Final answer:
The 'promotion order' in C is the rule set that defines how smaller integer types are converted to larger types in expressions to prevent data loss. It follows a specific hierarchy to ensure the largest sufficient data type is used.
Step-by-step explanation:
The promotion order in the C programming language refers to the set of rules used by the C compiler to convert smaller integer types into larger integer types during the evaluation of expressions. This process is known as integer promotion and is performed to avoid loss of data and to maintain consistency across different calculations.
When two operands of different types are used in an expression, the C standard specifies a set of conversions that are applied to the operands before the operation is performed. The usual arithmetic conversions are performed in the following order:
- Firstly, if either operand is of type long double, the other is converted to long double.
- Next, if not yet equal, and if either operand is double, the other is converted to double.
- Then, if not yet equal, and if either operand is float, the other is converted to float.
- In the absence of floating-point types, if either operand is unsigned long long, the other is converted to unsigned long long.
- If not, and if either operand is long long, the other is converted to long long.
- If not, and if either operand is unsigned long, the other is converted to unsigned long.
- If not, and if either operand is long, the other is converted to long.
- If not, and if either operand is unsigned int, the other is converted to unsigned int.
- Finally, if none of the above apply, both operands are promoted to int.
This hierarchical structure ensures that the result of the expression is of a type that is large enough to handle the result without causing an unintentional truncation or data loss.