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
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<HTML>
<HEAD>
<TITLE>AARM95 - Return Statements</TITLE>
<META NAME="Author" CONTENT="JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor">
<META NAME="GENERATOR" CONTENT="Arm_Form.Exe, Ada Reference Manual generator">
<STYLE type="text/css">
DIV.paranum {position: absolute; font-family: Arial, Helvetica, sans-serif; left: 0.5 em; top: auto}
TT {font-family: "Courier New", monospace}
DT {display: compact}
DIV.Normal {font-family: "Times New Roman", Times, serif; margin-bottom: 0.6em}
DIV.Wide {font-family: "Times New Roman", Times, serif; margin-top: 0.6em; margin-bottom: 0.6em}
DIV.Annotations {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-bottom: 0.6em}
DIV.WideAnnotations {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-top: 0.6em; margin-bottom: 0.6em}
DIV.Index {font-family: "Times New Roman", Times, serif}
DIV.SyntaxSummary {font-family: "Times New Roman", Times, serif; margin-left: 2.0em; margin-bottom: 0.4em}
DIV.Notes {font-family: "Times New Roman", Times, serif; margin-left: 2.0em; margin-bottom: 0.6em}
DIV.NotesHeader {font-family: "Times New Roman", Times, serif; margin-left: 2.0em}
DIV.SyntaxIndented {font-family: "Times New Roman", Times, serif; margin-left: 2.0em; margin-bottom: 0.4em}
DIV.Indented {font-family: "Times New Roman", Times, serif; margin-left: 6.0em; margin-bottom: 0.6em}
DIV.CodeIndented {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-bottom: 0.6em}
DIV.SmallIndented {font-family: "Times New Roman", Times, serif; margin-left: 10.0em; margin-bottom: 0.6em}
DIV.SmallCodeIndented {font-family: "Times New Roman", Times, serif; margin-left: 8.0em; margin-bottom: 0.6em}
DIV.Examples {font-family: "Courier New", monospace; margin-left: 2.0em; margin-bottom: 0.6em}
DIV.SmallExamples {font-family: "Courier New", monospace; font-size: 80%; margin-left: 7.5em; margin-bottom: 0.6em}
DIV.IndentedExamples {font-family: "Courier New", monospace; margin-left: 8.0em; margin-bottom: 0.6em}
DIV.SmallIndentedExamples {font-family: "Courier New", monospace; font-size: 80%; margin-left: 15.0em; margin-bottom: 0.6em}
UL.Bulleted {font-family: "Times New Roman", Times, serif; margin-left: 2.0em; margin-right: 2.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.SmallBulleted {font-family: "Times New Roman", Times, serif; margin-left: 6.0em; margin-right: 6.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.NestedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-right: 4.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.SmallNestedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 8.0em; margin-right: 8.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.IndentedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 8.0em; margin-right: 8.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.CodeIndentedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 6.0em; margin-right: 6.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.CodeIndentedNestedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 8.0em; margin-right: 8.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.SyntaxIndentedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-right: 4.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.NotesBulleted {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-right: 4.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.NotesNestedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 6.0em; margin-right: 6.0em; margin-top: 0em; margin-bottom: 0.5em}
DL.Hanging {font-family: "Times New Roman", Times, serif; margin-top: 0em; margin-bottom: 0.6em}
DD.Hanging {margin-left: 6.0em}
DL.IndentedHanging {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-top: 0em; margin-bottom: 0.6em}
DD.IndentedHanging {margin-left: 2.0em}
DL.HangingInBulleted {font-family: "Times New Roman", Times, serif; margin-left: 2.0em; margin-right: 2.0em; margin-top: 0em; margin-bottom: 0.5em}
DD.HangingInBulleted {margin-left: 4.0em}
DL.SmallHanging {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-top: 0em; margin-bottom: 0.6em}
DD.SmallHanging {margin-left: 7.5em}
DL.SmallIndentedHanging {font-family: "Times New Roman", Times, serif; margin-left: 8.0em; margin-top: 0em; margin-bottom: 0.6em}
DD.SmallIndentedHanging {margin-left: 2.0em}
DL.SmallHangingInBulleted {font-family: "Times New Roman", Times, serif; margin-left: 6.0em; margin-right: 6.0em; margin-top: 0em; margin-bottom: 0.5em}
DD.SmallHangingInBulleted {margin-left: 5.0em}
DL.Enumerated {font-family: "Times New Roman", Times, serif; margin-right: 0.0em; margin-top: 0em; margin-bottom: 0.5em}
DD.Enumerated {margin-left: 2.0em}
DL.SmallEnumerated {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-right: 4.0em; margin-top: 0em; margin-bottom: 0.5em}
DD.SmallEnumerated {margin-left: 2.5em}
DL.NestedEnumerated {font-family: "Times New Roman", Times, serif; margin-left: 2.0em; margin-right: 2.0em; margin-top: 0em; margin-bottom: 0.5em}
DL.SmallNestedEnumerated {font-family: "Times New Roman", Times, serif; margin-left: 6.0em; margin-right: 6.0em; margin-top: 0em; margin-bottom: 0.5em}
</STYLE>
</HEAD>
<BODY TEXT="#000000" BGCOLOR="#FFFFF0" LINK="#0000FF" VLINK="#800080" ALINK="#FF0000">
<P><A HREF="AA-TOC.html">Contents</A> <A HREF="AA-0-29.html">Index</A> <A HREF="AA-6-4-1.html">Previous</A> <A HREF="AA-6-6.html">Next</A></P>
<HR>
<H1> 6.5 Return Statements</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> A <FONT FACE="Arial, Helvetica">return_statement</FONT>
is used to complete the execution of the innermost enclosing <FONT FACE="Arial, Helvetica">subprogram_body</FONT>,
<FONT FACE="Arial, Helvetica">entry_body</FONT>, or <FONT FACE="Arial, Helvetica">accept_statement</FONT>.
</DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">return_statement<A NAME="I3164"></A>
::= </FONT><B>return</B> [<A NAME="I3165"></A><FONT FACE="Arial, Helvetica">expression</FONT>];</DIV>
<H4 ALIGN=CENTER>Name Resolution Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3166"></A>The <FONT FACE="Arial, Helvetica">expression</FONT>,
if any, of a <FONT FACE="Arial, Helvetica">return_statement</FONT> is
called the <I>return expression</I>. <A NAME="I3167"></A>The <I>result
subtype</I> of a function is the subtype denoted by the <FONT FACE="Arial, Helvetica">subtype_mark</FONT>
after the reserved word <B>return</B> in the profile of the function.
<A NAME="I3168"></A>The expected type for a return expression is the
result type of the corresponding function. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>To be honest: </B>The same
applies to generic functions. </FONT></DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3169"></A>A <FONT FACE="Arial, Helvetica">return_statement</FONT>
shall be within a callable construct, and it <I>applies to</I> the innermost
one. A <FONT FACE="Arial, Helvetica">return_statement</FONT> shall not
be within a body that is within the construct to which the <FONT FACE="Arial, Helvetica">return_statement</FONT>
applies.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> A function body shall contain at least one <FONT FACE="Arial, Helvetica">return_statement</FONT>
that applies to the function body, unless the function contains <FONT FACE="Arial, Helvetica">code_statement</FONT>s.
A <FONT FACE="Arial, Helvetica">return_statement</FONT> shall include
a return expression if and only if it applies to a function body. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>The requirement
that a function body has to have at least one <FONT FACE="Arial, Helvetica">return_statement</FONT>
is a ``helpful'' restriction. There was been some interest in lifting
this restriction, or allowing a raise statement to substitute for the
<FONT FACE="Arial, Helvetica">return_statement</FONT>. However, there
was enough interest in leaving it as is that we decided not to change
it. </FONT></DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3170"></A>For the execution of a <FONT FACE="Arial, Helvetica">return_statement</FONT>,
the <FONT FACE="Arial, Helvetica">expression</FONT> (if any) is first
evaluated and converted to the result subtype. <A NAME="I3171"></A></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The conversion
might raise Constraint_Error -- (see <A HREF="AA-4-6.html">4.6</A>).
</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> If the result type is class-wide, then the tag
of the result is the tag of the value of the <FONT FACE="Arial, Helvetica">expression</FONT>.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> If the result type
is a specific tagged type: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC><A NAME="I3172"></A><A NAME="I3173"></A>If it is limited,
then a check is made that the tag of the value of the return expression
identifies the result type. <A NAME="I3174"></A>Constraint_Error is raised
if this check fails.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>If it is nonlimited, then the tag of the result is that
of the result type. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>10.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>This is true
even if the tag of the return expression is different. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>These rules ensure
that a function whose result type is a specific tagged type always returns
an object whose tag is that of the result type. This is important for
dispatching on controlling result, and, if nonlimited, allows the caller
to allocate the appropriate amount of space to hold the value being returned
(assuming there are no discriminants). </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <A NAME="I3175"></A>A
type is a <I>return-by-reference</I> type if it is a descendant of one
of the following: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a tagged limited type;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a task or protected type;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a nonprivate type with the reserved word <B>limited</B>
in its declaration;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a composite type with a subcomponent of a return-by-reference
type;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a private type whose full type is a return-by-reference
type. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>16.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The above
rules are such that there are no "Ada 83" types other than
those containing tasks that are return-by-reference. This helps to minimize
upward incompatibilities relating to return-by-reference. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <A NAME="I3176"></A><A NAME="I3177"></A>If
the result type is a return-by-reference type, then a check is made that
the return expression is one of the following: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>18</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a <FONT FACE="Arial, Helvetica">name</FONT> that denotes
an object view whose accessibility level is not deeper than that of the
master that elaborated the function body; or</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>19</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a parenthesized expression or <FONT FACE="Arial, Helvetica">qualified_expression</FONT>
whose operand is one of these kinds of expressions.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>20</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3178"></A>The exception Program_Error
is raised if this check fails. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>Compare the
definition of return-by-reference with that of by-reference.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The return-by-reference types
are all limited types except those that are limited only because of a
limited private type with a nonlimited untagged full type. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B><A NAME="I3179"></A></FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.d</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>This check can often be performed
at compile time. It is defined to be a run-time check to avoid generic
contract model problems. In a future version of the standard, we anticipate
that function return of a local variable will be illegal for all limited
types, eliminating the need for the run-time check except for dereferences
of an access parameter. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>21</FONT></DIV>
<DIV Class="Normal"> For a function with a return-by-reference result
type the result is returned by reference; that is, the function call
denotes a constant view of the object associated with the value of the
return expression. <A NAME="I3180"></A>For any other function, the result
is returned by copy; that is, the converted value is assigned into an
anonymous constant created at the point of the <FONT FACE="Arial, Helvetica">return_statement</FONT>,
and the function call denotes that object. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>21.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The assignment
operation does the necessary value adjustment, as described in <A HREF="AA-7-6.html">7.6</A>,
``<A HREF="AA-7-6.html">User-Defined Assignment and Finalization</A>''.
<A HREF="AA-7-6-1.html">7.6.1</A> describes when the anonymous constant
is finalized. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>22</FONT></DIV>
<DIV Class="Normal"> Finally, a transfer of control is performed which
completes the execution of the callable construct to which the <FONT FACE="Arial, Helvetica">return_statement</FONT>
applies, and returns to the caller. </DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>23</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Examples of
return statements:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>24</FONT></DIV>
<DIV Class="Examples"><TT><B>return</B>; --<I> in a procedure body, <FONT FACE="Arial, Helvetica">entry_body</FONT>, or <FONT FACE="Arial, Helvetica">accept_statement</FONT></I><BR>
<B>return</B> Key_Value(Last_Index); --<I> in a function body</I></TT></DIV>
<H4 ALIGN=CENTER>Incompatibilities With Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>24.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I3181"></A>In Ada 95,
if the result type of a function has a part that is a task, then an attempt
to return a local variable will raise Program_Error. In Ada 83, if a
function returns a local variable containing a task, execution is erroneous
according to AI83-00867. However, there are other situations where functions
that return tasks (or that return a variant record only one of whose
variants includes a task) are correct in Ada 83 but will raise Program_Error
according to the new rules.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>24.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The rule change was made because
there will be more types (protected types, limited controlled types)
in Ada 95 for which it will be meaningless to return a local variable,
and making all of these erroneous is unacceptable. The current rule was
felt to be the simplest that kept upward incompatibilities to situations
involving returning tasks, which are quite rare. </FONT></DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>24.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>This clause has been moved here
from chapter 5, since it has mainly to do with subprograms.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>24.d</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>A function now creates an anonymous
object. This is necessary so that controlled types will work.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>24.e</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>We have clarified that a <FONT FACE="Arial, Helvetica">return_statement</FONT>
applies to a callable construct, not to a callable entity.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>24.f</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>There is no need to mention generics
in the rules about where a <FONT FACE="Arial, Helvetica">return_statement</FONT>
can appear and what it applies to; the phrase ``body of a subprogram
or generic subprogram'' is syntactic, and refers exactly to ``<FONT FACE="Arial, Helvetica">subprogram_body</FONT>''.
</FONT></DIV>
<HR>
<P><A HREF="AA-TOC.html">Contents</A> <A HREF="AA-0-29.html">Index</A> <A HREF="AA-6-4-1.html">Previous</A> <A HREF="AA-6-6.html">Next</A> <A HREF="AA-TTL.html">Legal</A></P>
</BODY>
</HTML>
|
