From a previous question:
If you attempt to pass a
printf, it'll be promoted to
printf receives it
printf() is a variadic function right? So does a variadic function promote a
float argument to a
double before passing it?
Best How To :
Yes, float arguments to variadic function are promoted to double.
The draft C99 standard section
220.127.116.11 Function calls says:
[...]and arguments that have type float are promoted to double. These are called the default argument promotions.[...]
from the draft C++ standard section
5.2.2 Function call:
[...]a floating point type that is subject to the floating point promotion (4.6), the value of the argument is converted to the promoted type before the call. [...]
A prvalue of type float can be converted to a prvalue of type double. The value is unchanged
cppreference covers the default conversions for variadic function in C++ well:
- std::nullptr_t is converted to void*
- float arguments are converted to double as in floating-point promotion
- bool, char, short, and unscoped enumerations are converted to int or wider integer types as in integer promotion
We can see in C and presumably in C++ this conversion was kept around for compatibility with K&R C, from Rationale for International Standard—Programming Languages—C (emphasis mine):
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.