11-03-11 - BoolYouMustCheck

I've got a lot of functions that return error codes (rather than throw or something). The problem with that is that it's very easy to just not check the error code and then you have incorrect code that can possibly break in a nasty way if the error case is hit and not detected.

One way to test this is like this :

class BoolYouMustCheck
{
private:
    bool m_b;
    mutable bool m_checked;

public :

    //BoolYouMustCheck() : m_b(false), m_checked(false) { }
    BoolYouMustCheck(bool b) : m_b(b), m_checked(false) { }
    
    ~BoolYouMustCheck()
    {
        ASSERT( m_checked );
    }
    
    operator bool () const
    {
        m_checked = true;
        return m_b;
    }

};

it's just a proxy for bool which will assert if it is assigned and never read.

So now you can take a function that returns an error condition, for example :

bool func1(int x)
{
    return ( x > 7 );
}

normally you could easily just call func1() and not check the value. But you change it to :

BoolYouMustCheck func2(int x)
{
    return ( x > 7 );
}

(in practice you probably just want to do #define bool BoolYouMustCheck)

Now you get :

{

    int y = clock();

    // asserts:
    func1(y);

    // asserts :
    bool b1 = func1(y);
    
    // okay :
    bool b2 = func1(y);
    if ( b2 )
        y++;
    
    // okay :
    if ( func1(y) )
        y++;
        
    return y;
}

which is kind of nice.

The only ugly thing is that the assert can be rather far removed from the line of code that caused the problem. In the first case (just calling func1 and doing nothing with the return value), you get an assert right away, because the returned class is destructed right away. But in the second case where you assign to b1, you don't get the assert until the end of function scope. I guess you could fix that by taking a stack trace in the constructor.

(note : if you want to intentionally ignore the return value b1 you can just add a line like (int) b1; to surpress the assert.