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| <h3 class="section">6.4 Nested Functions</h3> |
| |
| <p><a name="index-nested-functions-2191"></a><a name="index-downward-funargs-2192"></a><a name="index-thunks-2193"></a> |
| A <dfn>nested function</dfn> is a function defined inside another function. |
| (Nested functions are not supported for GNU C++.) The nested function's |
| name is local to the block where it is defined. For example, here we |
| define a nested function named <code>square</code>, and call it twice: |
| |
| <pre class="smallexample"> foo (double a, double b) |
| { |
| double square (double z) { return z * z; } |
| |
| return square (a) + square (b); |
| } |
| </pre> |
| <p>The nested function can access all the variables of the containing |
| function that are visible at the point of its definition. This is |
| called <dfn>lexical scoping</dfn>. For example, here we show a nested |
| function which uses an inherited variable named <code>offset</code>: |
| |
| <pre class="smallexample"> bar (int *array, int offset, int size) |
| { |
| int access (int *array, int index) |
| { return array[index + offset]; } |
| int i; |
| /* <span class="roman">...</span> */ |
| for (i = 0; i < size; i++) |
| /* <span class="roman">...</span> */ access (array, i) /* <span class="roman">...</span> */ |
| } |
| </pre> |
| <p>Nested function definitions are permitted within functions in the places |
| where variable definitions are allowed; that is, in any block, mixed |
| with the other declarations and statements in the block. |
| |
| <p>It is possible to call the nested function from outside the scope of its |
| name by storing its address or passing the address to another function: |
| |
| <pre class="smallexample"> hack (int *array, int size) |
| { |
| void store (int index, int value) |
| { array[index] = value; } |
| |
| intermediate (store, size); |
| } |
| </pre> |
| <p>Here, the function <code>intermediate</code> receives the address of |
| <code>store</code> as an argument. If <code>intermediate</code> calls <code>store</code>, |
| the arguments given to <code>store</code> are used to store into <code>array</code>. |
| But this technique works only so long as the containing function |
| (<code>hack</code>, in this example) does not exit. |
| |
| <p>If you try to call the nested function through its address after the |
| containing function has exited, all hell will break loose. If you try |
| to call it after a containing scope level has exited, and if it refers |
| to some of the variables that are no longer in scope, you may be lucky, |
| but it's not wise to take the risk. If, however, the nested function |
| does not refer to anything that has gone out of scope, you should be |
| safe. |
| |
| <p>GCC implements taking the address of a nested function using a technique |
| called <dfn>trampolines</dfn>. This technique was described in |
| <cite>Lexical Closures for C++</cite> (Thomas M. Breuel, USENIX |
| C++ Conference Proceedings, October 17-21, 1988). |
| |
| <p>A nested function can jump to a label inherited from a containing |
| function, provided the label was explicitly declared in the containing |
| function (see <a href="Local-Labels.html#Local-Labels">Local Labels</a>). Such a jump returns instantly to the |
| containing function, exiting the nested function which did the |
| <code>goto</code> and any intermediate functions as well. Here is an example: |
| |
| <pre class="smallexample"> bar (int *array, int offset, int size) |
| { |
| __label__ failure; |
| int access (int *array, int index) |
| { |
| if (index > size) |
| goto failure; |
| return array[index + offset]; |
| } |
| int i; |
| /* <span class="roman">...</span> */ |
| for (i = 0; i < size; i++) |
| /* <span class="roman">...</span> */ access (array, i) /* <span class="roman">...</span> */ |
| /* <span class="roman">...</span> */ |
| return 0; |
| |
| /* <span class="roman">Control comes here from </span><code>access</code><span class="roman"> |
| if it detects an error.</span> */ |
| failure: |
| return -1; |
| } |
| </pre> |
| <p>A nested function always has no linkage. Declaring one with |
| <code>extern</code> or <code>static</code> is erroneous. If you need to declare the nested function |
| before its definition, use <code>auto</code> (which is otherwise meaningless |
| for function declarations). |
| |
| <pre class="smallexample"> bar (int *array, int offset, int size) |
| { |
| __label__ failure; |
| auto int access (int *, int); |
| /* <span class="roman">...</span> */ |
| int access (int *array, int index) |
| { |
| if (index > size) |
| goto failure; |
| return array[index + offset]; |
| } |
| /* <span class="roman">...</span> */ |
| } |
| </pre> |
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