When used in an expression out << setfill(c) sets the fill character of the stream out to c.

Note: The current fill character may be obtained with std::ostream::fill.

Example:

#include <iostream>
#include <iomanip>
int main()
{
    std::cout << "default fill: " << std::setw(10) << 42 << '\\n'
          << "setfill('*'): " << std::setfill('*')
                              << std::setw(10) << 42 << '\\n';
}
//output::
//default fill:         42
//setfill('*'): ********42