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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
+
+*/