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4.5.3 Binary Adding Operators
Static Semantics
1
{binary
adding operator} {operator
(binary adding)} {+ operator}
{operator (+)} {plus
operator} {operator (plus)}
{- operator} {operator
(-)} {minus operator}
{operator (minus)} The
binary adding operators + (addition) and - (subtraction) are predefined
for every specific numeric type
T with their conventional meaning.
They have the following specifications:
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function "+"(Left, Right : T) return T
function "-"(Left, Right : T) return T
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{&
operator} {operator (&)}
{ampersand operator}
{operator (ampersand)}
{concatenation operator}
{operator (concatenation)}
{catenation operator: See concatenation
operator} The concatenation operators
& are predefined for every nonlimited, one-dimensional array type
T with component type
C. They have the following specifications:
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function "&"(Left : T; Right : T) return T
function "&"(Left : T; Right : C) return T
function "&"(Left : C; Right : T) return T
function "&"(Left : C; Right : C) return T
Dynamic Semantics
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{evaluation
(concatenation) [partial]} For the evaluation
of a concatenation with result type
T, if both operands are of
type
T, the result of the concatenation is a one-dimensional array
whose length is the sum of the lengths of its operands, and whose components
comprise the components of the left operand followed by the components
of the right operand. If the left operand is a null array, the result
of the concatenation is the right operand. Otherwise, the lower bound
of the result is determined as follows:
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- If the ultimate ancestor of the array type was defined
by a constrained_array_definition,
then the lower bound of the result is that of the index subtype;
6.a
Reason: This rule avoids
Constraint_Error when using concatenation on an array type whose first
subtype is constrained.
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- If the ultimate ancestor of the array type was defined
by an unconstrained_array_definition,
then the lower bound of the result is that of the left operand.
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[The upper bound is determined by the lower bound
and the length.]
{Index_Check [partial]}
{check, language-defined (Index_Check)}
A check is made that the upper bound of the result
of the concatenation belongs to the range of the index subtype, unless
the result is a null array.
{Constraint_Error (raised
by failure of run-time check)} Constraint_Error
is raised if this check fails.
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If either operand is of the component type
C,
the result of the concatenation is given by the above rules, using in
place of such an operand an array having this operand as its only component
(converted to the component subtype) and having the lower bound of the
index subtype of the array type as its lower bound.
{implicit
subtype conversion (operand of concatenation) [partial]} 9.a
Ramification: The conversion
might raise Constraint_Error. The conversion provides ``sliding'' for
the component in the case of an array-of-arrays, consistent with the
normal Ada 95 rules that allow sliding during parameter passing.
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{assignment operation
(during evaluation of concatenation)} The result
of a concatenation is defined in terms of an assignment to an anonymous object,
as for any function call (see
6.5).
10.a
Ramification: This implies that
value adjustment is performed as appropriate -- see 7.6.
We don't bother saying this for other predefined operators, even though they
are all function calls, because this is the only one where it matters. It is
the only one that can return a value having controlled parts.
11
15 As for all predefined
operators on modular types, the binary adding operators + and - on modular
types include a final reduction modulo the modulus if the result is outside
the base range of the type.
11.a
Implementation Note: A
full "modulus" operation need not be performed after addition
or subtraction of modular types. For binary moduli, a simple mask is
sufficient. For nonbinary moduli, a check after addition to see if the
value is greater than the high bound of the base range can be followed
by a conditional subtraction of the modulus. Conversely, a check after
subtraction to see if a "borrow" was performed can be followed
by a conditional addition of the modulus.
Examples
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Examples of
expressions involving binary adding operators:
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Z + 0.1 -- Z has to be of a real type
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"A" & "BCD" -- concatenation of two string literals
'A' & "BCD" -- concatenation of a character literal and a string literal
'A' & 'A' -- concatenation of two character literals
Inconsistencies With Ada 83
14.a
{inconsistencies with Ada 83}
The lower bound of the result of concatenation, for
a type whose first subtype is constrained, is now that of the index subtype.
This is inconsistent with Ada 83, but generally only for Ada 83 programs
that raise Constraint_Error. For example, the concatenation operator
in
14.b
X : array(1..10) of Integer;
begin
X := X(6..10) & X(1..5);
14.c
would raise Constraint_Error in
Ada 83 (because the bounds of the result of the concatenation would be
6..15, which is outside of 1..10), but would succeed and swap the halves
of X (as expected) in Ada 95.
Extensions to Ada 83
14.d
{extensions to Ada 83}
Concatenation is now useful for array types whose
first subtype is constrained. When the result type of a concatenation
is such an array type, Constraint_Error is avoided by effectively first
sliding the left operand (if nonnull) so that its lower bound is that
of the index subtype.
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