C++ IOS Library - Precision



The floating-point precision determines the maximum number of digits to be written on insertion operations to express floating-point values. How this is interpreted depends on whether the floatfield format flag is set to a specific notation (either fixed or scientific) or it is unset (using the default notation, which is not necessarily equivalent to either fixed nor scientific).

For the default locale −

  • Using the default floating-point notation, the precision field specifies the maximum number of meaningful digits to display in total counting both those before and those after the decimal point. Notice that it is not a minimum, and therefore it does not pad the displayed number with trailing zeros if the number can be displayed with less digits than the 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.


Following is the declaration for ios_base::precision function.

get (1)	streamsize precision() const;
set (2)	streamsize precision (streamsize prec);

The first form (1) returns the value of the current floating-point precision field for the stream.

The second form (2) also sets it to a new value.


prec − New value for the floating-point precision.

Return Value

The precision selected in the stream before the call.


Basic guarantee − if an exception is thrown, the stream is in a valid state.

Data races

Accesses (1) or modifies (2) the stream object. Concurrent access to the same stream object may cause data races.


In below example explains about ios_base::precision function.

#include <iostream>     

int main () {
   double f = 3.14159;
   std::cout.unsetf ( std::ios::floatfield );                
   std::cout << f << '\n';
   std::cout << f << '\n';
   std::cout.setf( std::ios::fixed, std:: ios::floatfield ); 
   std::cout << f << '\n';
   return 0;

Let us compile and run the above program, this will produce the following result −