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.The example consists of two classes:
Calculator
is the calculator widget, with all the calculator functionality.
Button
is the widget used for each of the calculator button. It derives fromQToolButton
.We will start by reviewing
Calculator
, then we will take a look atButton
.
Calculator Class Definition¶
class Calculator : public QWidget { Q_OBJECT public: Calculator(QWidget *parent = nullptr); private slots: void digitClicked(); void unaryOperatorClicked(); void additiveOperatorClicked(); void multiplicativeOperatorClicked(); void equalClicked(); void pointClicked(); void changeSignClicked(); void backspaceClicked(); void clear(); void clearAll(); void clearMemory(); void readMemory(); void setMemory(); void addToMemory();The
Calculator
class provides a simple calculator widget. It inherits fromQDialog
and has several private slots associated with the calculator’s buttons.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 0 to 9) append a digit to the current operand. For these, we connect multiple buttons to the same slot (e.g.,
digitClicked()
). The categories are digits, unary operators (Sqrt, x², 1/x), additive operators (+, -), and multiplicative operators (×, ÷). The other buttons have their own slots.private: Button *createButton(const QString &text, const char *member); void abortOperation(); bool calculate(double rightOperand, const QString &pendingOperator);The private
createButton()
function is used as part of the widget construction.abortOperation()
is called whenever a division by zero occurs or when a square root operation is applied to a negative number.calculate()
applies a binary operator (+, -, ×, or ÷).double sumInMemory; double sumSoFar; double factorSoFar; QString pendingAdditiveOperator; QString pendingMultiplicativeOperator; bool waitingForOperand;These variables, together with the contents of the calculator display (a
QLineEdit
), encode the state of the calculator:
sumInMemory
contains the value stored in the calculator’s memory (using MS, M+, or MC).
sumSoFar
stores the value accumulated so far. When the user clicks =,sumSoFar
is recomputed and shown on the display. Clear All resetssumSoFar
to zero.
factorSoFar
stores a temporary value when doing multiplications and divisions.
pendingAdditiveOperator
stores the last additive operator clicked by the user.
pendingMultiplicativeOperator
stores the last multiplicative operator clicked by the user.
waitingForOperand
istrue
when the calculator is expecting the user to start typing an operand.Additive and multiplicative operators are treated differently because they have different precedences. For example, 1 + 2 ÷ 3 is interpreted as 1 + (2 ÷ 3) because ÷ has higher precedence than +.
The table below shows the evolution of the calculator state as the user enters a mathematical expression.
User Input
Display
Sum so Far
Add. Op.
Factor so Far
Mult. Op.
Waiting for Operand?
0
0
true
1
1
0
false
1 +
1
1
true
1 + 2
2
1
false
1 + 2 ÷
2
1
2
÷
true
1 + 2 ÷ 3
3
1
2
÷
false
1 + 2 ÷ 3 -
1.66667
1.66667
true
1 + 2 ÷ 3 - 4
4
1.66667
false
1 + 2 ÷ 3 - 4 =
-2.33333
0
true
Unary operators, such as Sqrt, require no special handling; they can be applied immediately since the operand is already known when the operator button is clicked.
QLineEdit *display; enum { NumDigitButtons = 10 }; Button *digitButtons[NumDigitButtons]; };Finally, we declare the variables associated with the display and the buttons used to display numerals.
Calculator Class Implementation¶
Calculator::Calculator(QWidget *parent) : QWidget(parent), sumInMemory(0.0), sumSoFar(0.0) , factorSoFar(0.0), waitingForOperand(true) {In the constructor, we initialize the calculator’s state. The
pendingAdditiveOperator
andpendingMultiplicativeOperator
variables don’t need to be initialized explicitly, because theQString
constructor initializes them to empty strings. It is also possible to initialize those variable directly in the header. This is calledmember-initializaton
and avoids a long initialization list.display = new QLineEdit("0"); display->setReadOnly(true); display->setAlignment(Qt::AlignRight); display->setMaxLength(15); QFont font = display->font(); font.setPointSize(font.pointSize() + 8); display->setFont(font);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
display
‘s font by 8 points.for (int i = 0; i < NumDigitButtons; ++i) digitButtons[i] = createButton(QString::number(i), SLOT(digitClicked())); Button *pointButton = createButton(tr("."), SLOT(pointClicked())); Button *changeSignButton = createButton(tr("\302\261"), SLOT(changeSignClicked())); Button *backspaceButton = createButton(tr("Backspace"), SLOT(backspaceClicked())); Button *clearButton = createButton(tr("Clear"), SLOT(clear())); Button *clearAllButton = createButton(tr("Clear All"), SLOT(clearAll())); Button *clearMemoryButton = createButton(tr("MC"), SLOT(clearMemory())); Button *readMemoryButton = createButton(tr("MR"), SLOT(readMemory())); Button *setMemoryButton = createButton(tr("MS"), SLOT(setMemory())); Button *addToMemoryButton = createButton(tr("M+"), SLOT(addToMemory())); Button *divisionButton = createButton(tr("\303\267"), SLOT(multiplicativeOperatorClicked())); Button *timesButton = createButton(tr("\303\227"), SLOT(multiplicativeOperatorClicked())); Button *minusButton = createButton(tr("-"), SLOT(additiveOperatorClicked())); Button *plusButton = createButton(tr("+"), SLOT(additiveOperatorClicked())); Button *squareRootButton = createButton(tr("Sqrt"), SLOT(unaryOperatorClicked())); Button *powerButton = createButton(tr("x\302\262"), SLOT(unaryOperatorClicked())); Button *reciprocalButton = createButton(tr("1/x"), SLOT(unaryOperatorClicked())); Button *equalButton = createButton(tr("="), SLOT(equalClicked()));For each button, we call the private
createButton()
function with the proper text label and a slot to connect to the button.QGridLayout *mainLayout = new QGridLayout; mainLayout->setSizeConstraint(QLayout::SetFixedSize); mainLayout->addWidget(display, 0, 0, 1, 6); mainLayout->addWidget(backspaceButton, 1, 0, 1, 2); mainLayout->addWidget(clearButton, 1, 2, 1, 2); mainLayout->addWidget(clearAllButton, 1, 4, 1, 2); mainLayout->addWidget(clearMemoryButton, 2, 0); mainLayout->addWidget(readMemoryButton, 3, 0); mainLayout->addWidget(setMemoryButton, 4, 0); mainLayout->addWidget(addToMemoryButton, 5, 0); for (int i = 1; i < NumDigitButtons; ++i) { int row = ((9 - i) / 3) + 2; int column = ((i - 1) % 3) + 1; mainLayout->addWidget(digitButtons[i], row, column); } mainLayout->addWidget(digitButtons[0], 5, 1); mainLayout->addWidget(pointButton, 5, 2); mainLayout->addWidget(changeSignButton, 5, 3); mainLayout->addWidget(divisionButton, 2, 4); mainLayout->addWidget(timesButton, 3, 4); mainLayout->addWidget(minusButton, 4, 4); mainLayout->addWidget(plusButton, 5, 4); mainLayout->addWidget(squareRootButton, 2, 5); mainLayout->addWidget(powerButton, 3, 5); mainLayout->addWidget(reciprocalButton, 4, 5); mainLayout->addWidget(equalButton, 5, 5); setLayout(mainLayout); setWindowTitle(tr("Calculator")); }The layout is handled by a single
QGridLayout
. ThesetSizeConstraint()
call ensures that theCalculator
widget is always shown as its optimal size (itssize hint
), preventing the user from resizing the calculator. The size hint is determined by the size andsize 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
addWidget()
. Thedisplay
,backspaceButton
,clearButton
, andclearAllButton
widgets occupy more than one column; for these we must also pass a row span and a column span.void Calculator::digitClicked() { Button *clickedButton = qobject_cast<Button *>(sender()); int digitValue = clickedButton->text().toInt(); if (display->text() == "0" && digitValue == 0.0) return; if (waitingForOperand) { display->clear(); waitingForOperand = false; } display->setText(display->text() + QString::number(digitValue)); }Pressing one of the calculator’s digit buttons will emit the button’s
clicked()
signal, which will trigger thedigitClicked()
slot.First, we find out which button sent the signal using
sender()
. This function returns the sender as aQObject
pointer. Since we know that the sender is aButton
object, we can safely cast theQObject
. We could have used a C-style cast or a C++static_cast<>()
, but as a defensive programming technique we use aqobject_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 usingtext()
.The slot needs to consider two situations in particular. If
display
contains “0” and the user clicks the 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.
void Calculator::unaryOperatorClicked() { Button *clickedButton = qobject_cast<Button *>(sender()); QString clickedOperator = clickedButton->text(); double operand = display->text().toDouble(); double result = 0.0; if (clickedOperator == tr("Sqrt")) { if (operand < 0.0) { abortOperation(); return; } result = std::sqrt(operand); } else if (clickedOperator == tr("x\302\262")) { result = std::pow(operand, 2.0); } else if (clickedOperator == tr("1/x")) { if (operand == 0.0) { abortOperation(); return; } result = 1.0 / operand; } display->setText(QString::number(result)); waitingForOperand = true; }The
unaryOperatorClicked()
slot is called whenever one of the unary operator buttons is clicked. Again a pointer to the clicked button is retrieved usingsender()
. The operator is extracted from the button’s text and stored inclickedOperator
. The operand is obtained fromdisplay
.Then we perform the operation. If Sqrt is applied to a negative number or 1/x to zero, we call
abortOperation()
. If everything goes well, we display the result of the operation in the line edit and we setwaitingForOperand
totrue
. 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.void Calculator::additiveOperatorClicked() { Button *clickedButton = qobject_cast<Button *>(sender()); if (!clickedButton) return; QString clickedOperator = clickedButton->text(); double operand = display->text().toDouble();The
additiveOperatorClicked()
slot is called when the user clicks the + or - 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:
if (!pendingMultiplicativeOperator.isEmpty()) { if (!calculate(operand, pendingMultiplicativeOperator)) { abortOperation(); return; } display->setText(QString::number(factorSoFar)); operand = factorSoFar; factorSoFar = 0.0; pendingMultiplicativeOperator.clear(); }If × or ÷ has been clicked earlier, without clicking = afterward, the current value in the display is the right operand of the × or ÷ operator and we can finally perform the operation and update the display.
if (!pendingAdditiveOperator.isEmpty()) { if (!calculate(operand, pendingAdditiveOperator)) { abortOperation(); return; } display->setText(QString::number(sumSoFar)); } else { sumSoFar = operand; }If + or - has been clicked earlier,
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,sumSoFar
is simply set to be the text in the display.pendingAdditiveOperator = clickedOperator; waitingForOperand = true; }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
pendingAdditiveOperator
variable. We will apply the operation later, when we have a right operand, withsumSoFar
as the left operand.void Calculator::multiplicativeOperatorClicked() { Button *clickedButton = qobject_cast<Button *>(sender()); if (!clickedButton) return; QString clickedOperator = clickedButton->text(); double operand = display->text().toDouble(); if (!pendingMultiplicativeOperator.isEmpty()) { if (!calculate(operand, pendingMultiplicativeOperator)) { abortOperation(); return; } display->setText(QString::number(factorSoFar)); } else { factorSoFar = operand; } pendingMultiplicativeOperator = clickedOperator; waitingForOperand = true; }The
multiplicativeOperatorClicked()
slot is similar toadditiveOperatorClicked()
. We don’t need to worry about pending additive operators here, because multiplicative operators have precedence over additive operators.void Calculator::equalClicked() { double operand = display->text().toDouble(); if (!pendingMultiplicativeOperator.isEmpty()) { if (!calculate(operand, pendingMultiplicativeOperator)) { abortOperation(); return; } operand = factorSoFar; factorSoFar = 0.0; pendingMultiplicativeOperator.clear(); } if (!pendingAdditiveOperator.isEmpty()) { if (!calculate(operand, pendingAdditiveOperator)) { abortOperation(); return; } pendingAdditiveOperator.clear(); } else { sumSoFar = operand; } display->setText(QString::number(sumSoFar)); sumSoFar = 0.0; waitingForOperand = true; }Like in
additiveOperatorClicked()
, we start by handling any pending multiplicative and additive operators. Then we displaysumSoFar
and reset the variable to zero. Resetting the variable to zero is necessary to avoid counting the value twice.void Calculator::pointClicked() { if (waitingForOperand) display->setText("0"); if (!display->text().contains('.')) display->setText(display->text() + tr(".")); waitingForOperand = false; }The
pointClicked()
slot adds a decimal point to the content indisplay
.void Calculator::changeSignClicked() { QString text = display->text(); double value = text.toDouble(); if (value > 0.0) { text.prepend(tr("-")); } else if (value < 0.0) { text.remove(0, 1); } display->setText(text); }The
changeSignClicked()
slot changes the sign of the value indisplay
. 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).void Calculator::backspaceClicked() { if (waitingForOperand) return; QString text = display->text(); text.chop(1); if (text.isEmpty()) { text = "0"; waitingForOperand = true; } display->setText(text); }The
backspaceClicked()
removes the rightmost character in the display. If we get an empty string, we show “0” and setwaitingForOperand
totrue
.void Calculator::clear() { if (waitingForOperand) return; display->setText("0"); waitingForOperand = true; }The
clear()
slot resets the current operand to zero. It is equivalent to clicking Backspace enough times to erase the entire operand.void Calculator::clearAll() { sumSoFar = 0.0; factorSoFar = 0.0; pendingAdditiveOperator.clear(); pendingMultiplicativeOperator.clear(); display->setText("0"); waitingForOperand = true; }The
clearAll()
slot resets the calculator to its initial state.void Calculator::clearMemory() { sumInMemory = 0.0; } void Calculator::readMemory() { display->setText(QString::number(sumInMemory)); waitingForOperand = true; } void Calculator::setMemory() { equalClicked(); sumInMemory = display->text().toDouble(); } void Calculator::addToMemory() { equalClicked(); sumInMemory += display->text().toDouble(); }The
clearMemory()
slot erases the sum kept in memory,readMemory()
displays the sum as an operand,setMemory()
replace the sum in memory with the current sum, andaddToMemory()
adds the current value to the value in memory. ForsetMemory()
andaddToMemory()
, we start by callingequalClicked()
to updatesumSoFar
and the value in the display.Button *Calculator::createButton(const QString &text, const char *member) { Button *button = new Button(text); connect(button, SIGNAL(clicked()), this, member); return button; }The private
createButton()
function is called from the constructor to create calculator buttons.void Calculator::abortOperation() { clearAll(); display->setText(tr("####")); }The private
abortOperation()
function is called whenever a calculation fails. It resets the calculator state and displays “####”.bool Calculator::calculate(double rightOperand, const QString &pendingOperator) { if (pendingOperator == tr("+")) { sumSoFar += rightOperand; } else if (pendingOperator == tr("-")) { sumSoFar -= rightOperand; } else if (pendingOperator == tr("\303\227")) { factorSoFar *= rightOperand; } else if (pendingOperator == tr("\303\267")) { if (rightOperand == 0.0) return false; factorSoFar /= rightOperand; } return true; }The private
calculate()
function performs a binary operation. The right operand is given byrightOperand
. For additive operators, the left operand issumSoFar
; for multiplicative operators, the left operand isfactorSoFar
. The function returnfalse
if a division by zero occurs.
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