aboutsummaryrefslogtreecommitdiffstats
path: root/docs/source/lang/cpp/goto.rst
blob: 2c0b3b0bea8107ab6123a7b155b8b7bc546e5998 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
.. highlight:: cpp

.. _lang-goto:

Labels and ``goto``
===================

A *label* gives a name to a line of code within a function.  You can
label a line by writing a name for it, then a colon (``:``), before
the line starts.  The ``goto`` keyword allows program flow to transfer
to a labeled line from anywhere within the same function.

.. warning:: The use of ``goto`` is discouraged in C and C++
   programming.  It is *never necessary* to use ``goto`` to write a
   program.

   Unless you know what you're doing, using ``goto`` tends to
   encourage code which is harder to debug and understand than
   programs without ``goto`` that do the same thing.  That said,
   however, it's sometimes useful; :ref:`see below <goto-when-to-use>`
   for a concrete example.

Using Labels and goto
---------------------

Labels and ``goto`` are probably best explained through example.
Let's start with an example of how to label lines.  The first line
(``int x = analogRead(some_pin);``) in the :ref:`loop <lang-loop>`
function below has label ``readpin``.  The third line (``delay(x);``)
has label ``startdelay``.  The second line (``SerialUSB.println(x);``)
does not have a label::

       void loop() {
       readpin:
              int x = analogRead(some_pin);
              SerialUSB.println(x); // for debugging
       startdelay:
              delay(x);
              // ... more code ...
       }

Anything which can be a :ref:`variable <lang-variables>` name can
be a label.

Let's say that we wanted to print ``x`` only if it was very large, say
at least 2000.  We might want to do this just so anybody watching on a
:ref:`serial monitor <ide-serial-monitor>` would know they were in for
a longer wait than usual.  We can accomplish this through the use of a
``goto`` statement that skips the printing if ``x`` is less than
2000::

       void loop() {
       readpin:
              int x = analogRead(some_pin);
              if (x < 2000) {
                  goto startdelay;
              }
              SerialUSB.println(x); // for debugging
       startdelay:
              delay(x);
              // ... more code ...
       }

In this modified program, whenever ``x`` is less than 2000, the body
of the :ref:`if <lang-if>` statement in the second line is
executed.  The ``goto`` statement inside the ``if`` body skips
straight to the line labeled ``startdelay``, passing over the line
doing the printing.

A ``goto`` does not have to "move forwards"; it can go "backwards",
too.  For example, the following program prints "5" forever (why?)::

    void loop() {
    printfive:
        SerialUSB.println(5);
        goto printfive;
        SerialUSB.println(6);
    }

.. _goto-when-to-use:

When to Use goto
----------------

As mentioned above, use of ``goto`` is `generally discouraged
<http://en.wikipedia.org/wiki/Goto#Criticism_and_decline>`_.  However,
when used with care, ``goto`` can simplify certain programs.  One
important use case for ``goto`` is breaking out of deeply nested
:ref:`for <lang-for>` loops or :ref:`if <lang-if>` logic blocks.
Here's an example::

    for(int r = 0; r < 255; r++) {
        for(int g = 255; g > -1; g--) {
            for(int b = 0; b < 255; b++) {
                if (analogRead(0) > 250) {
                    goto bailout;
                }
                // more statements ...
            }
            // innermost loop ends here
        }
    }
    bailout:
    // more code here

In the above example, whenever the :ref:`analog reading
<lang-analogread>` on pin 0 was greater than 250, the program would
jump to the line labeled ``bailout``, exiting all three loops at once.

While there is already a :ref:`break <lang-break>` keyword for
breaking out of a loop, it will only break out of the *innermost*
loop.  So, if instead of saying "``goto bailout;``", there was a
"``break;``" instead, the program would only exit from the loop with
header "``for(int b = 0; b < 255; b++)``".  The program would continue
at the line which reads "``// innermost loop ends here``", which is
clearly undesirable if you wanted to leave all three loops at once.

More examples of when ``goto`` is a good choice are given in Donald
Knuth's paper, "Structured Programming with go to Statements"; see
below for a link.

See Also
--------

- Dijkstra, Edsger W. `Go To Statement Considered Harmful <http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.92.4846&rep=rep1&type=pdf>`_ (PDF)

- Knuth, Donald. `Structured Programming with go to Statements <http://pplab.snu.ac.kr/courses/adv_pl05/papers/p261-knuth.pdf>`_ (PDF)

.. include:: /arduino-cc-attribution.txt