Argument Promotion Rules and Implicit Conversions¹

1. Promotions and conversions are complex topics discussed in Section 4 and the beginning of Section 5 of the C++ standard. You can purchase a copy of the standard at bit.ly/CPlusPlus11Standard.

Sometimes, argument values that do not correspond precisely to the parameter types in the function prototype can be converted by the compiler to the proper type before the function is called. These conversions occur as specified by C++’s promotion rules. The promotion rules indicate the implicit conversions that the compiler can perform between fundamental types. An int can be converted to a double. A double can also be converted to an int but the fractional part of the double value is truncated. Keep in mind that double variables can hold numbers of much greater magnitude than int variables, so the loss of data may be considerable. Values may also be modified when converting large integer types to small integer types (e.g., long to short), signed to unsigned or unsigned to signed. Unsigned integers range from 0 to approximately twice the positive range of the corresponding signed type.

The promotion rules apply to expressions containing values of two or more data types; such expressions are also referred to as mixed-type expressions. The type of each value in a mixed-type expression is promoted to the “highest” type in the expression (actually a temporary version of each value is created and used for the expression—the original values remain unchanged). Promotion also occurs when the type of a function argument does not match the parameter type specified in the function definition or prototype. Figure 6.5 lists the arithmetic data types in order from “highest type” to “lowest type.”

Fig. 6.5. Promotion hierarchy for arithmetic data types.