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
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<HTML>
<HEAD>
<TITLE>Ada95 - Assignment 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="RM-TOC.html">Contents</A> <A HREF="RM-0-29.html">Index</A> <A HREF="RM-5-1.html">Previous</A> <A HREF="RM-5-3.html">Next</A></P>
<HR>
<H1> 5.2 Assignment Statements</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> An <FONT FACE="Arial, Helvetica">assignment_statement</FONT>
replaces the current value of a variable with the result of evaluating
an <FONT FACE="Arial, Helvetica">expression</FONT>. </DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">assignment_statement<A NAME="I2798"></A>
::= </FONT><BR>
<I>variable_</I><A NAME="I2799"></A><FONT FACE="Arial, Helvetica">name</FONT> := <A NAME="I2800"></A><FONT FACE="Arial, Helvetica">expression</FONT>;</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Normal"> The execution of an <FONT FACE="Arial, Helvetica">assignment_statement</FONT>
includes the evaluation of the <FONT FACE="Arial, Helvetica">expression</FONT>
and the <I>assignment</I> of the value of the <FONT FACE="Arial, Helvetica">expression</FONT>
into the <I>target</I>. <A NAME="I2801"></A><A NAME="I2802"></A>An assignment
operation (as opposed to an <FONT FACE="Arial, Helvetica">assignment_statement</FONT>)
is performed in other contexts as well, including object initialization
and by-copy parameter passing. <A NAME="I2803"></A><A NAME="I2804"></A>The
<I>target</I> of an assignment operation is the view of the object to
which a value is being assigned; the target of an <FONT FACE="Arial, Helvetica">assignment_statement</FONT>
is the variable denoted by the <I>variable_</I><FONT FACE="Arial, Helvetica">name</FONT>.
</DIV>
<H4 ALIGN=CENTER>Name Resolution Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> <A NAME="I2805"></A>The <I>variable_</I><FONT FACE="Arial, Helvetica">name</FONT>
of an <FONT FACE="Arial, Helvetica">assignment_statement</FONT> is expected
to be of any nonlimited type. <A NAME="I2806"></A>The expected type for
the <FONT FACE="Arial, Helvetica">expression</FONT> is the type of the
target. </DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> The target denoted by the <I>variable_</I><FONT FACE="Arial, Helvetica">name</FONT>
shall be a variable.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> If the target is of a tagged class-wide type <I>T</I>'Class,
then the <FONT FACE="Arial, Helvetica">expression</FONT> shall either
be dynamically tagged, or of type <I>T</I> and tag-indeterminate (see
<A HREF="RM-3-9-2.html">3.9.2</A>). </DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> <A NAME="I2807"></A>For the execution of an <FONT FACE="Arial, Helvetica">assignment_statement</FONT>,
the <I>variable_</I><FONT FACE="Arial, Helvetica">name</FONT> and the
<FONT FACE="Arial, Helvetica">expression</FONT> are first evaluated in
an arbitrary order. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> When the type of
the target is class-wide: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC><A NAME="I2808"></A>If the <FONT FACE="Arial, Helvetica">expression</FONT>
is tag-indeterminate (see <A HREF="RM-3-9-2.html">3.9.2</A>), then the
controlling tag value for the <FONT FACE="Arial, Helvetica">expression</FONT>
is the tag of the target; </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC><A NAME="I2809"></A><A NAME="I2810"></A><A NAME="I2811"></A>Otherwise
(the <FONT FACE="Arial, Helvetica">expression</FONT> is dynamically tagged),
a check is made that the tag of the value of the <FONT FACE="Arial, Helvetica">expression</FONT>
is the same as that of the target; if this check fails, Constraint_Error
is raised. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal"> The value of the <FONT FACE="Arial, Helvetica">expression</FONT>
is converted to the subtype of the target. The conversion might raise
an exception (see <A HREF="RM-4-6.html">4.6</A>). <A NAME="I2812"></A></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Normal"> In cases involving controlled types, the target
is finalized, and an anonymous object might be used as an intermediate
in the assignment, as described in <A HREF="RM-7-6-1.html">7.6.1</A>,
``<A HREF="RM-7-6-1.html">Completion and Finalization</A>''. <A NAME="I2813"></A><A NAME="I2814"></A>In
any case, the converted value of the <FONT FACE="Arial, Helvetica">expression</FONT>
is then <I>assigned</I> to the target, which consists of the following
two steps: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>The value of the target becomes the converted value.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>If any part of the target is controlled, its value is adjusted
as explained in clause <A HREF="RM-7-6.html">7.6</A>. <A NAME="I2815"></A></LI></UL>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>2 The tag of an object never
changes; in particular, an <FONT FACE="Arial, Helvetica">assignment_statement</FONT>
does not change the tag of the target.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>3 The values of the discriminants
of an object designated by an access value cannot be changed (not even
by assigning a complete value to the object itself) since such objects
are always constrained; however, subcomponents of such objects may be
unconstrained. </FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Examples of
assignment statements:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>18</FONT></DIV>
<DIV Class="Examples"><TT>Value := Max_Value - 1;<BR>
Shade := Blue;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>19</FONT></DIV>
<DIV Class="Examples"><TT>Next_Frame(F)(M, N) := 2.5; --<I> see <A HREF="RM-4-1-1.html">4.1.1</A></I><BR>
U := Dot_Product(V, W); --<I> see <A HREF="RM-6-3.html">6.3</A></I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20</FONT></DIV>
<DIV Class="Examples"><TT>Writer := (Status => Open, Unit => Printer, Line_Count => 60); --<I> see <A HREF="RM-3-8-1.html">3.8.1</A></I><BR>
Next_Car.<B>all</B> := (72074, <B>null</B>); --<I> see <A HREF="RM-3-10-1.html">3.10.1</A></I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>21</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> <I>Examples involving
scalar subtype conversions:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>22</FONT></DIV>
<DIV Class="Examples"><TT>I, J : Integer <B>range</B> 1 .. 10 := 5;<BR>
K : Integer <B>range</B> 1 .. 20 := 15;<BR>
...</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>23</FONT></DIV>
<DIV Class="Examples"><TT>I := J; --<I> identical ranges</I><BR>
K := J; --<I> compatible ranges</I><BR>
J := K; --<I> will raise Constraint_Error if K > 10</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>24</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> <I>Examples involving
array subtype conversions:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>25</FONT></DIV>
<DIV Class="Examples"><TT>A : String(1 .. 31);<BR>
B : String(3 .. 33);<BR>
...</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>26</FONT></DIV>
<DIV Class="Examples"><TT>A := B; --<I> same number of components</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>27</FONT></DIV>
<DIV Class="Examples"><TT>A(1 .. 9) := "tar sauce";<BR>
A(4 .. 12) := A(1 .. 9); --<I> A(1 .. 12) = "tartar sauce"</I></TT></DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>28</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>4 <I>Notes on the examples:</I>
<FONT FACE="Arial, Helvetica">Assignment_statement</FONT>s are allowed
even in the case of overlapping slices of the same array, because the
<I>variable_</I><FONT FACE="Arial, Helvetica">name</FONT> and <FONT FACE="Arial, Helvetica">expression</FONT>
are both evaluated before copying the value into the variable. In the
above example, an implementation yielding A(1 .. 12) = "tartartartar"
would be incorrect. </FONT></DIV>
<HR>
<P><A HREF="RM-TOC.html">Contents</A> <A HREF="RM-0-29.html">Index</A> <A HREF="RM-5-1.html">Previous</A> <A HREF="RM-5-3.html">Next</A> <A HREF="RM-TTL.html">Legal</A></P>
</BODY>
</HTML>
|
