4.5 Operators and Expression Evaluation
The language
defines the following six categories of operators (given in order of
increasing precedence). The corresponding
operator_symbols,
and only those, can be used as
designators
in declarations of functions for user-defined operators. See
6.6,
“
Overloading of Operators”.
Syntax
logical_operator ::= and |
or |
xor
relational_operator ::= = | /= | < | <= | > | >=
binary_adding_operator ::= + | – | &
unary_adding_operator ::= + | –
multiplying_operator ::= * | / |
mod |
rem
highest_precedence_operator ::= ** |
abs |
not
Static Semantics
For a sequence of operators of the same precedence
level, the operators are associated with their operands in textual order
from left to right. Parentheses can be used to impose specific associations.
For each
form of type definition, certain of the above operators are
predefined;
that is, they are implicitly declared immediately after the type definition.
For
each such implicit operator declaration, the parameters are called Left
and Right for
binary operators; the single parameter is called
Right for
unary operators. An expression of the form X op Y, where
op is a binary operator, is equivalent to a
function_call
of the form "op"(X, Y). An expression of the form op Y, where
op is a unary operator, is equivalent to a
function_call
of the form "op"(Y). The predefined operators and their effects
are described in subclauses
4.5.1 through
4.5.6.
Dynamic Semantics
The predefined operations on
integer types either yield the mathematically correct result or raise
the exception Constraint_Error. For implementations that support the
Numerics Annex, the predefined operations on real types yield results
whose accuracy is defined in
Annex G, or raise
the exception Constraint_Error.
Implementation Requirements
The implementation of a predefined
operator that delivers a result of an integer or fixed point type may
raise Constraint_Error only if the result is outside the base range of
the result type.
The implementation of a predefined
operator that delivers a result of a floating point type may raise Constraint_Error
only if the result is outside the safe range of the result type.
Implementation Permissions
For a sequence of predefined operators of the same
precedence level (and in the absence of parentheses imposing a specific
association), an implementation may impose any association of the operators
with operands so long as the result produced is an allowed result for
the left-to-right association, but ignoring the potential for failure
of language-defined checks in either the left-to-right or chosen order
of association.
NOTE The two operands of an expression
of the form X op Y, where op is a binary operator, are evaluated in an
arbitrary order, as for any
function_call
(see
6.4).
Examples
Examples of precedence:
not Sunny or Warm -- same as (not Sunny) or Warm
X > 4.0 and Y > 0.0 -- same as (X > 4.0) and (Y > 0.0)
-4.0*A**2 -- same as –(4.0 * (A**2))
abs(1 + A) + B -- same as (abs (1 + A)) + B
Y**(-3) -- parentheses are necessary
A / B * C -- same as (A/B)*C
A + (B + C) -- evaluate B + C before adding it to A
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