Detecting precision loss when converting from double to float

Question

I am writing a piece of code in which i have to convert from double to float values. I am using boost::numeric_cast to do this conversion which will alert me of any overflow/underflow. However i am also interested in knowing if that conversion resulted in some precision loss or not.

For example

    double source =  1988.1012;
    float dest = numeric_cast<float>(source);

Produces dest which has value 1988.1

Is there any way available in which i can detect this kind of precision loss/rounding

Answer 1

float dest = numeric_cast<float>(source);
double residual = source - numeric_cast<double>(dest);

Hence, residual contains the "loss" you're looking for.

Answer 2

You could cast the float back to a double and compare this double to the original - that should give you a fair indication as to whether there was a loss of precision.