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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 : Treturn T
function "-"(Left, Right : Treturn 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 : Treturn T
function "&"(Left : T; Right : Creturn T
function "&"(Left : C; Right : Treturn T
function "&"(Left : C; Right : Creturn 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:
Reason: This rule avoids Constraint_Error when using concatenation on an array type whose first subtype is constrained.
   [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.
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.
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 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;
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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