### 10.6 Expressions

The internal representation for expressions is for the most part quite straightforward. However, there are a few facts that one must bear in mind. In particular, the expression “tree” is actually a directed acyclic graph. (For example there may be many references to the integer constant zero throughout the source program; many of these will be represented by the same expression node.) You should not rely on certain kinds of node being shared, nor should you rely on certain kinds of nodes being unshared.

The following macros can be used with all expression nodes:

`TREE_TYPE`
  Returns the type of the expression. This value may not be precisely the same type that would be given the expression in the original program.

In what follows, some nodes that one might expect to always have type `bool` are documented to have either integral or boolean type. At some point in the future, the C front end may also make use of this same intermediate representation, and at this point these nodes will certainly have integral type. The previous sentence is not meant to imply that the C++ front end does not or will not give these nodes integral type.

Below, we list the various kinds of expression nodes. Except where noted otherwise, the operands to an expression are accessed using the `TREE_OPERAND` macro. For example, to access the first operand to a binary plus expression `expr`, use:

         TREE_OPERAND (expr, 0)

As this example indicates, the operands are zero-indexed.