conditional-expression: logical-or-expression logical-or-expression ? expression : assignment-expression

Conditional expressions group right-to-left.

The first expression is
contextually converted to bool.

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.

Every
value computation and side effect associated with the
first expression is sequenced before every value computation and side
effect associated with the second or third expression.

If either the second or the third operand has type void,
one of the following shall hold:

- 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.
- Both the second and the third operands have type void; the
result is of type void and is a prvalue. [ Note ]

Otherwise, if the second and third operand are glvalue bit-fields
of the same value category and
of types cv1 T and cv2 T, respectively,
the operands are considered to be of type cv T
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.

[ Note

: *end note*

]Properties such as access, whether an operand is a bit-field, or whether
a conversion function is deleted are ignored for that determination.

—
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 an xvalue, the target type is “rvalue reference to T2”, subject to the constraint that the reference must bind directly.
- 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:
- if T1 and T2 are the same class type (ignoring cv-qualification) and T2 is at least as cv-qualified as T1, the target type is T2,
- otherwise, if T2 is a base class of T1, the target type is cv1 T2, where cv1 denotes the cv-qualifiers of T1,
- otherwise, the target type is the type that E2 would have after applying the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions.

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.

Otherwise, the result is a prvalue.

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.

Lvalue-to-rvalue, array-to-pointer,
and function-to-pointer standard conversions are
performed on the second and third operands.

After those conversions, one
of the following shall hold:

- The second and third operands have the same type; the result is of that type and the result object is initialized using the selected operand.
- 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 type; pointer conversions, function pointer conversions, and qualification conversions are performed to bring them to their composite pointer type.The result is of the composite pointer 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.