Question

I’m making a simple command line Hangman game.

void Hangman::printStatus()
{
    cout << "Lives remaining: " << livesRemaining << endl;
    cout << getFormattedAnswer() << endl;
}

string Hangman::getFormattedAnswer()
{
    return getFormattedAnswerFrom(correctAnswer.begin(), correctAnswer.end());
}

string Hangman::getFormattedAnswerFrom(string::const_iterator begin, string::const_iterator end)
{
    return begin == end? "" : displayChar(*begin) + getFormattedAnswerFrom(++begin, end);
}

char Hangman::displayChar(const char c)
{
    return c;
}

(Eventually, I’ll change this so displayChar() displays a - or a character if the user has guessed it, but for simplicity now I’m just returning everything.)

When I build and run this from VS 2010, I get a popup box:

Debug Assertion Failed!

xstring Line: 78

Expression: string iterator not
dereferenceable

What am I doing wrong?

Answer 1

The problem is in the evaluation of:

displayChar(*begin) + getFormattedAnswerFrom(++begin, end)

In executing this statement, it is evident that your compiler is first incrementing begin, returning the “next” begin for use as the first argument to getFormattedAnswerFrom, and then dereferencing begin for the argument to displayChar.

When begin is one behind end, then begin != end so displayChar(*begin) + getFormattedAnswerFrom(++begin, end) will run. Your compiler increments begin, so now begin == end, and the dereference of begin is invalid.

See also: Order of evaluation in C++ function parameters