Is C / C ++ different from a function compared to an expression?

Question

#include <stdio.h> double metersToFeet(double meters) { return meters / 0.3048; } int main() { printf("%u\n", (unsigned char)(char)(45.72 / 0.3048)); printf("%u\n", (unsigned char)(char)metersToFeet(45.72)); return 0; } 

This program outputs (both on GCC and Clang):

 127 150 

Why am I getting two different numbers?

Answer 1

The real answer (150) exceeds the range of a char (in a normal system) if it is signed. This conversion causes undefined behavior ; the compiler can do whatever he likes.

From standard C99, 6.3.1.4:

When the final value of the real floating type is converted to an integer type other than _Bool, the fractional part is discarded (i.e. the value is truncated to zero). If the value of the integral part cannot be represented as an integer type, the behavior is undefined.

Answer 2

45.72 / 0.3048 150 .

The char type on your platform is apparently 8, but a signed type. 150 is out of this range. This means that the behavior is undefined. The rest follows.

For example, I get 127,127 in GCC through ideone.com and 150,150 in MSVC. You got 127 150 , which is funny, but not surprising.