18. Trees: The intermediate representation used by the C and C++ front-ends

This chapter documents the internal representation used by GCC and C++ to represent C and C++ source programs. When presented with a C or C++ source program, GCC parses the program, performs semantic analysis (including the generation of error messages), and then produces the internal representation described here. This representation contains a complete representation for the entire translation unit provided as input to the front-end. This representation is then typically processed by a code-generator in order to produce machine code, but could also be used in the creation of source browsers, intelligent editors, automatic documentation generators, interpreters, and any other programs needing the ability to process C or C++ code.

This chapter explains the internal representation. In particular, it documents the internal representation for C and C++ source constructs, and the macros, functions, and variables that can be used to access these constructs. The C++ representation which is largely a superset of the representation used in the C front-end. There is only one construct used in C that does not appear in the C++ front-end and that is the GNU "nested function" extension. Many of the macros documented here do not apply in C because the corresponding language constructs do not appear in C.

If you are developing a "back-end", be it is a code-generator or some other tool, that uses this representation, you may occasionally find that you need to ask questions not easily answered by the functions and macros available here. If that situation occurs, it is quite likely that GCC already supports the functionality you desire, but that the interface is simply not documented here. In that case, you should ask the GCC maintainers (via mail to gcc@gcc.gnu.org) about documenting the functionality you require. Similarly, if you find yourself writing functions that do not deal directly with your back-end, but instead might be useful to other people using the GCC front-end, you should submit your patches for inclusion in GCC.

18.1 Deficiencies		Topics net yet covered in this document.
18.2 Overview		All about trees.
18.3 Types		Fundamental and aggregate types.
18.4 Scopes		Namespaces and classes.
18.6 Functions		Overloading, function bodies, and linkage.
18.5 Declarations		Type declarations and variables.
18.7 Expressions		From typeid to throw.