It is
evaluated and if it is true, the result of the conditional
expression is the value of the second expression, otherwise that of the
third expression.
Only one of the second and third expressions is
evaluated.
The first expression is sequenced before
the second or third expression ([intro.execution]).
The second or the third operand (but not both) is a (possibly
parenthesized) throw-expression; the result
is of the type and value category of the other. The conditional-expression
is a bit-field if that operand is a bit-field.
Otherwise, if the second and third operand are glvalue bit-fields
of the same value category and
of types cv1T and cv2T, respectively,
the operands are considered to be of type cvT
for the remainder of this subclause,
where cv is the union of cv1 and cv2.
Otherwise, if the second and third operand have different types and
either has (possibly cv-qualified) class type, or if both
are glvalues of the same value category and the same type except for
cv-qualification, an attempt is made to
form an implicit conversion sequence from
each of those operands to the type of the other.
Properties such as access, whether an operand is a bit-field, or whether
a conversion function is deleted are ignored for that determination.
— end note
]
Attempts are made to form an implicit conversion sequence
from an operand expression E1 of type T1
to a target type related to the type T2
of the operand expression E2 as follows:
If E2 is an lvalue, the target type is
“lvalue reference to T2”, subject to the
constraint that in the conversion the reference must bind
directly ([dcl.init.ref]) to a glvalue.
If E2 is a prvalue or if neither of the conversion sequences above can be
formed and at least one of the operands has (possibly cv-qualified) class type:
Using this process, it is determined whether an implicit conversion
sequence can be formed from the second operand
to the target type determined for the third operand, and vice versa.
If both sequences can be formed, or one can be formed but it is the
ambiguous conversion sequence, the program is ill-formed.
If no conversion sequence can be formed, the operands are left unchanged
and further checking is performed as described below.
Otherwise, if exactly one conversion sequence can be formed,
that conversion is applied to the chosen operand
and the converted operand is used in place of the original operand for
the remainder of this subclause.
If the second and third operands are glvalues of the same value category
and have the same type, the
result is of that type and value category and it is a bit-field if the
second or the third operand is a bit-field, or if both are bit-fields.
If the second and third operands do
not have the same type, and either has (possibly cv-qualified) class
type, overload resolution is used to determine the conversions (if any)
to be applied to the operands ([over.match.oper], [over.built]).
If the overload resolution fails, the program is ill-formed.
Otherwise,
the conversions thus determined are applied, and the converted operands
are used in place of the original operands for the remainder of this
subclause.
The second and third operands have arithmetic or enumeration type;
the usual arithmetic conversions are performed to bring them to a common
type, and the result is of that type.
One or both of the second and third operands have pointer-to-member type;
pointer to member conversions ([conv.mem]),
function pointer conversions ([conv.fctptr]), and
qualification conversions ([conv.qual])
are performed to bring them to their composite
pointer type ([expr.type]). The result is of the composite pointer type.
Both the second and third operands have type std::nullptr_t or one has
that type and the other is a null pointer constant. The result is of type
std::nullptr_t.