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6.6 Overloading of Operators

An operator is a function whose designator is an operator_symbol. [Operators, like other functions, may be overloaded.] 

Name Resolution Rules

Each use of a unary or binary operator is equivalent to a function_call with function_prefix being the corresponding operator_symbol, and with (respectively) one or two positional actual parameters being the operand(s) of the operator (in order). 
To be honest: {AI05-0299-1} We also use the term operator (in Clause 4 and in 6.1) to refer to one of the syntactic categories defined in 4.5, “Operators and Expression Evaluation” whose names end with “_operator:” logical_operator, relational_operator, binary_adding_operator, unary_adding_operator, multiplying_operator, and highest_precedence_operator.
Discussion: {AI05-0005-1} This equivalence extends to uses of function_call in most other language rules. However, as often happens, the equivalence is not perfect, as operator calls are not a name, while a function_call is a name. Thus, operator calls cannot be used in contexts that require a name (such as a rename of an object). A direct fix for this problem would be very disruptive, and thus we have not done that. However, qualifying an operator call can be used as a workaround in contexts that require a name

Legality Rules

{AI05-0143-1} The subprogram_specification of a unary or binary operator shall have one or two parameters, respectively. The parameters shall be of mode in. A generic function instantiation whose designator is an operator_symbol is only allowed if the specification of the generic function has the corresponding number of parameters, and they are all of mode in.
Default_expressions are not allowed for the parameters of an operator (whether the operator is declared with an explicit subprogram_specification or by a generic_instantiation).
An explicit declaration of "/=" shall not have a result type of the predefined type Boolean. 

Static Semantics

{AI05-0128-1} An explicit declaration of "=" whose result type is Boolean implicitly declares an operator "/=" that gives the complementary result. 
Discussion: {AI05-0128-1} A "/=" defined by this rule is considered user-defined, which means that it will be inherited by a derived type. “User-defined” means “not language-defined” for the purposes of inheritance, that is anything other than predefined operators.
10  The operators "+" and "–" are both unary and binary operators, and hence may be overloaded with both one- and two-parameter functions. 


Examples of user-defined operators: 
function "+" (Left, Right : Matrix) return Matrix;
function "+" (Left, Right : Vector) return Vector;

--  assuming that A, B, and C are of the type Vector
--  the following two statements are equivalent:

A := B + C;
A := "+"(B, C);

Extensions to Ada 83

Explicit declarations of "=" are now permitted for any combination of parameter and result types.
Explicit declarations of "/=" are now permitted, so long as the result type is not Boolean. 

Wording Changes from Ada 2005

{AI05-0128-1} Correction: Corrected the wording so that only explicit declarations of "=" cause an implicit declaration of "/="; otherwise, we could get multiple implicit definitions of "/=" without an obvious way to chose between them.
{AI05-0143-1} Added wording so that operators only allow parameters of mode in. This was made necessary by the elimination elsewhere of the restriction that function parameters be only of mode in

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