13.7.7. Specifying Boolean Format (boolalpha)

C++ provides data type bool, whose values may be false or true, as a preferred alternative to the old style of using 0 to indicate false and nonzero to indicate true. A bool variable outputs as 0 or 1 by default. However, we can use stream manipulator boolalpha to set the output stream to display bool values as the strings "true" and "false". Use stream manipulator noboolalpha to set the output stream to display bool values as integers (i.e., the default setting). The program of Fig. 13.20 demonstrates these stream manipulators. Line 11 displays the bool value, which line 8 sets to true, as an integer. Line 15 uses manipulator boolalpha to display the bool value as a string. Lines 18–19 then change the bool’s value and use manipulator noboolalpha, so line 22 can display the bool value as an integer. Line 26 uses manipulator boolalpha to display the bool value as a string. Both boolalpha and noboolalpha are sticky settings.

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 1   // Fig. 13.20: fig13_20.cpp
 2   // Stream manipulators boolalpha and noboolalpha.
 3   #include <iostream>
 4   using namespace std;
 5
 6   int main()
 7   {
 8      bool booleanValue = true;
 9
10      // display default true booleanValue
11      cout << "booleanValue is " << booleanValue << endl;
12
13      // display booleanValue after using boolalpha
14      cout << "booleanValue (after using boolalpha) is "
15         << boolalpha << booleanValue << endl << endl;
16
17      cout << "switch booleanValue and use noboolalpha" << endl;
18      booleanValue = false; // change booleanValue
19      cout << noboolalpha << endl; // use noboolalpha
20
21      // display default false booleanValue after using noboolalpha
22      cout << "booleanValue is " << booleanValue << endl;
23
24      // display booleanValue after using boolalpha again
25      cout << "booleanValue (after using boolalpha) is "
26         << boolalpha << booleanValue << endl;
27   } // end main

booleanValue is 1
booleanValue (after using boolalpha) is true

switch booleanValue and use noboolalpha

booleanValue is 0
booleanValue (after using boolalpha) is false

Fig. 13.20. Stream manipulators boolalpha and noboolalpha.

Image Good Programming Practice 13.1

Displaying bool values as true or false, rather than nonzero or 0, respectively, makes program outputs clearer.

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