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It derives from QToolButton. \endlist We will start by reviewing \c Calculator, then we will take a look at \c Button. \section1 Calculator Class Definition \snippet examples/widgets/calculator/calculator.h 0 The \c Calculator class provides a simple calculator widget. It inherits from QDialog and has several private slots associated with the calculator's buttons. QObject::eventFilter() is reimplemented to handle mouse events on the calculator's display. Buttons are grouped in categories according to their behavior. For example, all the digit buttons (labeled \gui 0 to \gui 9) append a digit to the current operand. For these, we connect multiple buttons to the same slot (e.g., \c digitClicked()). The categories are digits, unary operators (\gui{Sqrt}, \gui{x\unicode{178}}, \gui{1/x}), additive operators (\gui{+}, \gui{-}), and multiplicative operators (\gui{\unicode{215}}, \gui{\unicode{247}}). The other buttons have their own slots. \snippet examples/widgets/calculator/calculator.h 1 \snippet examples/widgets/calculator/calculator.h 2 The private \c createButton() function is used as part of the widget construction. \c abortOperation() is called whenever a division by zero occurs or when a square root operation is applied to a negative number. \c calculate() applies a binary operator (\gui{+}, \gui{-}, \gui{\unicode{215}}, or \gui{\unicode{247}}). \snippet examples/widgets/calculator/calculator.h 3 \snippet examples/widgets/calculator/calculator.h 4 \snippet examples/widgets/calculator/calculator.h 5 \snippet examples/widgets/calculator/calculator.h 6 \snippet examples/widgets/calculator/calculator.h 7 \snippet examples/widgets/calculator/calculator.h 8 These variables, together with the contents of the calculator display (a QLineEdit), encode the state of the calculator: \list \o \c sumInMemory contains the value stored in the calculator's memory (using \gui{MS}, \gui{M+}, or \gui{MC}). \o \c sumSoFar stores the value accumulated so far. When the user clicks \gui{=}, \c sumSoFar is recomputed and shown on the display. \gui{Clear All} resets \c sumSoFar to zero. \o \c factorSoFar stores a temporary value when doing multiplications and divisions. \o \c pendingAdditiveOperator stores the last additive operator clicked by the user. \o \c pendingMultiplicativeOperator stores the last multiplicative operator clicked by the user. \o \c waitingForOperand is \c true when the calculator is expecting the user to start typing an operand. \endlist Additive and multiplicative operators are treated differently because they have different precedences. For example, \gui{1 + 2 \unicode{247} 3} is interpreted as \gui{1 + (2 \unicode{247} 3)} because \gui{\unicode{247}} has higher precedence than \gui{+}. The table below shows the evolution of the calculator state as the user enters a mathematical expression. \table \header \o User Input \o Display \o Sum so Far \o Add. Op. \o Factor so Far \o Mult. Op. \o Waiting for Operand? \row \o \o 0 \o 0 \o \o \o \o \c true \row \o \gui{1} \o 1 \o 0 \o \o \o \o \c false \row \o \gui{1 +} \o 1 \o 1 \o \gui{+} \o \o \o \c true \row \o \gui{1 + 2} \o 2 \o 1 \o \gui{+} \o \o \o \c false \row \o \gui{1 + 2 \unicode{247}} \o 2 \o 1 \o \gui{+} \o 2 \o \gui{\unicode{247}} \o \c true \row \o \gui{1 + 2 \unicode{247} 3} \o 3 \o 1 \o \gui{+} \o 2 \o \gui{\unicode{247}} \o \c false \row \o \gui{1 + 2 \unicode{247} 3 -} \o 1.66667 \o 1.66667 \o \gui{-} \o \o \o \c true \row \o \gui{1 + 2 \unicode{247} 3 - 4} \o 4 \o 1.66667 \o \gui{-} \o \o \o \c false \row \o \gui{1 + 2 \unicode{247} 3 - 4 =} \o -2.33333 \o 0 \o \o \o \o \c true \endtable Unary operators, such as \gui Sqrt, require no special handling; they can be applied immediately since the operand is already known when the operator button is clicked. \snippet examples/widgets/calculator/calculator.h 9 \codeline \snippet examples/widgets/calculator/calculator.h 10 Finally, we declare the variables associated with the display and the buttons used to display numerals. \section1 Calculator Class Implementation \snippet examples/widgets/calculator/calculator.cpp 0 In the constructor, we initialize the calculator's state. The \c pendingAdditiveOperator and \c pendingMultiplicativeOperator variables don't need to be initialized explicitly, because the QString constructor initializes them to empty strings. \snippet examples/widgets/calculator/calculator.cpp 1 \snippet examples/widgets/calculator/calculator.cpp 2 We create the QLineEdit representing the calculator's display and set up some of its properties. In particular, we set it to be read-only. We also enlarge \c{display}'s font by 8 points. \snippet examples/widgets/calculator/calculator.cpp 4 For each button, we call the private \c createButton() function with the proper text label and a slot to connect to the button. \snippet examples/widgets/calculator/calculator.cpp 5 \snippet examples/widgets/calculator/calculator.cpp 6 The layout is handled by a single QGridLayout. The QLayout::setSizeConstraint() call ensures that the \c Calculator widget is always shown as its optimal size (its \l{QWidget::sizeHint()}{size hint}), preventing the user from resizing the calculator. The size hint is determined by the size and \l{QWidget::sizePolicy()}{size policy} of the child widgets. Most child widgets occupy only one cell in the grid layout. For these, we only need to pass a row and a column to QGridLayout::addWidget(). The \c display, \c backspaceButton, \c clearButton, and \c clearAllButton widgets occupy more than one column; for these we must also pass a row span and a column span. \snippet examples/widgets/calculator/calculator.cpp 7 Pressing one of the calculator's digit buttons will emit the button's \l{QToolButton::clicked()}{clicked()} signal, which will trigger the \c digitClicked() slot. First, we find out which button sent the signal using QObject::sender(). This function returns the sender as a QObject pointer. Since we know that the sender is a \c Button object, we can safely cast the QObject. We could have used a C-style cast or a C++ \c static_cast<>(), but as a defensive programming technique we use a \l qobject_cast(). The advantage is that if the object has the wrong type, a null pointer is returned. Crashes due to null pointers are much easier to diagnose than crashes due to unsafe casts. Once we have the button, we extract the operator using QToolButton::text(). The slot needs to consider two situations in particular. If \c display contains "0" and the user clicks the \gui{0} button, it would be silly to show "00". And if the calculator is in a state where it is waiting for a new operand, the new digit is the first digit of that new operand; in that case, any result of a previous calculation must be cleared first. At the end, we append the new digit to the value in the display. \snippet examples/widgets/calculator/calculator.cpp 8 \snippet examples/widgets/calculator/calculator.cpp 9 The \c unaryOperatorClicked() slot is called whenever one of the unary operator buttons is clicked. Again a pointer to the clicked button is retrieved using QObject::sender(). The operator is extracted from the button's text and stored in \c clickedOperator. The operand is obtained from \c display. Then we perform the operation. If \gui Sqrt is applied to a negative number or \gui{1/x} to zero, we call \c abortOperation(). If everything goes well, we display the result of the operation in the line edit and we set \c waitingForOperand to \c true. This ensures that if the user types a new digit, the digit will be considered as a new operand, instead of being appended to the current value. \snippet examples/widgets/calculator/calculator.cpp 10 \snippet examples/widgets/calculator/calculator.cpp 11 The \c additiveOperatorClicked() slot is called when the user clicks the \gui{+} or \gui{-} button. Before we can actually do something about the clicked operator, we must handle any pending operations. We start with the multiplicative operators, since these have higher precedence than additive operators: \snippet examples/widgets/calculator/calculator.cpp 12 \snippet examples/widgets/calculator/calculator.cpp 13 If \gui{\unicode{215}} or \gui{\unicode{247}} has been clicked earlier, without clicking \gui{=} afterward, the current value in the display is the right operand of the \gui{\unicode{215}} or \gui{\unicode{247}} operator and we can finally perform the operation and update the display. \snippet examples/widgets/calculator/calculator.cpp 14 \snippet examples/widgets/calculator/calculator.cpp 15 If \gui{+} or \gui{-} has been clicked earlier, \c sumSoFar is the left operand and the current value in the display is the right operand of the operator. If there is no pending additive operator, \c sumSoFar is simply set to be the text in the display. \snippet examples/widgets/calculator/calculator.cpp 16 \snippet examples/widgets/calculator/calculator.cpp 17 Finally, we can take care of the operator that was just clicked. Since we don't have the right-hand operand yet, we store the clicked operator in the \c pendingAdditiveOperator variable. We will apply the operation later, when we have a right operand, with \c sumSoFar as the left operand. \snippet examples/widgets/calculator/calculator.cpp 18 The \c multiplicativeOperatorClicked() slot is similar to \c additiveOperatorClicked(). We don't need to worry about pending additive operators here, because multiplicative operators have precedence over additive operators. \snippet examples/widgets/calculator/calculator.cpp 20 Like in \c additiveOperatorClicked(), we start by handing any pending multiplicative and additive operators. Then we display \c sumSoFar and reset the variable to zero. Resetting the variable to zero is necessary to avoid counting the value twice. \snippet examples/widgets/calculator/calculator.cpp 22 The \c pointClicked() slot adds a decimal point to the content in \c display. \snippet examples/widgets/calculator/calculator.cpp 24 The \c changeSignClicked() slot changes the sign of the value in \c display. If the current value is positive, we prepend a minus sign; if the current value is negative, we remove the first character from the value (the minus sign). \snippet examples/widgets/calculator/calculator.cpp 26 The \c backspaceClicked() removes the rightmost character in the display. If we get an empty string, we show "0" and set \c waitingForOperand to \c true. \snippet examples/widgets/calculator/calculator.cpp 28 The \c clear() slot resets the current operand to zero. It is equivalent to clicking \gui Backspace enough times to erase the entire operand. \snippet examples/widgets/calculator/calculator.cpp 30 The \c clearAll() slot resets the calculator to its initial state. \snippet examples/widgets/calculator/calculator.cpp 32 The \c clearMemory() slot erases the sum kept in memory, \c readMemory() displays the sum as an operand, \c setMemory() replace the sum in memory with the current sum, and \c addToMemory() adds the current value to the value in memory. For \c setMemory() and \c addToMemory(), we start by calling \c equalClicked() to update \c sumSoFar and the value in the display. \snippet examples/widgets/calculator/calculator.cpp 34 The private \c createButton() function is called from the constructor to create calculator buttons. \snippet examples/widgets/calculator/calculator.cpp 36 The private \c abortOperation() function is called whenever a calculation fails. It resets the calculator state and displays "####". \snippet examples/widgets/calculator/calculator.cpp 38 The private \c calculate() function performs a binary operation. The right operand is given by \c rightOperand. For additive operators, the left operand is \c sumSoFar; for multiplicative operators, the left operand is \c factorSoFar. The function return \c false if a division by zero occurs. \section1 Button Class Definition Let's now take a look at the \c Button class: \snippet examples/widgets/calculator/button.h 0 The \c Button class has a convenience constructor that takes a text label and a parent widget, and it reimplements QWidget::sizeHint() to provide more space around the text than the amount QToolButton normally provides. \section1 Button Class Implementation \snippet examples/widgets/calculator/button.cpp 0 The buttons' appearance is determined by the layout of the calculator widget through the size and \l{QWidget::sizePolicy}{size policy} of the layout's child widgets. The call to the \l{QWidget::setSizePolicy()}{setSizePolicy()} function in the constructor ensures that the button will expand horizontally to fill all the available space; by default, \l{QToolButton}s don't expand to fill available space. Without this call, the different buttons in a same column would have different widths. \snippet examples/widgets/calculator/button.cpp 1 \snippet examples/widgets/calculator/button.cpp 2 In \l{QWidget::sizeHint()}{sizeHint()}, we try to return a size that looks good for most buttons. We reuse the size hint of the base class (QToolButton) but modify it in the following ways: \list \o We add 20 to the \l{QSize::height()}{height} component of the size hint. \o We make the \l{QSize::width()}{width} component of the size hint at least as much as the \l{QSize::width()}{height}. \endlist This ensures that with most fonts, the digit and operator buttons will be square, without truncating the text on the \gui{Backspace}, \gui{Clear}, and \gui{Clear All} buttons. The screenshot below shows how the \c Calculator widget would look like if we \e didn't set the horizontal size policy to QSizePolicy::Expanding in the constructor and if we didn't reimplement QWidget::sizeHint(). \image calculator-ugly.png The Calculator example with default size policies and size hints */