What is the role of **std::setprecision()** without **std::fixed** in c++?

Question

As shown in the tutorial http://www.cplusplus.com/reference/iomanip/setprecision/

// setprecision example
#include      // std::cout, std::fixed
         


        

Answer 1

According to http://en.cppreference.com/w/cpp/io/ios_base/precision, the precision decides how many digits are printed, not how many digits after the floating point are printed:

std::ios_base::precision

Manages the precision (i.e. how many digits are generated) of floating point output

This explains the rounding.

Yes, using std::fixed will change the interpretation of the floatfield precision. According to http://www.cplusplus.com/reference/ios/ios_base/precision/:

In both the fixed and scientific notations, the precision field specifies exactly how many digits to display after the decimal point, even if this includes trailing decimal zeros. The digits before the decimal point are not relevant for the precision in this case.