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<title>GNU Compiler Collection (GCC) Internals: Statements for C++</title>
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<a name="Statements-for-C_002b_002b"></a>
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<div class="header">
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<p>
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Next: <a href="C_002b_002b-Expressions.html#C_002b_002b-Expressions" accesskey="n" rel="next">C++ Expressions</a>, Previous: <a href="Functions-for-C_002b_002b.html#Functions-for-C_002b_002b" accesskey="p" rel="prev">Functions for C++</a>, Up: <a href="C-and-C_002b_002b-Trees.html#C-and-C_002b_002b-Trees" accesskey="u" rel="up">C and C++ Trees</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Statements-for-C_002b_002b-1"></a>
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<h4 class="subsection">10.10.5 Statements for C++</h4>
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<a name="index-statements-1"></a>
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<a name="index-BREAK_005fSTMT"></a>
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<a name="index-CLEANUP_005fSTMT"></a>
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<a name="index-CLEANUP_005fDECL"></a>
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<a name="index-CLEANUP_005fEXPR"></a>
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<a name="index-CONTINUE_005fSTMT"></a>
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<a name="index-DECL_005fSTMT"></a>
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<a name="index-DECL_005fSTMT_005fDECL"></a>
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<a name="index-DO_005fSTMT"></a>
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<a name="index-DO_005fBODY"></a>
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<a name="index-DO_005fCOND"></a>
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<a name="index-EMPTY_005fCLASS_005fEXPR"></a>
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<a name="index-EXPR_005fSTMT"></a>
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<a name="index-EXPR_005fSTMT_005fEXPR"></a>
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<a name="index-FOR_005fSTMT"></a>
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<a name="index-FOR_005fINIT_005fSTMT"></a>
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<a name="index-FOR_005fCOND"></a>
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<a name="index-FOR_005fEXPR"></a>
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<a name="index-FOR_005fBODY"></a>
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<a name="index-HANDLER"></a>
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<a name="index-IF_005fSTMT"></a>
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<a name="index-IF_005fCOND"></a>
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<a name="index-THEN_005fCLAUSE"></a>
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<a name="index-ELSE_005fCLAUSE"></a>
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<a name="index-RETURN_005fSTMT"></a>
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<a name="index-RETURN_005fEXPR"></a>
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<a name="index-SUBOBJECT"></a>
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<a name="index-SUBOBJECT_005fCLEANUP"></a>
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<a name="index-SWITCH_005fSTMT"></a>
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<a name="index-SWITCH_005fCOND"></a>
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<a name="index-SWITCH_005fBODY"></a>
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<a name="index-TRY_005fBLOCK"></a>
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<a name="index-TRY_005fSTMTS"></a>
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<a name="index-TRY_005fHANDLERS"></a>
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<a name="index-HANDLER_005fPARMS"></a>
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<a name="index-HANDLER_005fBODY"></a>
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<a name="index-USING_005fSTMT"></a>
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<a name="index-WHILE_005fSTMT"></a>
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<a name="index-WHILE_005fBODY"></a>
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<a name="index-WHILE_005fCOND"></a>
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<p>A function that has a definition in the current translation unit will
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have a non-<code>NULL</code> <code>DECL_INITIAL</code>. However, back ends should not make
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use of the particular value given by <code>DECL_INITIAL</code>.
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</p>
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<p>The <code>DECL_SAVED_TREE</code> macro will give the complete body of the
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function.
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</p>
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<a name="Statements-2"></a>
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<h4 class="subsubsection">10.10.5.1 Statements</h4>
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<p>There are tree nodes corresponding to all of the source-level
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statement constructs, used within the C and C++ frontends. These are
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enumerated here, together with a list of the various macros that can
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be used to obtain information about them. There are a few macros that
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can be used with all statements:
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</p>
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<dl compact="compact">
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<dt><code>STMT_IS_FULL_EXPR_P</code>
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<a name="index-STMT_005fIS_005fFULL_005fEXPR_005fP"></a>
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</dt>
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<dd><p>In C++, statements normally constitute “full expressions”; temporaries
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created during a statement are destroyed when the statement is complete.
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However, G++ sometimes represents expressions by statements; these
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statements will not have <code>STMT_IS_FULL_EXPR_P</code> set. Temporaries
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created during such statements should be destroyed when the innermost
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enclosing statement with <code>STMT_IS_FULL_EXPR_P</code> set is exited.
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</p>
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</dd>
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</dl>
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<p>Here is the list of the various statement nodes, and the macros used to
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access them. This documentation describes the use of these nodes in
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non-template functions (including instantiations of template functions).
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In template functions, the same nodes are used, but sometimes in
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slightly different ways.
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</p>
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<p>Many of the statements have substatements. For example, a <code>while</code>
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loop will have a body, which is itself a statement. If the substatement
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is <code>NULL_TREE</code>, it is considered equivalent to a statement
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consisting of a single <code>;</code>, i.e., an expression statement in which
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the expression has been omitted. A substatement may in fact be a list
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of statements, connected via their <code>TREE_CHAIN</code>s. So, you should
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always process the statement tree by looping over substatements, like
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this:
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</p><div class="smallexample">
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<pre class="smallexample">void process_stmt (stmt)
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tree stmt;
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{
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while (stmt)
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{
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switch (TREE_CODE (stmt))
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{
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case IF_STMT:
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process_stmt (THEN_CLAUSE (stmt));
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/* <span class="roman">More processing here.</span> */
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break;
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…
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}
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stmt = TREE_CHAIN (stmt);
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}
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}
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</pre></div>
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<p>In other words, while the <code>then</code> clause of an <code>if</code> statement
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in C++ can be only one statement (although that one statement may be a
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compound statement), the intermediate representation will sometimes use
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several statements chained together.
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</p>
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<dl compact="compact">
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<dt><code>BREAK_STMT</code></dt>
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<dd>
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<p>Used to represent a <code>break</code> statement. There are no additional
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fields.
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</p>
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</dd>
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<dt><code>CILK_SPAWN_STMT</code></dt>
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<dd>
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<p>Used to represent a spawning function in the Cilk Plus language extension.
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This tree has one field that holds the name of the spawning function.
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<code>_Cilk_spawn</code> can be written in C in the following way:
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</p>
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<div class="smallexample">
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<pre class="smallexample"><code>_Cilk_spawn</code> <function_name> (<parameters>);
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</pre></div>
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<p>Detailed description for usage and functionality of <code>_Cilk_spawn</code> can be
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found at <a href="https://www.cilkplus.org">https://www.cilkplus.org</a>.
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</p>
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</dd>
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<dt><code>CILK_SYNC_STMT</code></dt>
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<dd>
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<p>This statement is part of the Cilk Plus language extension. It indicates that
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the current function cannot continue in parallel with its spawned children.
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There are no additional fields. <code>_Cilk_sync</code> can be written in C in the
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following way:
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</p>
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<div class="smallexample">
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<pre class="smallexample"><code>_Cilk_sync</code>;
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</pre></div>
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</dd>
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<dt><code>CLEANUP_STMT</code></dt>
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<dd>
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<p>Used to represent an action that should take place upon exit from the
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enclosing scope. Typically, these actions are calls to destructors for
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local objects, but back ends cannot rely on this fact. If these nodes
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are in fact representing such destructors, <code>CLEANUP_DECL</code> will be
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the <code>VAR_DECL</code> destroyed. Otherwise, <code>CLEANUP_DECL</code> will be
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<code>NULL_TREE</code>. In any case, the <code>CLEANUP_EXPR</code> is the
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expression to execute. The cleanups executed on exit from a scope
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should be run in the reverse order of the order in which the associated
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<code>CLEANUP_STMT</code>s were encountered.
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</p>
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</dd>
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<dt><code>CONTINUE_STMT</code></dt>
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<dd>
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<p>Used to represent a <code>continue</code> statement. There are no additional
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fields.
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</p>
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</dd>
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<dt><code>CTOR_STMT</code></dt>
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<dd>
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<p>Used to mark the beginning (if <code>CTOR_BEGIN_P</code> holds) or end (if
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<code>CTOR_END_P</code> holds of the main body of a constructor. See also
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<code>SUBOBJECT</code> for more information on how to use these nodes.
|
</p>
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</dd>
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<dt><code>DO_STMT</code></dt>
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<dd>
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<p>Used to represent a <code>do</code> loop. The body of the loop is given by
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<code>DO_BODY</code> while the termination condition for the loop is given by
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<code>DO_COND</code>. The condition for a <code>do</code>-statement is always an
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expression.
|
</p>
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</dd>
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<dt><code>EMPTY_CLASS_EXPR</code></dt>
|
<dd>
|
<p>Used to represent a temporary object of a class with no data whose
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address is never taken. (All such objects are interchangeable.) The
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<code>TREE_TYPE</code> represents the type of the object.
|
</p>
|
</dd>
|
<dt><code>EXPR_STMT</code></dt>
|
<dd>
|
<p>Used to represent an expression statement. Use <code>EXPR_STMT_EXPR</code> to
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obtain the expression.
|
</p>
|
</dd>
|
<dt><code>FOR_STMT</code></dt>
|
<dd>
|
<p>Used to represent a <code>for</code> statement. The <code>FOR_INIT_STMT</code> is
|
the initialization statement for the loop. The <code>FOR_COND</code> is the
|
termination condition. The <code>FOR_EXPR</code> is the expression executed
|
right before the <code>FOR_COND</code> on each loop iteration; often, this
|
expression increments a counter. The body of the loop is given by
|
<code>FOR_BODY</code>. Note that <code>FOR_INIT_STMT</code> and <code>FOR_BODY</code>
|
return statements, while <code>FOR_COND</code> and <code>FOR_EXPR</code> return
|
expressions.
|
</p>
|
</dd>
|
<dt><code>HANDLER</code></dt>
|
<dd>
|
<p>Used to represent a C++ <code>catch</code> block. The <code>HANDLER_TYPE</code>
|
is the type of exception that will be caught by this handler; it is
|
equal (by pointer equality) to <code>NULL</code> if this handler is for all
|
types. <code>HANDLER_PARMS</code> is the <code>DECL_STMT</code> for the catch
|
parameter, and <code>HANDLER_BODY</code> is the code for the block itself.
|
</p>
|
</dd>
|
<dt><code>IF_STMT</code></dt>
|
<dd>
|
<p>Used to represent an <code>if</code> statement. The <code>IF_COND</code> is the
|
expression.
|
</p>
|
<p>If the condition is a <code>TREE_LIST</code>, then the <code>TREE_PURPOSE</code> is
|
a statement (usually a <code>DECL_STMT</code>). Each time the condition is
|
evaluated, the statement should be executed. Then, the
|
<code>TREE_VALUE</code> should be used as the conditional expression itself.
|
This representation is used to handle C++ code like this:
|
</p>
|
<p>C++ distinguishes between this and <code>COND_EXPR</code> for handling templates.
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">if (int i = 7) …
|
</pre></div>
|
|
<p>where there is a new local variable (or variables) declared within the
|
condition.
|
</p>
|
<p>The <code>THEN_CLAUSE</code> represents the statement given by the <code>then</code>
|
condition, while the <code>ELSE_CLAUSE</code> represents the statement given
|
by the <code>else</code> condition.
|
</p>
|
</dd>
|
<dt><code>SUBOBJECT</code></dt>
|
<dd>
|
<p>In a constructor, these nodes are used to mark the point at which a
|
subobject of <code>this</code> is fully constructed. If, after this point, an
|
exception is thrown before a <code>CTOR_STMT</code> with <code>CTOR_END_P</code> set
|
is encountered, the <code>SUBOBJECT_CLEANUP</code> must be executed. The
|
cleanups must be executed in the reverse order in which they appear.
|
</p>
|
</dd>
|
<dt><code>SWITCH_STMT</code></dt>
|
<dd>
|
<p>Used to represent a <code>switch</code> statement. The <code>SWITCH_STMT_COND</code>
|
is the expression on which the switch is occurring. See the documentation
|
for an <code>IF_STMT</code> for more information on the representation used
|
for the condition. The <code>SWITCH_STMT_BODY</code> is the body of the switch
|
statement. The <code>SWITCH_STMT_TYPE</code> is the original type of switch
|
expression as given in the source, before any compiler conversions.
|
</p>
|
</dd>
|
<dt><code>TRY_BLOCK</code></dt>
|
<dd><p>Used to represent a <code>try</code> block. The body of the try block is
|
given by <code>TRY_STMTS</code>. Each of the catch blocks is a <code>HANDLER</code>
|
node. The first handler is given by <code>TRY_HANDLERS</code>. Subsequent
|
handlers are obtained by following the <code>TREE_CHAIN</code> link from one
|
handler to the next. The body of the handler is given by
|
<code>HANDLER_BODY</code>.
|
</p>
|
<p>If <code>CLEANUP_P</code> holds of the <code>TRY_BLOCK</code>, then the
|
<code>TRY_HANDLERS</code> will not be a <code>HANDLER</code> 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,
|
<code>terminate</code> must be called.
|
</p>
|
</dd>
|
<dt><code>USING_STMT</code></dt>
|
<dd><p>Used to represent a <code>using</code> directive. The namespace is given by
|
<code>USING_STMT_NAMESPACE</code>, which will be a NAMESPACE_DECL. This node
|
is needed inside template functions, to implement using directives
|
during instantiation.
|
</p>
|
</dd>
|
<dt><code>WHILE_STMT</code></dt>
|
<dd>
|
<p>Used to represent a <code>while</code> loop. The <code>WHILE_COND</code> is the
|
termination condition for the loop. See the documentation for an
|
<code>IF_STMT</code> for more information on the representation used for the
|
condition.
|
</p>
|
<p>The <code>WHILE_BODY</code> is the body of the loop.
|
</p>
|
</dd>
|
</dl>
|
|
<hr>
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<div class="header">
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<p>
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Next: <a href="C_002b_002b-Expressions.html#C_002b_002b-Expressions" accesskey="n" rel="next">C++ Expressions</a>, Previous: <a href="Functions-for-C_002b_002b.html#Functions-for-C_002b_002b" accesskey="p" rel="prev">Functions for C++</a>, Up: <a href="C-and-C_002b_002b-Trees.html#C-and-C_002b_002b-Trees" accesskey="u" rel="up">C and C++ Trees</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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