code{y} is a binfo representing a
base-class @code{B} of a derived class @code{D}, then @code{BINFO_TYPE
(y)} will be @code{B}, and @code{TYPE_BINFO (BINFO_TYPE (y))} will be
@code{B} as its own base-class, rather than as a base-class of @code{D}.

The access to a base type can be found with @code{BINFO_BASE_ACCESS}.
This will produce @code{access_public_node}, @code{access_private_node}
or @code{access_protected_node}.  If bases are always public,
@code{BINFO_BASE_ACCESSES} may be @code{NULL}.

@code{BINFO_VIRTUAL_P} is used to specify whether the binfo is inherited
virtually or not.  The other flags, @code{BINFO_MARKED_P} and
@code{BINFO_FLAG_1} to @code{BINFO_FLAG_6} can be used for language
specific use.

The following macros can be used on a tree node representing a class-type.

@ftable @code
@item LOCAL_CLASS_P
This predicate holds if the class is local class @emph{i.e.}@: declared
inside a function body.

@item TYPE_POLYMORPHIC_P
This predicate holds if the class has at least one virtual function
(declared or inherited).

@item TYPE_HAS_DEFAULT_CONSTRUCTOR
This predicate holds whenever its argument represents a class-type with
default constructor.

@item CLASSTYPE_HAS_MUTABLE
@itemx TYPE_HAS_MUTABLE_P
These predicates hold for a class-type having a mutable data member.

@item CLASSTYPE_NON_POD_P
This predicate holds only for class-types that are not PODs.

@item TYPE_HAS_NEW_OPERATOR
This predicate holds for a class-type that defines
@code{operator new}.

@item TYPE_HAS_ARRAY_NEW_OPERATOR
This predicate holds for a class-type for which
@code{operator new[]} is defined.

@item TYPE_OVERLOADS_CALL_EXPR
This predicate holds for class-type for which the function call
@code{operator()} is overloaded.

@item TYPE_OVERLOADS_ARRAY_REF
This predicate holds for a class-type that overloads
@code{operator[]}

@item TYPE_OVERLOADS_ARROW
This predicate holds for a class-type for which @code{operator->} is
overloaded.

@end ftable

@c ---------------------------------------------------------------------
@c Declarations
@c ---------------------------------------------------------------------

@node Declarations
@section Declarations
@cindex declaration
@cindex variable
@cindex type declaration
@tindex LABEL_DECL
@tindex CONST_DECL
@tindex TYPE_DECL
@tindex VAR_DECL
@tindex PARM_DECL
@tindex FIELD_DECL
@tindex NAMESPACE_DECL
@tindex RESULT_DECL
@tindex TEMPLATE_DECL
@tindex THUNK_DECL
@tindex USING_DECL
@findex THUNK_DELTA
@findex DECL_INITIAL
@findex DECL_SIZE
@findex DECL_ALIGN
@findex DECL_EXTERNAL

This section covers the various kinds of declarations that appear in the
internal representation, except for declarations of functions
(represented by @code{FUNCTION_DECL} nodes), which are described in
@ref{Functions}.

@menu
* Working with declarations::  Macros and functions that work on
declarations.
* Internal structure:: How declaration nodes are represented. 
@end menu

@node Working with declarations
@subsection Working with declarations

Some macros can be used with any kind of declaration.  These include:
@ftable @code
@item DECL_NAME
This macro returns an @code{IDENTIFIER_NODE} giving the name of the
entity.

@item TREE_TYPE
This macro returns the type of the entity declared.

@item TREE_FILENAME
This macro returns the name of the file in which the entity was
declared, as a @code{char*}.  For an entity declared implicitly by the
compiler (like @code{__builtin_memcpy}), this will be the string
@code{"<internal>"}.

@item TREE_LINENO
This macro returns the line number at which the entity was declared, as
an @code{int}.

@item DECL_ARTIFICIAL
This predicate holds if the declaration was implicitly generated by the
compiler.  For example, this predicate will hold of an implicitly
declared member function, or of the @code{TYPE_DECL} implicitly
generated for a class type.  Recall that in C++ code like:
@smallexample
struct S @{@};
@end smallexample
@noindent
is roughly equivalent to C code like:
@smallexample
struct S @{@};
typedef struct S S;
@end smallexample
The implicitly generated @code{typedef} declaration is represented by a
@code{TYPE_DECL} for which @code{DECL_ARTIFICIAL} holds.

@item DECL_NAMESPACE_SCOPE_P
This predicate holds if the entity was declared at a namespace scope.

@item DECL_CLASS_SCOPE_P
This predicate holds if the entity was declared at a class scope.

@item DECL_FUNCTION_SCOPE_P
This predicate holds if the entity was declared inside a function
body.

@end ftable

The various kinds of declarations include:
@table @code
@item LABEL_DECL
These nodes are used to represent labels in function bodies.  For more
information, see @ref{Functions}.  These nodes only appear in block
scopes.

@item CONST_DECL
These nodes are used to represent enumeration constants.  The value of
the constant is given by @code{DECL_INITIAL} which will be an
@code{INTEGER_CST} with the same type as the @code{TREE_TYPE} of the
@code{CONST_DECL}, i.e., an @code{ENUMERAL_TYPE}.

@item RESULT_DECL
These nodes represent the value returned by a function.  When a value is
assigned to a @code{RESULT_DECL}, that indicates that the value should
be returned, via bitwise copy, by the function.  You can use
@code{DECL_SIZE} and @code{DECL_ALIGN} on a @code{RESULT_DECL}, just as
with a @code{VAR_DECL}.