Casting float to int inconsistent across MinGw and Clang - c++

Question

Using C++, I'm trying to cast a float value to an int using these instructions :
#include <iostream>
int main() {
float NbrToCast = 1.8f;
int TmpNbr = NbrToCast * 10;
std::cout << TmpNbr << "\n";
}
I understand the value 1.8 cannot be precisely represented as a float and is actually stored as 1.79999995.
Thus, I would expect that multiplying this value by ten, would result to 17.99999995 and then casting it to an int would give 17.
When compiling and running this code with MinGW (v4.9.2 32bits) on Windows 7, I get the expected result (17).
When compiling and running this code with CLang (v600.0.57) on my Mac (OS X 10.11), I get 18as a result, which is not what I was expecting but which seems more correct in a mathematical way !
Why do I get this difference ?
Is there a way to have a consistent behavior no matter the OS or the compiler ?

Answer 1

Like Yuushi said in the comments, I guess the rounding rules may differ for each compiler. Having a portable solution on such a topic probably means you need to write your own rounding method.
So in your case you probably need to check the value of the digit after 7 and increment the value or not. Let's say something like:
int main() {
float NbrToCast = 1.8f;
float TmpNbr = NbrToCast * 10;
std::cout << RoundingFloatToInt(TmpNbr) << "\n";
}
int RoundingFloatToInt(const float &val)
{
float intPart, fractPart;
fractpart = modf (val, &intpart);
int result = intPart;
if (fractpart > 0.5)
{
result++;
}
return result;
}
(code not tested at all but you have the idea)
If you need performance, it's probably not great but I think it should be portable.