GNU Compiler Collection (GCC) Internals: Statements for C++

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#### 10.10.5 Statements for C++

A function that has a definition in the current translation unit will have a non-`NULL` `DECL_INITIAL`. However, back ends should not make use of the particular value given by `DECL_INITIAL`.

The `DECL_SAVED_TREE` macro will give the complete body of the function.

#### 10.10.5.1 Statements

There are tree nodes corresponding to all of the source-level statement constructs, used within the C and C++ frontends. These are enumerated here, together with a list of the various macros that can be used to obtain information about them. There are a few macros that can be used with all statements:

`STMT_IS_FULL_EXPR_P`

  In C++, statements normally constitute “full expressions”; temporaries created during a statement are destroyed when the statement is complete. However, G++ sometimes represents expressions by statements; these statements will not have `STMT_IS_FULL_EXPR_P` set. Temporaries created during such statements should be destroyed when the innermost enclosing statement with `STMT_IS_FULL_EXPR_P` set is exited.

Here is the list of the various statement nodes, and the macros used to access them. This documentation describes the use of these nodes in non-template functions (including instantiations of template functions). In template functions, the same nodes are used, but sometimes in slightly different ways.

Many of the statements have substatements. For example, a `while` loop will have a body, which is itself a statement. If the substatement is `NULL_TREE`, it is considered equivalent to a statement consisting of a single `;`, i.e., an expression statement in which the expression has been omitted. A substatement may in fact be a list of statements, connected via their `TREE_CHAIN`s. So, you should always process the statement tree by looping over substatements, like this:

    void process_stmt (stmt)
         tree stmt;
    {
      while (stmt)
        {
          switch (TREE_CODE (stmt))
            {
            case IF_STMT:
              process_stmt (THEN_CLAUSE (stmt));
              /* More processing here.  */
              break;

            …
            }

          stmt = TREE_CHAIN (stmt);
        }
    }

In other words, while the `then` clause of an `if` statement in C++ can be only one statement (although that one statement may be a compound statement), the intermediate representation will sometimes use several statements chained together.

`BREAK_STMT`

  Used to represent a `break` statement. There are no additional fields.

`CILK_SPAWN_STMT`

  Used to represent a spawning function in the Cilk Plus language extension. This tree has one field that holds the name of the spawning function. `_Cilk_spawn` can be written in C in the following way:

      _Cilk_spawn <function_name> (<parameters>);

  Detailed description for usage and functionality of `_Cilk_spawn` can be found at http://www.cilkplus.org

`CILK_SYNC_STMT`

  This statement is part of the Cilk Plus language extension. It indicates that the current function cannot continue in parallel with its spawned children. There are no additional fields. `_Cilk_sync` can be written in C in the following way:

      _Cilk_sync;

`CLEANUP_STMT`

  Used to represent an action that should take place upon exit from the enclosing scope. Typically, these actions are calls to destructors for local objects, but back ends cannot rely on this fact. If these nodes are in fact representing such destructors, `CLEANUP_DECL` will be the `VAR_DECL` destroyed. Otherwise, `CLEANUP_DECL` will be `NULL_TREE`. In any case, the `CLEANUP_EXPR` is the expression to execute. The cleanups executed on exit from a scope should be run in the reverse order of the order in which the associated `CLEANUP_STMT`s were encountered.

`CONTINUE_STMT`

  Used to represent a `continue` statement. There are no additional fields.

`CTOR_STMT`

  Used to mark the beginning (if `CTOR_BEGIN_P` holds) or end (if `CTOR_END_P` holds of the main body of a constructor. See also `SUBOBJECT` for more information on how to use these nodes.

`DO_STMT`

  Used to represent a `do` loop. The body of the loop is given by `DO_BODY` while the termination condition for the loop is given by `DO_COND`. The condition for a `do`-statement is always an expression.

`EMPTY_CLASS_EXPR`

  Used to represent a temporary object of a class with no data whose address is never taken. (All such objects are interchangeable.) The `TREE_TYPE` represents the type of the object.

`EXPR_STMT`

  Used to represent an expression statement. Use `EXPR_STMT_EXPR` to obtain the expression.

`FOR_STMT`

  Used to represent a `for` statement. The `FOR_INIT_STMT` is the initialization statement for the loop. The `FOR_COND` is the termination condition. The `FOR_EXPR` is the expression executed right before the `FOR_COND` on each loop iteration; often, this expression increments a counter. The body of the loop is given by `FOR_BODY`. Note that `FOR_INIT_STMT` and `FOR_BODY` return statements, while `FOR_COND` and `FOR_EXPR` return expressions.

`HANDLER`

  Used to represent a C++ `catch` block. The `HANDLER_TYPE` is the type of exception that will be caught by this handler; it is equal (by pointer equality) to `NULL` if this handler is for all types. `HANDLER_PARMS` is the `DECL_STMT` for the catch parameter, and `HANDLER_BODY` is the code for the block itself.

`IF_STMT`

  Used to represent an `if` statement. The `IF_COND` is the expression.

  If the condition is a `TREE_LIST`, then the `TREE_PURPOSE` is a statement (usually a `DECL_STMT`). Each time the condition is evaluated, the statement should be executed. Then, the `TREE_VALUE` should be used as the conditional expression itself. This representation is used to handle C++ code like this:

  C++ distinguishes between this and `COND_EXPR` for handling templates.

      if (int i = 7) …

  where there is a new local variable (or variables) declared within the condition.

  The `THEN_CLAUSE` represents the statement given by the `then` condition, while the `ELSE_CLAUSE` represents the statement given by the `else` condition.

`SUBOBJECT`

  In a constructor, these nodes are used to mark the point at which a subobject of `this` is fully constructed. If, after this point, an exception is thrown before a `CTOR_STMT` with `CTOR_END_P` set is encountered, the `SUBOBJECT_CLEANUP` must be executed. The cleanups must be executed in the reverse order in which they appear.

`SWITCH_STMT`

  Used to represent a `switch` statement. The `SWITCH_STMT_COND` is the expression on which the switch is occurring. See the documentation for an `IF_STMT` for more information on the representation used for the condition. The `SWITCH_STMT_BODY` is the body of the switch statement. The `SWITCH_STMT_TYPE` is the original type of switch expression as given in the source, before any compiler conversions.

`TRY_BLOCK`

  Used to represent a `try` block. The body of the try block is given by `TRY_STMTS`. Each of the catch blocks is a `HANDLER` node. The first handler is given by `TRY_HANDLERS`. Subsequent handlers are obtained by following the `TREE_CHAIN` link from one handler to the next. The body of the handler is given by `HANDLER_BODY`.

  If `CLEANUP_P` holds of the `TRY_BLOCK`, then the `TRY_HANDLERS` will not be a `HANDLER` node. Instead, it will be an expression that should be executed if an exception is thrown in the try block. It must rethrow the exception after executing that code. And, if an exception is thrown while the expression is executing, `terminate` must be called.

`USING_STMT`

  Used to represent a `using` directive. The namespace is given by `USING_STMT_NAMESPACE`, which will be a NAMESPACE_DECL. This node is needed inside template functions, to implement using directives during instantiation.

`WHILE_STMT`

  Used to represent a `while` loop. The `WHILE_COND` is the termination condition for the loop. See the documentation for an `IF_STMT` for more information on the representation used for the condition.

  The `WHILE_BODY` is the body of the loop.

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