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:
    Option()
      : _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

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