7 Expressions [expr]

7.4 Usual arithmetic conversions [expr.arith.conv]

1
#
Many binary operators that expect operands of arithmetic or enumeration type cause conversions and yield result types in a similar way.
The purpose is to yield a common type, which is also the type of the result.
This pattern is called the usual arithmetic conversions, which are defined as follows:
  • (1.1)
    If either operand is of scoped enumeration type, no conversions are performed; if the other operand does not have the same type, the expression is ill-formed.
  • (1.2)
    Otherwise, if either operand is of floating-point type, the following rules are applied:
    • (1.2.1)
      If both operands have the same type, no further conversion is needed.
    • (1.2.2)
      Otherwise, if one of the operands is of a non-floating-point type, that operand is converted to the type of the operand with the floating-point type.
    • (1.2.3)
      Otherwise, if the floating-point conversion ranks ([conv.rank]) of the types of the operands are ordered but not equal, then the operand of the type with the lesser floating-point conversion rank is converted to the type of the other operand.
    • (1.2.4)
      Otherwise, if the floating-point conversion ranks of the types of the operands are equal, then the operand with the lesser floating-point conversion subrank ([conv.rank]) is converted to the type of the other operand.
    • (1.2.5)
      Otherwise, the expression is ill-formed.
  • (1.3)
    Otherwise, each operand is converted to a common type C.
    The integral promotion rules ([conv.prom]) are used to determine a type T1 and type T2 for each operand.48
    Then the following rules are applied to determine C:
    • (1.3.1)
      If T1 and T2 are the same type, C is that type.
    • (1.3.2)
      Otherwise, if T1 and T2 are both signed integer types or are both unsigned integer types, C is the type with greater rank.
    • (1.3.3)
      Otherwise, let U be the unsigned integer type and S be the signed integer type.
      • (1.3.3.1)
        If U has rank greater than or equal to the rank of S, C is U.
      • (1.3.3.2)
        Otherwise, if S can represent all of the values of U, C is S.
      • (1.3.3.3)
        Otherwise, C is the unsigned integer type corresponding to S.
2
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If one operand is of enumeration type and the other operand is of a different enumeration type or a floating-point type, this behavior is deprecated ([depr.arith.conv.enum]).
48)48)
As a consequence, operands of type bool, char8_t, char16_t, char32_t, wchar_t, or of enumeration type are converted to some integral type.