5 Expressions [expr]

5.16 Conditional operator [expr.cond]

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 (Clause [conv]). 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, then the lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), and function-to-pointer ([conv.func]) standard conversions are performed on the second and third operands, and one of the following shall hold:

• The second or the third operand (but not both) is a throw-expression ([except.throw]); the result is of the type of the other and is a prvalue.

• Both the second and the third operands have type void; the result is of type void and is a prvalue. [ Note: This includes the case where both operands are throw-expressions.  — end note ]

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 convert each of those operands to the type of the other. The process for determining whether an operand expression E1 of type T1 can be converted to match an operand expression E2 of type T2 is defined as follows:

• If E2 is an lvalue: E1 can be converted to match E2 if E1 can be implicitly converted (Clause [conv]) to the type “lvalue reference to T2”, subject to the constraint that in the conversion the reference must bind directly ([dcl.init.ref]) to an lvalue.

• If E2 is an xvalue: E1 can be converted to match E2 if E1 can be implicitly converted to the type “rvalue reference to T2”, subject to the constraint that the reference must bind directly.

• If E2 is an rvalue or if neither of the conversions above can be done and at least one of the operands has (possibly cv-qualified) class type:

• if E1 and E2 have class type, and the underlying class types are the same or one is a base class of the other: E1 can be converted to match E2 if the class of T2 is the same type as, or a base class of, the class of T1, and the cv-qualification of T2 is the same cv-qualification as, or a greater cv-qualification than, the cv-qualification of T1. If the conversion is applied, E1 is changed to a prvalue of type T2 by copy-initializing a temporary of type T2 from E1 and using that temporary as the converted operand.

• Otherwise (i.e., if E1 or E2 has a nonclass type, or if they both have class types but the underlying classes are not either the same or one a base class of the other): E1 can be converted to match E2 if E1 can be implicitly converted to the type that expression E2 would have if E2 were converted to a prvalue (or the type it has, if E2 is a prvalue).

Using this process, it is determined whether the second operand can be converted to match the third operand, and whether the third operand can be converted to match the second operand. If both can be converted, or one can be converted but the conversion is ambiguous, the program is ill-formed. If neither can be converted, the operands are left unchanged and further checking is performed as described below. If exactly one conversion is possible, 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 section.

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 section.

Lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), and function-to-pointer ([conv.func]) 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. If the operands have class type, the result is a prvalue temporary of the result type, which is copy-initialized from either the second operand or the third operand depending on the value of the first 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.

• The second and third operands have pointer type, or one has pointer type and the other is a null pointer constant, or both are null pointer constants, at least one of which is non-integral; pointer conversions ([conv.ptr]) and qualification conversions ([conv.qual]) are performed to bring them to their composite pointer type ([expr.rel]). The result is of the composite pointer type.

• The second and third operands have pointer to member type, or one has pointer to member type and the other is a null pointer constant; pointer to member conversions ([conv.mem]) and qualification conversions ([conv.qual]) are performed to bring them to a common type, whose cv-qualification shall match the cv-qualification of either the second or the third operand. The result is of the common type.