flags.cpp

Output stream objects, cout, for example, maintain their current configuration or settings with a set of 1-bit flags. Programmers can dynamically (i.e., while a program is running) change the flags with the functions and symbolic constants illustrated in this section. The program demonstrates three stream member functions: setf (set flags), unsetf (unset flags), and flags, and three symbolic constants: ios::hex, ios::oct, and ios::dec. ios, sort for input/output system, is the name of a class, and :: is the scope resolution operator. Together, they bind the names hex, oct, and dec to the ios class - the class defining the constants.

Programmers use the flag functions and constants to adjust the base or radix in which the stream object displays data. Consequently, the operation setf(ios::dec | ios::oct | ios::hex) function, is meaningless as it's not possible to display data in multiple bases or radicies simultaneously. (Nevertheless, the statement compiles and runs without causing a runtime error, but its behavior is implementation-dependent.) Alternatively, the OR'ing the three flags together makes sense: unsetf(ios::dec | ios::oct | ios::hex) clears all three flags. The presence of ios::dec in the unsetf argument notwithstanding, the stream displays output in decimal until the hex manipulator in the next statement switches the stream to hexadecimal output.

#include <iostream>
using namespace std;

int main()
{
	unsigned default_flags = cout.flags();		// gets the default formating flags

	cout << hex << default_flags << endl;

	cout.setf(ios::dec | ios::oct | ios::hex);	// makes no sense
	cout << hex << cout.flags() << endl;

	cout.unsetf(ios::dec | ios::oct | ios::hex);	// useful
	cout << 123 << endl;
	cout << hex << cout.flags() << endl;

	cout.flags(default_flags);			// restores the default flags
	cout << hex << cout.flags() << endl;

	return 0;
}

201 = 0010 0000 0001


e01 = 1110 0000 0001


123
1 = 0000 0000 0001


201 = 0010 0000 0001