4.5.3 Binary Adding Operators
Static Semantics
{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:
function "+"(Left, Right : T) return T
function "-"(Left, Right : T) return T
{&
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:
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
{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:
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;
Reason: This rule avoids Constraint_Error
when using concatenation on an array type whose first subtype is constrained.
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.
[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.
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]} 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.
{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).
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.
14 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.
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
Examples of expressions
involving binary adding operators:
Z + 0.1 -- Z has to be of a real type
"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
{
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
X : array(1..10) of Integer;
begin
X := X(6..10) & X(1..5);
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
{
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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