The Superstition of Adding 0.5 Then Casting to int for Rounding a Float Value

When looking at other people’s code, occasionally you come across this kind of code for rounding:

float f;
// ...
int r = static_cast<int>(f + 0.5f);

This approach is often found in books and is not necessarily incorrect for specific use cases.

I used to use this “trick” myself in the past.

However, I believe that this code should not be written in general.

Firstly, this code does not work correctly when f is negative:

float f = -1.1f;
// ...
int r = static_cast<int>(f + 0.5f);  // r = 0

In C++, you can simply #include <cmath> and use std::nearbyint() instead:

float f = -1.1f;
// ...
int r = std::nearbyint(f);  // r = -1

Depending on your needs, you can also use std::rint() or std::round().

In most programming languages, similar functions should be available.

By using these functions, your intentions become clear, and the risk of introducing unnecessary bugs is eliminated.

Therefore, I encourage people to use these functions actively.

• Programming