libzypp  15.28.6
Code Pitfalls - Frequently made mistakes

Comparing TriBool values

Comparing two TriBool values is not as easy as it might look like, because the TriBool::operator== and TriBool::operator!= return a TriBool.

For example is (indeterminate==indeterminate) not true, but indeterminate. That's why the following snippet does not do what the author expected:

// buggy option class
struct Option
{
public:
: _value( indeterminate )
{}
bool get() const
{ return ( _value == indeterminate ) ? true : bool(_value); }
void set( bool new_r )
{ _value = new_r; }
private:
tribool _value;
};

You find that get() returns false as long as the option is unset, and not true as the code suggests. That's because (_value==indeterminate) returns indeterminate.

Note
Always use indeterminate(_value) to test whether a TriBools value is indeterminate:
bool get() const
{ return indeterminate( _value ) ? true : bool(_value); }
tribool _value;
...
if ( indeterminate( _value ) )
{ ... } // indeterminate
else if ( _value )
{ ... } // true
else
{ ... } // false