14.11 Conditional logical operators

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本文详细解释了条件逻辑运算符&&及||的工作原理及其在编程中的应用方式。这两种运算符被称作“短路”逻辑运算符，它们是常规逻辑运算符&及|的条件版本。文章还探讨了如何通过重载常规逻辑运算符来间接实现条件逻辑运算符的重载。

The && and || operators are called the conditional logical operators. They
are also called the "shortcircuiting" logical operators.
conditional-and-expression:
inclusive-or-expression
conditional-and-expression && inclusive-or-expression
conditional-or-expression:
conditional-and-expression
conditional-or-expression || conditional-and-expression
The && and || operators are conditional versions of the & and | operators:
?The operation x && y corresponds to the operation x & y, except that y is
evaluated only if x is true.
?The operation x || y corresponds to the operation x | y, except that y is
evaluated only if x is
false.
An operation of the form x && y or x || y is processed by applying overload
resolution (?4.2.4) as if the
operation was written x & y or x | y. Then,
?If overload resolution fails to find a single best operator, or if
overload resolution selects one of the
predefined integer logical operators, a compile-time error occurs.
?Otherwise, if the selected operator is one of the predefined boolean
logical operators (?4.10.2), the
operation is processed as described in ?4.11.1.
?Otherwise, the selected operator is a user-defined operator, and the
operation is processed as described
in ?4.11.2.
It is not possible to directly overload the conditional logical operators.
However, because the conditional
logical operators are evaluated in terms of the regular logical operators,
overloads of the regular logical
operators are, with certain restrictions, also considered overloads of the
conditional logical operators. This is
described further in ?4.11.2.
14.11.1 Boolean conditional logical operators
When the operands of && or || are of type bool, or when the operands are of
types that do not define an
applicable operator & or operator |, but do define implicit conversions to
bool, the operation is
processed as follows:
?The operation x && y is evaluated as x ? y : false. In other words, x is
first evaluated and
converted to type bool. Then, if x is true, y is evaluated and converted to
type bool, and this becomes
the result of the operation. Otherwise, the result of the operation is
false.
?The operation x || y is evaluated as x ? true : y. In other words, x is
first evaluated and converted
to type bool. Then, if x is true, the result of the operation is true.
Otherwise, y is evaluated and
converted to type bool, and this becomes the result of the operation.