敬业的IT人 互联网 佚名 2008-2-27 10:16:13

　　A boolean-expression is an expression that yields a result of type bool.
boolean-expression:
expression
The controlling conditional expression of an if-statement (?5.7.1),
while-statement (?5.8.1), do-statement
(?5.8.2), or for-statement (?5.8.3) is a boolean-expression. The
controlling conditional expression of the
?: operator (?4.12) follows the same rules as a boolean-expression, but
for reasons of operator precedence
is classified as a conditional-or-expression.
A boolean-expression is required to be of a type that can be implicitly
converted to bool or of a type that
implements operator true. [Note: As required by ?7.9.1, any type that
implements operator true
must also implement operator false. end note] If neither requirement is
satisfied, a compile-time error
occurs.
When a boolean expression is of a type that cannot be implicitly converted
to bool but does implement
operator true, then following evaluation of the expression, the operator
true implementation
provided by that type is invoked to produce a bool value.
[Note: The DBBool struct type in ?8.4.2 provides an example of a type that
implements operator true
and operator false. end note]

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