5.2 Assignment Statements
Syntax
The execution of an
assignment_statement
includes the evaluation of the
expression
and the
assignment of the value of the
expression
into the
target.
An assignment
operation (as opposed to an
assignment_statement)
is performed in other contexts as well, including object initialization
and by-copy parameter passing.
The
target of an assignment operation is the view of the object to
which a value is being assigned; the target of an
assignment_statement
is the variable denoted by the
variable_name.
Name Resolution Rules
Legality Rules
The target denoted by the
variable_name
shall be a variable of a nonlimited type.
If the target is of a tagged class-wide type
T'Class,
then the
expression
shall either be dynamically tagged, or of type
T and tag-indeterminate
(see
3.9.2).
Dynamic Semantics
When the type of the
target is class-wide:
If the
expression
is tag-indeterminate (see
3.9.2), then the
controlling tag value for the
expression
is the tag of the target;
Otherwise
(the
expression
is dynamically tagged), a check is made that the tag of the value of
the
expression
is the same as that of the target; if this check fails, Constraint_Error
is raised.
The value of the
expression
is converted to the subtype of the target. The conversion might raise
an exception (see
4.6).
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
7.6.1, “
Completion
and Finalization”.
In
any case, the converted value of the
expression
is then
assigned to the target, which consists of the following
two steps:
The value of the target becomes the converted value.
If any part of the target is controlled, its value
is adjusted as explained in subclause
7.6.
2 The tag of an object never changes; in
particular, an
assignment_statement
does not change the tag of the target.
This paragraph was
deleted.
Examples
Examples of assignment
statements:
Value := Max_Value - 1;
Shade := Blue;
Next_Frame(F)(M, N) := 2.5; --
see 4.1.1
U := Dot_Product(V, W); --
see 6.3
Writer := (Status => Open, Unit => Printer, Line_Count => 60); --
see 3.8.1
Next_Car.
all := (72074,
null); --
see 3.10.1
Examples involving
scalar subtype conversions:
I, J : Integer range 1 .. 10 := 5;
K : Integer range 1 .. 20 := 15;
...
I := J; -- identical ranges
K := J; -- compatible ranges
J := K; -- will raise Constraint_Error if K > 10
Examples involving
array subtype conversions:
A : String(1 .. 31);
B : String(3 .. 33);
...
A := B; -- same number of components
A(1 .. 9) := "tar sauce";
A(4 .. 12) := A(1 .. 9); -- A(1 .. 12) = "tartar sauce"
3
Notes on the examples: Assignment_statements
are allowed even in the case of overlapping slices of the same array,
because the
variable_name
and
expression
are both evaluated before copying the value into the variable. In the
above example, an implementation yielding A(1 .. 12) = "tartartartar"
would be incorrect.
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