Do All Variadic Functions Promote `float` Arguments to `double`?
Nov 23, 2024 am 06:21 AMPromotion of Float Argument to Double in Variadic Functions
In a previous discussion, it was noted that printf() promotes a float argument to a double before accepting it. This raises the question: do all variadic functions perform this promotion?
Variadic Argument Promotion
Yes, according to both the C99 and C standards, float arguments passed to variadic functions are promoted to double.
Standard Specifications
- C99 (section 6.5.2.2): "arguments that have type float are promoted to double."
- C (section 5.2.2): "a floating point type... is converted to the promoted type before the call."
- C (section 4.6): "A prvalue of type float can be converted to a prvalue of type double. The value is unchanged."
cppreference on Variadic Function Conversions
cppreference provides a summary of the default conversions for variadic functions in C :
- std::nullptr_t to void*
- Float arguments to double
- Integer promotion for bool, char, short, and enumerations
Origin of Promotion
This promotion was introduced in C for compatibility with the K&R C standard, despite its potential drawbacks. As the C Rationale states:
"For compatibility with past practice, all argument promotions occur as described in K&R in the absence of a prototype declaration, including the not always desirable promotion of float to double."
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