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All types have corresponding tree nodes. However, you should not assume that there is exactly one tree node corresponding to each type. There are often several nodes each of which correspond to the same type.

For the most part, different kinds of types have different tree codes. (For example, pointer types use a POINTER_TYPE code while arrays use an ARRAY_TYPE code.) However, pointers to member functions use the RECORD_TYPE code. Therefore, when writing a switch statement that depends on the code associated with a particular type, you should take care to handle pointers to member functions under the RECORD_TYPE case label.

In C++, an array type is not qualified; rather the type of the array elements is qualified. This situation is reflected in the intermediate representation. The macros described here will always examine the qualification of the underlying element type when applied to an array type. (If the element type is itself an array, then the recursion continues until a non-array type is found, and the qualification of this type is examined.) So, for example, CP_TYPE_CONST_P will hold of the type const int ()[7], denoting an array of seven ints.

The following functions and macros deal with cv-qualification of types:

This macro returns the set of type qualifiers applied to this type. This value is TYPE_UNQUALIFIED if no qualifiers have been applied. The TYPE_QUAL_CONST bit is set if the type is const-qualified. The TYPE_QUAL_VOLATILE bit is set if the type is volatile-qualified. The TYPE_QUAL_RESTRICT bit is set if the type is restrict-qualified.
This macro holds if the type is const-qualified.
This macro holds if the type is volatile-qualified.
This macro holds if the type is restrict-qualified.
This predicate holds for a type that is const-qualified, but not volatile-qualified; other cv-qualifiers are ignored as well: only the const-ness is tested.
This macro returns the unqualified version of a type. It may be applied to an unqualified type, but it is not always the identity function in that case.

A few other macros and functions are usable with all types:

The number of bits required to represent the type, represented as an INTEGER_CST. For an incomplete type, TYPE_SIZE will be NULL_TREE.
The alignment of the type, in bits, represented as an int.
This macro returns a declaration (in the form of a TYPE_DECL) for the type. (Note this macro does not return a IDENTIFIER_NODE, as you might expect, given its name!) You can look at the DECL_NAME of the TYPE_DECL to obtain the actual name of the type. The TYPE_NAME will be NULL_TREE for a type that is not a built-in type, the result of a typedef, or a named class type.
This predicate holds if the type is an integral type. Notice that in C++, enumerations are not integral types.
This predicate holds if the type is an integral type (in the C++ sense) or a floating point type.
This predicate holds for a class-type.
This predicate holds for a built-in type.
This predicate holds if the type is a pointer to data member.
This predicate holds if the type is a pointer type, and the pointee is not a data member.
This predicate holds for a pointer to function type.
This predicate holds for a pointer to object type. Note however that it does not hold for the generic pointer to object type void *. You may use TYPE_PTROBV_P to test for a pointer to object type as well as void *.
This predicate takes two types as input, and holds if they are the same type. For example, if one type is a typedef for the other, or both are typedefs for the same type. This predicate also holds if the two trees given as input are simply copies of one another; i.e., there is no difference between them at the source level, but, for whatever reason, a duplicate has been made in the representation. You should never use == (pointer equality) to compare types; always use same_type_p instead.

Detailed below are the various kinds of types, and the macros that can be used to access them. Although other kinds of types are used elsewhere in G++, the types described here are the only ones that you will encounter while examining the intermediate representation.

Used to represent the void type.
Used to represent the various integral types, including char, short, int, long, and long long. This code is not used for enumeration types, nor for the bool type. Note that GCC's CHAR_TYPE node is not used to represent char. The TYPE_PRECISION is the number of bits used in the representation, represented as an unsigned int. (Note that in the general case this is not the same value as TYPE_SIZE; suppose that there were a 24-bit integer type, but that alignment requirements for the ABI required 32-bit alignment. Then, TYPE_SIZE would be an INTEGER_CST for 32, while TYPE_PRECISION would be 24.) The integer type is unsigned if TREE_UNSIGNED holds; otherwise, it is signed.

The TYPE_MIN_VALUE is an INTEGER_CST for the smallest integer that may be represented by this type. Similarly, the TYPE_MAX_VALUE is an INTEGER_CST for the largest integer that may be represented by this type.

Used to represent the float, double, and long double types. The number of bits in the floating-point representation is given by TYPE_PRECISION, as in the INTEGER_TYPE case.
Used to represent GCC built-in __complex__ data types. The TREE_TYPE is the type of the real and imaginary parts.
Used to represent an enumeration type. The TYPE_PRECISION gives (as an int), the number of bits used to represent the type. If there are no negative enumeration constants, TREE_UNSIGNED will hold. The minimum and maximum enumeration constants may be obtained with TYPE_MIN_VALUE and TYPE_MAX_VALUE, respectively; each of these macros returns an INTEGER_CST.

The actual enumeration constants themselves may be obtained by looking at the TYPE_VALUES. This macro will return a TREE_LIST, containing the constants. The TREE_PURPOSE of each node will be an IDENTIFIER_NODE giving the name of the constant; the TREE_VALUE will be an INTEGER_CST giving the value assigned to that constant. These constants will appear in the order in which they were declared. The TREE_TYPE of each of these constants will be the type of enumeration type itself.

Used to represent the bool type.
Used to represent pointer types, and pointer to data member types. The TREE_TYPE gives the type to which this type points. If the type is a pointer to data member type, then TYPE_PTRMEM_P will hold. For a pointer to data member type of the form T X::*, TYPE_PTRMEM_CLASS_TYPE will be the type X, while TYPE_PTRMEM_POINTED_TO_TYPE will be the type T.
Used to represent reference types. The TREE_TYPE gives the type to which this type refers.
Used to represent the type of non-member functions and of static member functions. The TREE_TYPE gives the return type of the function. The TYPE_ARG_TYPES are a TREE_LIST of the argument types. The TREE_VALUE of each node in this list is the type of the corresponding argument; the TREE_PURPOSE is an expression for the default argument value, if any. If the last node in the list is void_list_node (a TREE_LIST node whose TREE_VALUE is the void_type_node), then functions of this type do not take variable arguments. Otherwise, they do take a variable number of arguments.

Note that in C (but not in C++) a function declared like void f() is an unprototyped function taking a variable number of arguments; the TYPE_ARG_TYPES of such a function will be NULL.

Used to represent the type of a non-static member function. Like a FUNCTION_TYPE, the return type is given by the TREE_TYPE. The type of *this, i.e., the class of which functions of this type are a member, is given by the TYPE_METHOD_BASETYPE. The TYPE_ARG_TYPES is the parameter list, as for a FUNCTION_TYPE, and includes the this argument.
Used to represent array types. The TREE_TYPE gives the type of the elements in the array. If the array-bound is present in the type, the TYPE_DOMAIN is an INTEGER_TYPE whose TYPE_MIN_VALUE and TYPE_MAX_VALUE will be the lower and upper bounds of the array, respectively. The TYPE_MIN_VALUE will always be an INTEGER_CST for zero, while the TYPE_MAX_VALUE will be one less than the number of elements in the array, i.e., the highest value which may be used to index an element in the array.
Used to represent struct and class types, as well as pointers to member functions and similar constructs in other languages. TYPE_FIELDS contains the items contained in this type, each of which can be a FIELD_DECL, VAR_DECL, CONST_DECL, or TYPE_DECL. You may not make any assumptions about the ordering of the fields in the type or whether one or more of them overlap. If TYPE_PTRMEMFUNC_P holds, then this type is a pointer-to-member type. In that case, the TYPE_PTRMEMFUNC_FN_TYPE is a POINTER_TYPE pointing to a METHOD_TYPE. The METHOD_TYPE is the type of a function pointed to by the pointer-to-member function. If TYPE_PTRMEMFUNC_P does not hold, this type is a class type. For more information, see see Classes.
Used to represent union types. Similar to RECORD_TYPE except that all FIELD_DECL nodes in TYPE_FIELD start at bit position zero.
Used to represent part of a variant record in Ada. Similar to UNION_TYPE except that each FIELD_DECL has a DECL_QUALIFIER field, which contains a boolean expression that indicates whether the field is present in the object. The type will only have one field, so each field's DECL_QUALIFIER is only evaluated if none of the expressions in the previous fields in TYPE_FIELDS are nonzero. Normally these expressions will reference a field in the outer object using a PLACEHOLDER_EXPR.
This node is used to represent a type the knowledge of which is insufficient for a sound processing.
This node is used to represent a data member; for example a pointer-to-data-member is represented by a POINTER_TYPE whose TREE_TYPE is an OFFSET_TYPE. For a data member X::m the TYPE_OFFSET_BASETYPE is X and the TREE_TYPE is the type of m.
Used to represent a construct of the form typename T::A. The TYPE_CONTEXT is T; the TYPE_NAME is an IDENTIFIER_NODE for A. If the type is specified via a template-id, then TYPENAME_TYPE_FULLNAME yields a TEMPLATE_ID_EXPR. The TREE_TYPE is non-NULL if the node is implicitly generated in support for the implicit typename extension; in which case the TREE_TYPE is a type node for the base-class.
Used to represent the __typeof__ extension. The TYPE_FIELDS is the expression the type of which is being represented.

There are variables whose values represent some of the basic types. These include:

A node for void.
A node for int.
A node for unsigned int.
A node for char.

It may sometimes be useful to compare one of these variables with a type in hand, using same_type_p.