GNU Compiler Collection (GCC) Internals: Classes

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#### 10.10.3 Classes

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

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

Almost all non-function members are available on the `TYPE_FIELDS` list. Given one member, the next can be found by following the `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 ‘`DECL`’ nodes. A `FIELD_DECL` is used to represent a non-static data member, a `VAR_DECL` is used to represent a static data member, and a `TYPE_DECL` is used to represent a type. Note that the `CONST_DECL` for an enumeration constant will appear on this list, if the enumeration type was declared in the class. (Of course, the `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 `FIELD_DECL` for the “base-class portion” of an object.

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

The function members are available on the `TYPE_METHODS` list. Again, subsequent members are found by following the `TREE_CHAIN` field. If a function is overloaded, each of the overloaded functions appears; no `OVERLOAD` nodes appear on the `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 binfo, which can be obtained with `TYPE_BINFO`. Binfos are used to represent base-classes. The binfo given by `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 `BINFO_N_BASE_BINFOS`. The base binfos themselves are obtained with `BINFO_BASE_BINFO` and `BINFO_BASE_ITERATE`. To add a new binfo, use `BINFO_BASE_APPEND`. The vector of base binfos can be obtained with `BINFO_BASE_BINFOS`, but normally you do not need to use that. The class type associated with a binfo is given by `BINFO_TYPE`. It is not always the case that `BINFO_TYPE (TYPE_BINFO (x))`, because of typedefs and qualified types. Neither is it the case that `TYPE_BINFO (BINFO_TYPE (y))` is the same binfo as `y`. The reason is that if `y` is a binfo representing a base-class `B` of a derived class `D`, then `BINFO_TYPE (y)` will be `B`, and `TYPE_BINFO (BINFO_TYPE (y))` will be `B` as its own base-class, rather than as a base-class of `D`.

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

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

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

`LOCAL_CLASS_P`

  This predicate holds if the class is local class i.e. declared inside a function body.

`TYPE_POLYMORPHIC_P`

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

`TYPE_HAS_DEFAULT_CONSTRUCTOR`

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

`CLASSTYPE_HAS_MUTABLE`
`TYPE_HAS_MUTABLE_P`

  These predicates hold for a class-type having a mutable data member.

`CLASSTYPE_NON_POD_P`

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

`TYPE_HAS_NEW_OPERATOR`

  This predicate holds for a class-type that defines `operator new`.

`TYPE_HAS_ARRAY_NEW_OPERATOR`

  This predicate holds for a class-type for which `operator new[]` is defined.

`TYPE_OVERLOADS_CALL_EXPR`

  This predicate holds for class-type for which the function call `operator()` is overloaded.

`TYPE_OVERLOADS_ARRAY_REF`

  This predicate holds for a class-type that overloads `operator[]`

`TYPE_OVERLOADS_ARROW`

  This predicate holds for a class-type for which `operator->` is overloaded.

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