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<li class="level1"><a href="#calculator-class-definition">Calculator Class Definition</a></li>
<li class="level1"><a href="#calculator-class-implementation">Calculator Class Implementation</a></li>
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<li class="level1"><a href="#button-class-implementation">Button Class Implementation</a></li>
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<h1 class="title">Calculator Example</h1>
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<li><a href="widgets-calculator-button-cpp.html">widgets/calculator/button.cpp</a></li>
<li><a href="widgets-calculator-button-h.html">widgets/calculator/button.h</a></li>
<li><a href="widgets-calculator-calculator-cpp.html">widgets/calculator/calculator.cpp</a></li>
<li><a href="widgets-calculator-calculator-h.html">widgets/calculator/calculator.h</a></li>
<li><a href="widgets-calculator-main-cpp.html">widgets/calculator/main.cpp</a></li>
<li><a href="widgets-calculator-calculator-pro.html">widgets/calculator/calculator.pro</a></li>
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<p>The example shows how to use signals and slots to implement the functionality of a calculator widget, and how to use <a href="qgridlayout.html">QGridLayout</a> to place child widgets in a grid.<p class="centerAlign"><img src="images/calculator-example.png" alt="Screenshot of the Calculator example" /></p><p>The example consists of two classes:</p>
<ul>
<li><tt>Calculator</tt> is the calculator widget, with all the calculator functionality.</li>
<li><tt>Button</tt> is the widget used for each of the calculator button. It derives from <a href="qtoolbutton.html">QToolButton</a>.</li>
</ul>
<p>We will start by reviewing <tt>Calculator</tt>, then we will take a look at <tt>Button</tt>.</p>
<a name="calculator-class-definition"></a>
<h2>Calculator Class Definition</h2>
<pre class="cpp"> <span class="keyword">class</span> Calculator : <span class="keyword">public</span> <span class="type"><a href="qwidget.html">QWidget</a></span>
{
Q_OBJECT
<span class="keyword">public</span>:
Calculator(<span class="type"><a href="qwidget.html">QWidget</a></span> <span class="operator">*</span>parent <span class="operator">=</span> <span class="number">0</span>);
<span class="keyword">private</span> <span class="keyword">slots</span>:
<span class="type">void</span> digitClicked();
<span class="type">void</span> unaryOperatorClicked();
<span class="type">void</span> additiveOperatorClicked();
<span class="type">void</span> multiplicativeOperatorClicked();
<span class="type">void</span> equalClicked();
<span class="type">void</span> pointClicked();
<span class="type">void</span> changeSignClicked();
<span class="type">void</span> backspaceClicked();
<span class="type">void</span> clear();
<span class="type">void</span> clearAll();
<span class="type">void</span> clearMemory();
<span class="type">void</span> readMemory();
<span class="type">void</span> setMemory();
<span class="type">void</span> addToMemory();</pre>
<p>The <tt>Calculator</tt> class provides a simple calculator widget. It inherits from <a href="qdialog.html">QDialog</a> and has several private slots associated with the calculator's buttons. <a href="qobject.html#eventFilter">QObject::eventFilter</a>() is reimplemented to handle mouse events on the calculator's display.</p>
<p>Buttons are grouped in categories according to their behavior. For example, all the digit buttons (labeled <b>0</b> to <b>9</b>) append a digit to the current operand. For these, we connect multiple buttons to the same slot (e.g., <tt>digitClicked()</tt>). The categories are digits, unary operators (<b>Sqrt</b>, <b>x²</b>, <b>1/x</b>), additive operators (<b>+</b>, <b>-</b>), and multiplicative operators (<b>×</b>, <b>÷</b>). The other buttons have their own slots.</p>
<pre class="cpp"> <span class="keyword">private</span>:
Button <span class="operator">*</span>createButton(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>text<span class="operator">,</span> <span class="keyword">const</span> <span class="type">char</span> <span class="operator">*</span>member);
<span class="type">void</span> abortOperation();
<span class="type">bool</span> calculate(<span class="type">double</span> rightOperand<span class="operator">,</span> <span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>pendingOperator);</pre>
<p>The private <tt>createButton()</tt> function is used as part of the widget construction. <tt>abortOperation()</tt> is called whenever a division by zero occurs or when a square root operation is applied to a negative number. <tt>calculate()</tt> applies a binary operator (<b>+</b>, <b>-</b>, <b>×</b>, or <b>÷</b>).</p>
<pre class="cpp"> <span class="type">double</span> sumInMemory;
<span class="type">double</span> sumSoFar;
<span class="type">double</span> factorSoFar;
<span class="type"><a href="qstring.html">QString</a></span> pendingAdditiveOperator;
<span class="type"><a href="qstring.html">QString</a></span> pendingMultiplicativeOperator;
<span class="type">bool</span> waitingForOperand;</pre>
<p>These variables, together with the contents of the calculator display (a <a href="qlineedit.html">QLineEdit</a>), encode the state of the calculator:</p>
<ul>
<li><tt>sumInMemory</tt> contains the value stored in the calculator's memory (using <b>MS</b>, <b>M+</b>, or <b>MC</b>).</li>
<li><tt>sumSoFar</tt> stores the value accumulated so far. When the user clicks <b>=</b>, <tt>sumSoFar</tt> is recomputed and shown on the display. <b>Clear All</b> resets <tt>sumSoFar</tt> to zero.</li>
<li><tt>factorSoFar</tt> stores a temporary value when doing multiplications and divisions.</li>
<li><tt>pendingAdditiveOperator</tt> stores the last additive operator clicked by the user.</li>
<li><tt>pendingMultiplicativeOperator</tt> stores the last multiplicative operator clicked by the user.</li>
<li><tt>waitingForOperand</tt> is <tt>true</tt> when the calculator is expecting the user to start typing an operand.</li>
</ul>
<p>Additive and multiplicative operators are treated differently because they have different precedences. For example, <b>1 + 2 ÷ 3</b> is interpreted as <b>1 + (2 ÷ 3)</b> because <b>÷</b> has higher precedence than <b>+</b>.</p>
<p>The table below shows the evolution of the calculator state as the user enters a mathematical expression.</p>
<table class="generic">
<thead><tr class="qt-style"><th >User Input</th><th >Display</th><th >Sum so Far</th><th >Add. Op.</th><th >Factor so Far</th><th >Mult. Op.</th><th >Waiting for Operand?</th></tr></thead>
<tr valign="top" class="odd"><td ></td><td >0</td><td >0</td><td ></td><td ></td><td ></td><td ><tt>true</tt></td></tr>
<tr valign="top" class="even"><td ><b>1</b></td><td >1</td><td >0</td><td ></td><td ></td><td ></td><td ><tt>false</tt></td></tr>
<tr valign="top" class="odd"><td ><b>1 +</b></td><td >1</td><td >1</td><td ><b>+</b></td><td ></td><td ></td><td ><tt>true</tt></td></tr>
<tr valign="top" class="even"><td ><b>1 + 2</b></td><td >2</td><td >1</td><td ><b>+</b></td><td ></td><td ></td><td ><tt>false</tt></td></tr>
<tr valign="top" class="odd"><td ><b>1 + 2 ÷</b></td><td >2</td><td >1</td><td ><b>+</b></td><td >2</td><td ><b>÷</b></td><td ><tt>true</tt></td></tr>
<tr valign="top" class="even"><td ><b>1 + 2 ÷ 3</b></td><td >3</td><td >1</td><td ><b>+</b></td><td >2</td><td ><b>÷</b></td><td ><tt>false</tt></td></tr>
<tr valign="top" class="odd"><td ><b>1 + 2 ÷ 3 -</b></td><td >1.66667</td><td >1.66667</td><td ><b>-</b></td><td ></td><td ></td><td ><tt>true</tt></td></tr>
<tr valign="top" class="even"><td ><b>1 + 2 ÷ 3 - 4</b></td><td >4</td><td >1.66667</td><td ><b>-</b></td><td ></td><td ></td><td ><tt>false</tt></td></tr>
<tr valign="top" class="odd"><td ><b>1 + 2 ÷ 3 - 4 =</b></td><td >-2.33333</td><td >0</td><td ></td><td ></td><td ></td><td ><tt>true</tt></td></tr>
</table>
<p>Unary operators, such as <b>Sqrt</b>, require no special handling; they can be applied immediately since the operand is already known when the operator button is clicked.</p>
<pre class="cpp"> <span class="type"><a href="qlineedit.html">QLineEdit</a></span> <span class="operator">*</span>display;
<span class="keyword">enum</span> { NumDigitButtons <span class="operator">=</span> <span class="number">10</span> };
Button <span class="operator">*</span>digitButtons<span class="operator">[</span>NumDigitButtons<span class="operator">]</span>;
};</pre>
<p>Finally, we declare the variables associated with the display and the buttons used to display numerals.</p>
<a name="calculator-class-implementation"></a>
<h2>Calculator Class Implementation</h2>
<pre class="cpp"> Calculator<span class="operator">::</span>Calculator(<span class="type"><a href="qwidget.html">QWidget</a></span> <span class="operator">*</span>parent)
: <span class="type"><a href="qwidget.html">QWidget</a></span>(parent)
{
sumInMemory <span class="operator">=</span> <span class="number">0.0</span>;
sumSoFar <span class="operator">=</span> <span class="number">0.0</span>;
factorSoFar <span class="operator">=</span> <span class="number">0.0</span>;
waitingForOperand <span class="operator">=</span> <span class="keyword">true</span>;</pre>
<p>In the constructor, we initialize the calculator's state. The <tt>pendingAdditiveOperator</tt> and <tt>pendingMultiplicativeOperator</tt> variables don't need to be initialized explicitly, because the <a href="qstring.html">QString</a> constructor initializes them to empty strings.</p>
<pre class="cpp"> display <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qlineedit.html">QLineEdit</a></span>(<span class="string">"0"</span>);
display<span class="operator">-</span><span class="operator">></span>setReadOnly(<span class="keyword">true</span>);
display<span class="operator">-</span><span class="operator">></span>setAlignment(<span class="type"><a href="qt.html">Qt</a></span><span class="operator">::</span>AlignRight);
display<span class="operator">-</span><span class="operator">></span>setMaxLength(<span class="number">15</span>);
<span class="type"><a href="qfont.html">QFont</a></span> font <span class="operator">=</span> display<span class="operator">-</span><span class="operator">></span>font();
font<span class="operator">.</span>setPointSize(font<span class="operator">.</span>pointSize() <span class="operator">+</span> <span class="number">8</span>);
display<span class="operator">-</span><span class="operator">></span>setFont(font);</pre>
<p>We create the <a href="qlineedit.html">QLineEdit</a> representing the calculator's display and set up some of its properties. In particular, we set it to be read-only.</p>
<p>We also enlarge <tt>display</tt>'s font by 8 points.</p>
<pre class="cpp"> <span class="keyword">for</span> (<span class="type">int</span> i <span class="operator">=</span> <span class="number">0</span>; i <span class="operator"><</span> NumDigitButtons; <span class="operator">+</span><span class="operator">+</span>i) {
digitButtons<span class="operator">[</span>i<span class="operator">]</span> <span class="operator">=</span> createButton(<span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>number(i)<span class="operator">,</span> SLOT(digitClicked()));
}
Button <span class="operator">*</span>pointButton <span class="operator">=</span> createButton(tr(<span class="string">"."</span>)<span class="operator">,</span> SLOT(pointClicked()));
Button <span class="operator">*</span>changeSignButton <span class="operator">=</span> createButton(tr(<span class="string">"\261"</span>)<span class="operator">,</span> SLOT(changeSignClicked()));
Button <span class="operator">*</span>backspaceButton <span class="operator">=</span> createButton(tr(<span class="string">"Backspace"</span>)<span class="operator">,</span> SLOT(backspaceClicked()));
Button <span class="operator">*</span>clearButton <span class="operator">=</span> createButton(tr(<span class="string">"Clear"</span>)<span class="operator">,</span> SLOT(clear()));
Button <span class="operator">*</span>clearAllButton <span class="operator">=</span> createButton(tr(<span class="string">"Clear All"</span>)<span class="operator">,</span> SLOT(clearAll()));
Button <span class="operator">*</span>clearMemoryButton <span class="operator">=</span> createButton(tr(<span class="string">"MC"</span>)<span class="operator">,</span> SLOT(clearMemory()));
Button <span class="operator">*</span>readMemoryButton <span class="operator">=</span> createButton(tr(<span class="string">"MR"</span>)<span class="operator">,</span> SLOT(readMemory()));
Button <span class="operator">*</span>setMemoryButton <span class="operator">=</span> createButton(tr(<span class="string">"MS"</span>)<span class="operator">,</span> SLOT(setMemory()));
Button <span class="operator">*</span>addToMemoryButton <span class="operator">=</span> createButton(tr(<span class="string">"M+"</span>)<span class="operator">,</span> SLOT(addToMemory()));
Button <span class="operator">*</span>divisionButton <span class="operator">=</span> createButton(tr(<span class="string">"\367"</span>)<span class="operator">,</span> SLOT(multiplicativeOperatorClicked()));
Button <span class="operator">*</span>timesButton <span class="operator">=</span> createButton(tr(<span class="string">"\327"</span>)<span class="operator">,</span> SLOT(multiplicativeOperatorClicked()));
Button <span class="operator">*</span>minusButton <span class="operator">=</span> createButton(tr(<span class="string">"-"</span>)<span class="operator">,</span> SLOT(additiveOperatorClicked()));
Button <span class="operator">*</span>plusButton <span class="operator">=</span> createButton(tr(<span class="string">"+"</span>)<span class="operator">,</span> SLOT(additiveOperatorClicked()));
Button <span class="operator">*</span>squareRootButton <span class="operator">=</span> createButton(tr(<span class="string">"Sqrt"</span>)<span class="operator">,</span> SLOT(unaryOperatorClicked()));
Button <span class="operator">*</span>powerButton <span class="operator">=</span> createButton(tr(<span class="string">"x\262"</span>)<span class="operator">,</span> SLOT(unaryOperatorClicked()));
Button <span class="operator">*</span>reciprocalButton <span class="operator">=</span> createButton(tr(<span class="string">"1/x"</span>)<span class="operator">,</span> SLOT(unaryOperatorClicked()));
Button <span class="operator">*</span>equalButton <span class="operator">=</span> createButton(tr(<span class="string">"="</span>)<span class="operator">,</span> SLOT(equalClicked()));</pre>
<p>For each button, we call the private <tt>createButton()</tt> function with the proper text label and a slot to connect to the button.</p>
<pre class="cpp"> <span class="type"><a href="qgridlayout.html">QGridLayout</a></span> <span class="operator">*</span>mainLayout <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qgridlayout.html">QGridLayout</a></span>;
<span class="preprocessor">#if defined(Q_OS_SYMBIAN) || defined(Q_WS_MAEMO_5) || defined(Q_WS_SIMULATOR)</span>
mainLayout<span class="operator">-</span><span class="operator">></span>setSizeConstraint(<span class="type"><a href="qlayout.html">QLayout</a></span><span class="operator">::</span>SetNoConstraint);
<span class="preprocessor">#else</span>
mainLayout<span class="operator">-</span><span class="operator">></span>setSizeConstraint(<span class="type"><a href="qlayout.html">QLayout</a></span><span class="operator">::</span>SetFixedSize);
<span class="preprocessor">#endif</span>
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(display<span class="operator">,</span> <span class="number">0</span><span class="operator">,</span> <span class="number">0</span><span class="operator">,</span> <span class="number">1</span><span class="operator">,</span> <span class="number">6</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(backspaceButton<span class="operator">,</span> <span class="number">1</span><span class="operator">,</span> <span class="number">0</span><span class="operator">,</span> <span class="number">1</span><span class="operator">,</span> <span class="number">2</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(clearButton<span class="operator">,</span> <span class="number">1</span><span class="operator">,</span> <span class="number">2</span><span class="operator">,</span> <span class="number">1</span><span class="operator">,</span> <span class="number">2</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(clearAllButton<span class="operator">,</span> <span class="number">1</span><span class="operator">,</span> <span class="number">4</span><span class="operator">,</span> <span class="number">1</span><span class="operator">,</span> <span class="number">2</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(clearMemoryButton<span class="operator">,</span> <span class="number">2</span><span class="operator">,</span> <span class="number">0</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(readMemoryButton<span class="operator">,</span> <span class="number">3</span><span class="operator">,</span> <span class="number">0</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(setMemoryButton<span class="operator">,</span> <span class="number">4</span><span class="operator">,</span> <span class="number">0</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(addToMemoryButton<span class="operator">,</span> <span class="number">5</span><span class="operator">,</span> <span class="number">0</span>);
<span class="keyword">for</span> (<span class="type">int</span> i <span class="operator">=</span> <span class="number">1</span>; i <span class="operator"><</span> NumDigitButtons; <span class="operator">+</span><span class="operator">+</span>i) {
<span class="type">int</span> row <span class="operator">=</span> ((<span class="number">9</span> <span class="operator">-</span> i) <span class="operator">/</span> <span class="number">3</span>) <span class="operator">+</span> <span class="number">2</span>;
<span class="type">int</span> column <span class="operator">=</span> ((i <span class="operator">-</span> <span class="number">1</span>) <span class="operator">%</span> <span class="number">3</span>) <span class="operator">+</span> <span class="number">1</span>;
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(digitButtons<span class="operator">[</span>i<span class="operator">]</span><span class="operator">,</span> row<span class="operator">,</span> column);
}
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(digitButtons<span class="operator">[</span><span class="number">0</span><span class="operator">]</span><span class="operator">,</span> <span class="number">5</span><span class="operator">,</span> <span class="number">1</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(pointButton<span class="operator">,</span> <span class="number">5</span><span class="operator">,</span> <span class="number">2</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(changeSignButton<span class="operator">,</span> <span class="number">5</span><span class="operator">,</span> <span class="number">3</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(divisionButton<span class="operator">,</span> <span class="number">2</span><span class="operator">,</span> <span class="number">4</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(timesButton<span class="operator">,</span> <span class="number">3</span><span class="operator">,</span> <span class="number">4</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(minusButton<span class="operator">,</span> <span class="number">4</span><span class="operator">,</span> <span class="number">4</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(plusButton<span class="operator">,</span> <span class="number">5</span><span class="operator">,</span> <span class="number">4</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(squareRootButton<span class="operator">,</span> <span class="number">2</span><span class="operator">,</span> <span class="number">5</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(powerButton<span class="operator">,</span> <span class="number">3</span><span class="operator">,</span> <span class="number">5</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(reciprocalButton<span class="operator">,</span> <span class="number">4</span><span class="operator">,</span> <span class="number">5</span>);
mainLayout<span class="operator">-</span><span class="operator">></span>addWidget(equalButton<span class="operator">,</span> <span class="number">5</span><span class="operator">,</span> <span class="number">5</span>);
setLayout(mainLayout);
setWindowTitle(tr(<span class="string">"Calculator"</span>));
}</pre>
<p>The layout is handled by a single <a href="qgridlayout.html">QGridLayout</a>. The <a href="qlayout.html#sizeConstraint-prop">QLayout::setSizeConstraint</a>() call ensures that the <tt>Calculator</tt> widget is always shown as its optimal size (its <a href="qwidget.html#sizeHint-prop">size hint</a>), preventing the user from resizing the calculator. The size hint is determined by the size and <a href="qwidget.html#sizePolicy-prop">size policy</a> of the child widgets.</p>
<p>Most child widgets occupy only one cell in the grid layout. For these, we only need to pass a row and a column to <a href="qgridlayout.html#addWidget">QGridLayout::addWidget</a>(). The <tt>display</tt>, <tt>backspaceButton</tt>, <tt>clearButton</tt>, and <tt>clearAllButton</tt> widgets occupy more than one column; for these we must also pass a row span and a column span.</p>
<pre class="cpp"> <span class="type">void</span> Calculator<span class="operator">::</span>digitClicked()
{
Button <span class="operator">*</span>clickedButton <span class="operator">=</span> qobject_cast<span class="operator"><</span>Button <span class="operator">*</span><span class="operator">></span>(sender());
<span class="type">int</span> digitValue <span class="operator">=</span> clickedButton<span class="operator">-</span><span class="operator">></span>text()<span class="operator">.</span>toInt();
<span class="keyword">if</span> (display<span class="operator">-</span><span class="operator">></span>text() <span class="operator">=</span><span class="operator">=</span> <span class="string">"0"</span> <span class="operator">&</span><span class="operator">&</span> digitValue <span class="operator">=</span><span class="operator">=</span> <span class="number">0.0</span>)
<span class="keyword">return</span>;
<span class="keyword">if</span> (waitingForOperand) {
display<span class="operator">-</span><span class="operator">></span>clear();
waitingForOperand <span class="operator">=</span> <span class="keyword">false</span>;
}
display<span class="operator">-</span><span class="operator">></span>setText(display<span class="operator">-</span><span class="operator">></span>text() <span class="operator">+</span> <span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>number(digitValue));
}</pre>
<p>Pressing one of the calculator's digit buttons will emit the button's <a href="qabstractbutton.html#clicked">clicked()</a> signal, which will trigger the <tt>digitClicked()</tt> slot.</p>
<p>First, we find out which button sent the signal using <a href="qobject.html#sender">QObject::sender</a>(). This function returns the sender as a <a href="qobject.html">QObject</a> pointer. Since we know that the sender is a <tt>Button</tt> object, we can safely cast the <a href="qobject.html">QObject</a>. We could have used a C-style cast or a C++ <tt>static_cast<>()</tt>, but as a defensive programming technique we use a <a href="qobject.html#qobject_cast">qobject_cast</a>(). 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 <a href="qabstractbutton.html#text-prop">QToolButton::text</a>().</p>
<p>The slot needs to consider two situations in particular. If <tt>display</tt> contains "0" and the user clicks the <b>0</b> 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.</p>
<p>At the end, we append the new digit to the value in the display.</p>
<pre class="cpp"> <span class="type">void</span> Calculator<span class="operator">::</span>unaryOperatorClicked()
{
Button <span class="operator">*</span>clickedButton <span class="operator">=</span> qobject_cast<span class="operator"><</span>Button <span class="operator">*</span><span class="operator">></span>(sender());
<span class="type"><a href="qstring.html">QString</a></span> clickedOperator <span class="operator">=</span> clickedButton<span class="operator">-</span><span class="operator">></span>text();
<span class="type">double</span> operand <span class="operator">=</span> display<span class="operator">-</span><span class="operator">></span>text()<span class="operator">.</span>toDouble();
<span class="type">double</span> result <span class="operator">=</span> <span class="number">0.0</span>;
<span class="keyword">if</span> (clickedOperator <span class="operator">=</span><span class="operator">=</span> tr(<span class="string">"Sqrt"</span>)) {
<span class="keyword">if</span> (operand <span class="operator"><</span> <span class="number">0.0</span>) {
abortOperation();
<span class="keyword">return</span>;
}
result <span class="operator">=</span> sqrt(operand);
} <span class="keyword">else</span> <span class="keyword">if</span> (clickedOperator <span class="operator">=</span><span class="operator">=</span> tr(<span class="string">"x\262"</span>)) {
result <span class="operator">=</span> pow(operand<span class="operator">,</span> <span class="number">2.0</span>);
} <span class="keyword">else</span> <span class="keyword">if</span> (clickedOperator <span class="operator">=</span><span class="operator">=</span> tr(<span class="string">"1/x"</span>)) {
<span class="keyword">if</span> (operand <span class="operator">=</span><span class="operator">=</span> <span class="number">0.0</span>) {
abortOperation();
<span class="keyword">return</span>;
}
result <span class="operator">=</span> <span class="number">1.0</span> <span class="operator">/</span> operand;
}
display<span class="operator">-</span><span class="operator">></span>setText(<span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>number(result));
waitingForOperand <span class="operator">=</span> <span class="keyword">true</span>;
}</pre>
<p>The <tt>unaryOperatorClicked()</tt> slot is called whenever one of the unary operator buttons is clicked. Again a pointer to the clicked button is retrieved using <a href="qobject.html#sender">QObject::sender</a>(). The operator is extracted from the button's text and stored in <tt>clickedOperator</tt>. The operand is obtained from <tt>display</tt>.</p>
<p>Then we perform the operation. If <b>Sqrt</b> is applied to a negative number or <b>1/x</b> to zero, we call <tt>abortOperation()</tt>. If everything goes well, we display the result of the operation in the line edit and we set <tt>waitingForOperand</tt> to <tt>true</tt>. 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.</p>
<pre class="cpp"> <span class="type">void</span> Calculator<span class="operator">::</span>additiveOperatorClicked()
{
Button <span class="operator">*</span>clickedButton <span class="operator">=</span> qobject_cast<span class="operator"><</span>Button <span class="operator">*</span><span class="operator">></span>(sender());
<span class="type"><a href="qstring.html">QString</a></span> clickedOperator <span class="operator">=</span> clickedButton<span class="operator">-</span><span class="operator">></span>text();
<span class="type">double</span> operand <span class="operator">=</span> display<span class="operator">-</span><span class="operator">></span>text()<span class="operator">.</span>toDouble();</pre>
<p>The <tt>additiveOperatorClicked()</tt> slot is called when the user clicks the <b>+</b> or <b>-</b> button.</p>
<p>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:</p>
<pre class="cpp"> <span class="keyword">if</span> (<span class="operator">!</span>pendingMultiplicativeOperator<span class="operator">.</span>isEmpty()) {
<span class="keyword">if</span> (<span class="operator">!</span>calculate(operand<span class="operator">,</span> pendingMultiplicativeOperator)) {
abortOperation();
<span class="keyword">return</span>;
}
display<span class="operator">-</span><span class="operator">></span>setText(<span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>number(factorSoFar));
operand <span class="operator">=</span> factorSoFar;
factorSoFar <span class="operator">=</span> <span class="number">0.0</span>;
pendingMultiplicativeOperator<span class="operator">.</span>clear();
}</pre>
<p>If <b>×</b> or <b>÷</b> has been clicked earlier, without clicking <b>=</b> afterward, the current value in the display is the right operand of the <b>×</b> or <b>÷</b> operator and we can finally perform the operation and update the display.</p>
<pre class="cpp"> <span class="keyword">if</span> (<span class="operator">!</span>pendingAdditiveOperator<span class="operator">.</span>isEmpty()) {
<span class="keyword">if</span> (<span class="operator">!</span>calculate(operand<span class="operator">,</span> pendingAdditiveOperator)) {
abortOperation();
<span class="keyword">return</span>;
}
display<span class="operator">-</span><span class="operator">></span>setText(<span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>number(sumSoFar));
} <span class="keyword">else</span> {
sumSoFar <span class="operator">=</span> operand;
}</pre>
<p>If <b>+</b> or <b>-</b> has been clicked earlier, <tt>sumSoFar</tt> 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, <tt>sumSoFar</tt> is simply set to be the text in the display.</p>
<pre class="cpp"> pendingAdditiveOperator <span class="operator">=</span> clickedOperator;
waitingForOperand <span class="operator">=</span> <span class="keyword">true</span>;
}</pre>
<p>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 <tt>pendingAdditiveOperator</tt> variable. We will apply the operation later, when we have a right operand, with <tt>sumSoFar</tt> as the left operand.</p>
<pre class="cpp"> <span class="type">void</span> Calculator<span class="operator">::</span>multiplicativeOperatorClicked()
{
Button <span class="operator">*</span>clickedButton <span class="operator">=</span> qobject_cast<span class="operator"><</span>Button <span class="operator">*</span><span class="operator">></span>(sender());
<span class="type"><a href="qstring.html">QString</a></span> clickedOperator <span class="operator">=</span> clickedButton<span class="operator">-</span><span class="operator">></span>text();
<span class="type">double</span> operand <span class="operator">=</span> display<span class="operator">-</span><span class="operator">></span>text()<span class="operator">.</span>toDouble();
<span class="keyword">if</span> (<span class="operator">!</span>pendingMultiplicativeOperator<span class="operator">.</span>isEmpty()) {
<span class="keyword">if</span> (<span class="operator">!</span>calculate(operand<span class="operator">,</span> pendingMultiplicativeOperator)) {
abortOperation();
<span class="keyword">return</span>;
}
display<span class="operator">-</span><span class="operator">></span>setText(<span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>number(factorSoFar));
} <span class="keyword">else</span> {
factorSoFar <span class="operator">=</span> operand;
}
pendingMultiplicativeOperator <span class="operator">=</span> clickedOperator;
waitingForOperand <span class="operator">=</span> <span class="keyword">true</span>;
}</pre>
<p>The <tt>multiplicativeOperatorClicked()</tt> slot is similar to <tt>additiveOperatorClicked()</tt>. We don't need to worry about pending additive operators here, because multiplicative operators have precedence over additive operators.</p>
<pre class="cpp"> <span class="type">void</span> Calculator<span class="operator">::</span>equalClicked()
{
<span class="type">double</span> operand <span class="operator">=</span> display<span class="operator">-</span><span class="operator">></span>text()<span class="operator">.</span>toDouble();
<span class="keyword">if</span> (<span class="operator">!</span>pendingMultiplicativeOperator<span class="operator">.</span>isEmpty()) {
<span class="keyword">if</span> (<span class="operator">!</span>calculate(operand<span class="operator">,</span> pendingMultiplicativeOperator)) {
abortOperation();
<span class="keyword">return</span>;
}
operand <span class="operator">=</span> factorSoFar;
factorSoFar <span class="operator">=</span> <span class="number">0.0</span>;
pendingMultiplicativeOperator<span class="operator">.</span>clear();
}
<span class="keyword">if</span> (<span class="operator">!</span>pendingAdditiveOperator<span class="operator">.</span>isEmpty()) {
<span class="keyword">if</span> (<span class="operator">!</span>calculate(operand<span class="operator">,</span> pendingAdditiveOperator)) {
abortOperation();
<span class="keyword">return</span>;
}
pendingAdditiveOperator<span class="operator">.</span>clear();
} <span class="keyword">else</span> {
sumSoFar <span class="operator">=</span> operand;
}
display<span class="operator">-</span><span class="operator">></span>setText(<span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>number(sumSoFar));
sumSoFar <span class="operator">=</span> <span class="number">0.0</span>;
waitingForOperand <span class="operator">=</span> <span class="keyword">true</span>;
}</pre>
<p>Like in <tt>additiveOperatorClicked()</tt>, we start by handing any pending multiplicative and additive operators. Then we display <tt>sumSoFar</tt> and reset the variable to zero. Resetting the variable to zero is necessary to avoid counting the value twice.</p>
<pre class="cpp"> <span class="type">void</span> Calculator<span class="operator">::</span>pointClicked()
{
<span class="keyword">if</span> (waitingForOperand)
display<span class="operator">-</span><span class="operator">></span>setText(<span class="string">"0"</span>);
<span class="keyword">if</span> (<span class="operator">!</span>display<span class="operator">-</span><span class="operator">></span>text()<span class="operator">.</span>contains(<span class="string">"."</span>))
display<span class="operator">-</span><span class="operator">></span>setText(display<span class="operator">-</span><span class="operator">></span>text() <span class="operator">+</span> tr(<span class="string">"."</span>));
waitingForOperand <span class="operator">=</span> <span class="keyword">false</span>;
}</pre>
<p>The <tt>pointClicked()</tt> slot adds a decimal point to the content in <tt>display</tt>.</p>
<pre class="cpp"> <span class="type">void</span> Calculator<span class="operator">::</span>changeSignClicked()
{
<span class="type"><a href="qstring.html">QString</a></span> text <span class="operator">=</span> display<span class="operator">-</span><span class="operator">></span>text();
<span class="type">double</span> value <span class="operator">=</span> text<span class="operator">.</span>toDouble();
<span class="keyword">if</span> (value <span class="operator">></span> <span class="number">0.0</span>) {
text<span class="operator">.</span>prepend(tr(<span class="string">"-"</span>));
} <span class="keyword">else</span> <span class="keyword">if</span> (value <span class="operator"><</span> <span class="number">0.0</span>) {
text<span class="operator">.</span>remove(<span class="number">0</span><span class="operator">,</span> <span class="number">1</span>);
}
display<span class="operator">-</span><span class="operator">></span>setText(text);
}</pre>
<p>The <tt>changeSignClicked()</tt> slot changes the sign of the value in <tt>display</tt>. 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).</p>
<pre class="cpp"> <span class="type">void</span> Calculator<span class="operator">::</span>backspaceClicked()
{
<span class="keyword">if</span> (waitingForOperand)
<span class="keyword">return</span>;
<span class="type"><a href="qstring.html">QString</a></span> text <span class="operator">=</span> display<span class="operator">-</span><span class="operator">></span>text();
text<span class="operator">.</span>chop(<span class="number">1</span>);
<span class="keyword">if</span> (text<span class="operator">.</span>isEmpty()) {
text <span class="operator">=</span> <span class="string">"0"</span>;
waitingForOperand <span class="operator">=</span> <span class="keyword">true</span>;
}
display<span class="operator">-</span><span class="operator">></span>setText(text);
}</pre>
<p>The <tt>backspaceClicked()</tt> removes the rightmost character in the display. If we get an empty string, we show "0" and set <tt>waitingForOperand</tt> to <tt>true</tt>.</p>
<pre class="cpp"> <span class="type">void</span> Calculator<span class="operator">::</span>clear()
{
<span class="keyword">if</span> (waitingForOperand)
<span class="keyword">return</span>;
display<span class="operator">-</span><span class="operator">></span>setText(<span class="string">"0"</span>);
waitingForOperand <span class="operator">=</span> <span class="keyword">true</span>;
}</pre>
<p>The <tt>clear()</tt> slot resets the current operand to zero. It is equivalent to clicking <b>Backspace</b> enough times to erase the entire operand.</p>
<pre class="cpp"> <span class="type">void</span> Calculator<span class="operator">::</span>clearAll()
{
sumSoFar <span class="operator">=</span> <span class="number">0.0</span>;
factorSoFar <span class="operator">=</span> <span class="number">0.0</span>;
pendingAdditiveOperator<span class="operator">.</span>clear();
pendingMultiplicativeOperator<span class="operator">.</span>clear();
display<span class="operator">-</span><span class="operator">></span>setText(<span class="string">"0"</span>);
waitingForOperand <span class="operator">=</span> <span class="keyword">true</span>;
}</pre>
<p>The <tt>clearAll()</tt> slot resets the calculator to its initial state.</p>
<pre class="cpp"> <span class="type">void</span> Calculator<span class="operator">::</span>clearMemory()
{
sumInMemory <span class="operator">=</span> <span class="number">0.0</span>;
}
<span class="type">void</span> Calculator<span class="operator">::</span>readMemory()
{
display<span class="operator">-</span><span class="operator">></span>setText(<span class="type"><a href="qstring.html">QString</a></span><span class="operator">::</span>number(sumInMemory));
waitingForOperand <span class="operator">=</span> <span class="keyword">true</span>;
}
<span class="type">void</span> Calculator<span class="operator">::</span>setMemory()
{
equalClicked();
sumInMemory <span class="operator">=</span> display<span class="operator">-</span><span class="operator">></span>text()<span class="operator">.</span>toDouble();
}
<span class="type">void</span> Calculator<span class="operator">::</span>addToMemory()
{
equalClicked();
sumInMemory <span class="operator">+</span><span class="operator">=</span> display<span class="operator">-</span><span class="operator">></span>text()<span class="operator">.</span>toDouble();
}</pre>
<p>The <tt>clearMemory()</tt> slot erases the sum kept in memory, <tt>readMemory()</tt> displays the sum as an operand, <tt>setMemory()</tt> replace the sum in memory with the current sum, and <tt>addToMemory()</tt> adds the current value to the value in memory. For <tt>setMemory()</tt> and <tt>addToMemory()</tt>, we start by calling <tt>equalClicked()</tt> to update <tt>sumSoFar</tt> and the value in the display.</p>
<pre class="cpp"> Button <span class="operator">*</span>Calculator<span class="operator">::</span>createButton(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>text<span class="operator">,</span> <span class="keyword">const</span> <span class="type">char</span> <span class="operator">*</span>member)
{
Button <span class="operator">*</span>button <span class="operator">=</span> <span class="keyword">new</span> Button(text);
connect(button<span class="operator">,</span> SIGNAL(clicked())<span class="operator">,</span> <span class="keyword">this</span><span class="operator">,</span> member);
<span class="keyword">return</span> button;
}</pre>
<p>The private <tt>createButton()</tt> function is called from the constructor to create calculator buttons.</p>
<pre class="cpp"> <span class="type">void</span> Calculator<span class="operator">::</span>abortOperation()
{
clearAll();
display<span class="operator">-</span><span class="operator">></span>setText(tr(<span class="string">"####"</span>));
}</pre>
<p>The private <tt>abortOperation()</tt> function is called whenever a calculation fails. It resets the calculator state and displays "####".</p>
<pre class="cpp"> <span class="type">bool</span> Calculator<span class="operator">::</span>calculate(<span class="type">double</span> rightOperand<span class="operator">,</span> <span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>pendingOperator)
{
<span class="keyword">if</span> (pendingOperator <span class="operator">=</span><span class="operator">=</span> tr(<span class="string">"+"</span>)) {
sumSoFar <span class="operator">+</span><span class="operator">=</span> rightOperand;
} <span class="keyword">else</span> <span class="keyword">if</span> (pendingOperator <span class="operator">=</span><span class="operator">=</span> tr(<span class="string">"-"</span>)) {
sumSoFar <span class="operator">-</span><span class="operator">=</span> rightOperand;
} <span class="keyword">else</span> <span class="keyword">if</span> (pendingOperator <span class="operator">=</span><span class="operator">=</span> tr(<span class="string">"\327"</span>)) {
factorSoFar <span class="operator">*</span><span class="operator">=</span> rightOperand;
} <span class="keyword">else</span> <span class="keyword">if</span> (pendingOperator <span class="operator">=</span><span class="operator">=</span> tr(<span class="string">"\367"</span>)) {
<span class="keyword">if</span> (rightOperand <span class="operator">=</span><span class="operator">=</span> <span class="number">0.0</span>)
<span class="keyword">return</span> <span class="keyword">false</span>;
factorSoFar <span class="operator">/</span><span class="operator">=</span> rightOperand;
}
<span class="keyword">return</span> <span class="keyword">true</span>;
}</pre>
<p>The private <tt>calculate()</tt> function performs a binary operation. The right operand is given by <tt>rightOperand</tt>. For additive operators, the left operand is <tt>sumSoFar</tt>; for multiplicative operators, the left operand is <tt>factorSoFar</tt>. The function return <tt>false</tt> if a division by zero occurs.</p>
<a name="button-class-definition"></a>
<h2>Button Class Definition</h2>
<p>Let's now take a look at the <tt>Button</tt> class:</p>
<pre class="cpp"> <span class="keyword">class</span> Button : <span class="keyword">public</span> <span class="type"><a href="qtoolbutton.html">QToolButton</a></span>
{
Q_OBJECT
<span class="keyword">public</span>:
Button(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>text<span class="operator">,</span> <span class="type"><a href="qwidget.html">QWidget</a></span> <span class="operator">*</span>parent <span class="operator">=</span> <span class="number">0</span>);
<span class="type"><a href="qsize.html">QSize</a></span> sizeHint() <span class="keyword">const</span>;
};</pre>
<p>The <tt>Button</tt> class has a convenience constructor that takes a text label and a parent widget, and it reimplements <a href="qwidget.html#sizeHint-prop">QWidget::sizeHint</a>() to provide more space around the text than the amount <a href="qtoolbutton.html">QToolButton</a> normally provides.</p>
<a name="button-class-implementation"></a>
<h2>Button Class Implementation</h2>
<pre class="cpp"> Button<span class="operator">::</span>Button(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>text<span class="operator">,</span> <span class="type"><a href="qwidget.html">QWidget</a></span> <span class="operator">*</span>parent)
: <span class="type"><a href="qtoolbutton.html">QToolButton</a></span>(parent)
{
setSizePolicy(<span class="type"><a href="qsizepolicy.html">QSizePolicy</a></span><span class="operator">::</span>Expanding<span class="operator">,</span> <span class="type"><a href="qsizepolicy.html">QSizePolicy</a></span><span class="operator">::</span>Preferred);
setText(text);
}</pre>
<p>The buttons' appearance is determined by the layout of the calculator widget through the size and <a href="qwidget.html#sizePolicy-prop">size policy</a> of the layout's child widgets. The call to the <a href="qwidget.html#sizePolicy-prop">setSizePolicy()</a> function in the constructor ensures that the button will expand horizontally to fill all the available space; by default, <a href="qtoolbutton.html">QToolButton</a>s don't expand to fill available space. Without this call, the different buttons in a same column would have different widths.</p>
<pre class="cpp"> <span class="type"><a href="qsize.html">QSize</a></span> Button<span class="operator">::</span>sizeHint() <span class="keyword">const</span>
{
<span class="type"><a href="qsize.html">QSize</a></span> size <span class="operator">=</span> <span class="type"><a href="qtoolbutton.html">QToolButton</a></span><span class="operator">::</span>sizeHint();
size<span class="operator">.</span>rheight() <span class="operator">+</span><span class="operator">=</span> <span class="number">20</span>;
size<span class="operator">.</span>rwidth() <span class="operator">=</span> <a href="qtglobal.html#qMax">qMax</a>(size<span class="operator">.</span>width()<span class="operator">,</span> size<span class="operator">.</span>height());
<span class="keyword">return</span> size;
}</pre>
<p>In <a href="qwidget.html#sizeHint-prop">sizeHint()</a>, we try to return a size that looks good for most buttons. We reuse the size hint of the base class (<a href="qtoolbutton.html">QToolButton</a>) but modify it in the following ways:</p>
<ul>
<li>We add 20 to the <a href="qsize.html#height">height</a> component of the size hint.</li>
<li>We make the <a href="qsize.html#width">width</a> component of the size hint at least as much as the <a href="qsize.html#width">height</a>.</li>
</ul>
<p>This ensures that with most fonts, the digit and operator buttons will be square, without truncating the text on the <b>Backspace</b>, <b>Clear</b>, and <b>Clear All</b> buttons.</p>
<p>The screenshot below shows how the <tt>Calculator</tt> widget would look like if we <i>didn't</i> set the horizontal size policy to <a href="qsizepolicy.html#Policy-enum">QSizePolicy::Expanding</a> in the constructor and if we didn't reimplement <a href="qwidget.html#sizeHint-prop">QWidget::sizeHint</a>().</p>
<p class="centerAlign"><img src="images/calculator-ugly.png" alt="The Calculator example with default size policies and size hints" /></p></div>
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