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/*!

    \example widgets/calculator

    \title Calculator Example

    The example shows how to use signals and slots to implement the

    functionality of a calculator widget, and how to use QGridLayout

    to place child widgets in a grid.

    \image calculator-example.png Screenshot of the Calculator example

    The example consists of two classes:

    \list

    \o \c Calculator is the calculator widget, with all the

       calculator functionality.

    \o \c Button is the widget used for each of the calculator

       button. 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

*/