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C++ Operator Precedence [C++ Reference]
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<li class="level2"><div class="li"><span class="li"><a href="#c_operator_precedence" class="toc">C++ Operator Precedence</a></span></div></li>
<li class="level2"><div class="li"><span class="li"><a href="#order_of_evaluation_and_of_side_effects" class="toc">Order of Evaluation and of Side Effects</a></span></div></li>
<li class="level2"><div class="li"><span class="li"><a href="#overloading_of_operators" class="toc">Overloading of Operators</a></span></div></li></ul>
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<h2><a name="c_operator_precedence" id="c_operator_precedence">C++ Operator Precedence</a></h2>
<div class="level2">
<p>
The operators at the top of this list are evaluated first. Operators within a group have the same precedence. All operators have left-to-right associativity unless otherwise noted.
</p>
<p>
<br/>
</p>
<table class="inline">
<tr class="row0">
<th class="col0 centeralign"> Precedence </th><th class="col1 centeralign"> Operator </th><th class="col2 centeralign"> Description </th><th class="col3 centeralign"> Example </th><th class="col4 centeralign"> Overloadable </th><th class="col5 centeralign"> Associativity </th>
</tr>
<tr class="row1">
<td class="col0">1</td><td class="col1"><code>::</code></td><td class="col2">Scope resolution operator</td><td class="col3">Class::age = 2;</td><td class="col4">no</td><td class="col5">left to right</td>
</tr>
<tr class="row2">
<td class="col0">2</td><td class="col1"><code>()</code><br/>
<code>()</code><br/>
<code>[]</code><br/>
<code>-></code><br/>
<code>.</code><br/>
<code>++</code><br/>
<code>--</code><br/>
<code>dynamic_cast</code><br/>
<code>static_cast</code><br/>
<code>reinterpret_cast</code><br/>
<code>const_cast</code><br/>
<code>typeid</code></td><td class="col2">Function call<br/>
Member initalization <br/>
Array access<br/>
Member access from a pointer<br/>
Member access from an object<br/>
Post-increment<br/>
Post-decrement<br/>
Runtime-checked type conversion<br/>
Unchecked type conversion<br/>
Reinterpreting type conversion<br/>
Cast away/Add constness<br/>
Get type information</td><td class="col3">isdigit('1')<br/>
c_tor(int x, int y) : _x(x), _y(y * 10) {}<br/>
array[4] = 2;<br/>
ptr->age = 34;<br/>
obj.age = 34;<br/>
for (int i = 0; i < 10; i++) cout << i;<br/>
for (int i = 10; i > 0; i--) cout << i;<br/>
Y& y = dynamic_cast<Y&>(x);<br/>
Y& y = static_cast<Y&>(x);<br/>
int const* p = reinterpret_cast<int const*>(0x1234);<br/>
int* q = const_cast<int*>(p);<br/>
std::type_info const& t = typeid(x);</td><td class="col4">yes<br/>
yes<br/>
yes<br/>
yes<br/>
no<br/>
yes<br/>
yes<br/>
no<br/>
no<br/>
no<br/>
no<br/>
no</td><td class="col5">left to right</td>
</tr>
<tr class="row3">
<td class="col0">3</td><td class="col1"><code>!</code><br/>
<code>not</code><br/>
<code>~</code><br/>
<code>compl</code><br/>
<code>++</code><br/>
<code>--</code><br/>
<code>-</code><br/>
<code>+</code><br/>
<code>*</code><br/>
<code>&</code><br/>
<code>sizeof</code><br/>
<code>new</code><br/>
<code>new []</code><br/>
<code>delete</code><br/>
<code>delete []</code><br/>
<code>(type)</code></td><td class="col2">Logical negation<br/>
Alternate spelling for ! <br/>
Bitwise complement<br/>
Alternate spelling for ~ <br/>
Pre-increment<br/>
Pre-decrement<br/>
Unary minus<br/>
Unary plus<br/>
Dereference<br/>
Address of <br/>
Size (of the type) of the operand in bytes<br/>
Dynamic memory allocation<br/>
Dynamic memory allocation of array<br/>
Deallocating the memory<br/>
Deallocating the memory of array<br/>
Cast to a given type</td><td class="col3">if (!done) …<br/>
<br/>
flags = ~flags;<br/>
<br/>
for (i = 0; i < 10; ++i) cout << i;<br/>
for (i = 10; i > 0; --i) cout << i;<br/>
int i = -1;<br/>
int i = +1;<br/>
int data = *intPtr;<br/>
int *intPtr = &data;<br/>
size_t s = sizeof(int);<br/>
long* pVar = new long; <br/>
long* array = new long[20];<br/>
delete pVar;<br/>
delete [] array;<br/>
int i = (int)floatNum;</td><td class="col4">yes<br/>
<br/>
yes<br/>
<br/>
yes<br/>
yes<br/>
yes<br/>
yes<br/>
yes<br/>
yes <br/>
no<br/>
yes<br/>
yes<br/>
yes<br/>
yes<br/>
yes</td><td class="col5">right to left</td>
</tr>
<tr class="row4">
<td class="col0">4</td><td class="col1"><code>->*</code><br/>
<code>.*</code></td><td class="col2">Member pointer selector<br/>
Member object selector</td><td class="col3">ptr->*var = 24;<br/>
obj.*var = 24;</td><td class="col4">yes<br/>
no</td><td class="col5">left to right</td>
</tr>
<tr class="row5">
<td class="col0">5</td><td class="col1"><code>*</code><br/>
<code>/</code><br/>
<code>%</code></td><td class="col2">Multiplication<br/>
Division<br/>
Modulus</td><td class="col3">int i = 2 * 4;<br/>
float f = 10.0 / 3.0;<br/>
int rem = 4 % 3;</td><td class="col4">yes<br/>
yes<br/>
yes</td><td class="col5">left to right</td>
</tr>
<tr class="row6">
<td class="col0">6</td><td class="col1"><code>+</code><br/>
<code>-</code></td><td class="col2">Addition<br/>
Subtraction</td><td class="col3">int i = 2 + 3;<br/>
int i = 5 - 1;</td><td class="col4">yes<br/>
yes</td><td class="col5">left to right</td>
</tr>
<tr class="row7">
<td class="col0">7</td><td class="col1"><code><<</code><br/>
<code>>></code></td><td class="col2">Bitwise shift left<br/>
Bitwise shift right</td><td class="col3">int flags = 33 << 1;<br/>
int flags = 33 >> 1;</td><td class="col4">yes<br/>
yes</td><td class="col5">left to right</td>
</tr>
<tr class="row8">
<td class="col0">8</td><td class="col1"><code><</code><br/>
<code><=</code><br/>
<code>></code><br/>
<code>>=</code></td><td class="col2">Comparison less-than<br/>
Comparison less-than-or-equal-to<br/>
Comparison greater-than<br/>
Comparison greater-than-or-equal-to</td><td class="col3">if (i < 42) …<br/>
if (i <= 42) ...<br/>
if (i > 42) …<br/>
if (i >= 42) ...</td><td class="col4">yes<br/>
yes<br/>
yes<br/>
yes</td><td class="col5">left to right</td>
</tr>
<tr class="row9">
<td class="col0">9</td><td class="col1"><code>==</code><br/>
<code>eq</code><br/>
<code>!=</code><br/>
<code>not_eq</code></td><td class="col2">Comparison equal-to<br/>
Alternate spelling for == <br/>
Comparison not-equal-to<br/>
Alternate spelling for != </td><td class="col3">if (i == 42) ...<br/>
<br/>
if (i != 42) …<br/>
</td><td class="col4">yes<br/>
-<br/>
yes<br/>
</td><td class="col5">left to right</td>
</tr>
<tr class="row10">
<td class="col0">10</td><td class="col1"><code>&</code><br/>
<code>bitand</code></td><td class="col2">Bitwise AND<br/>
Alternate spelling for & </td><td class="col3">flags = flags & 42;<br/>
</td><td class="col4">yes<br/>
</td><td class="col5">left to right</td>
</tr>
<tr class="row11">
<td class="col0">11</td><td class="col1"><code>^</code><br/>
<code>xor</code></td><td class="col2">Bitwise exclusive OR (XOR)<br/>
Alternate spelling for ^ </td><td class="col3">flags = flags ^ 42;<br/>
</td><td class="col4">yes<br/>
</td><td class="col5">left to right</td>
</tr>
<tr class="row12">
<td class="col0">12</td><td class="col1"><code>|</code><br/>
<code>bitor</code></td><td class="col2">Bitwise inclusive (normal) OR<br/>
Alternate spelling for |</td><td class="col3">flags = flags | 42;<br/>
</td><td class="col4">yes<br/>
</td><td class="col5">left to right</td>
</tr>
<tr class="row13">
<td class="col0">13</td><td class="col1"><code>&&</code><br/>
<code>and</code></td><td class="col2">Logical AND<br/>
Alternate spelling for && </td><td class="col3">if (conditionA && conditionB) …<br/>
</td><td class="col4">yes<br/>
</td><td class="col5">left to right</td>
</tr>
<tr class="row14">
<td class="col0">14</td><td class="col1"><code>||</code><br/>
<code>or</code></td><td class="col2">Logical OR<br/>
Alternate spelling for ||</td><td class="col3">if (conditionA || conditionB) ...<br/>
</td><td class="col4">yes<br/>
</td><td class="col5">left to right</td>
</tr>
<tr class="row15">
<td class="col0">15</td><td class="col1"><code>? :</code></td><td class="col2">Ternary conditional (if-then-else)</td><td class="col3">int i = a > b ? a : b;</td><td class="col4">no</td><td class="col5">right to left</td>
</tr>
<tr class="row16">
<td class="col0">16</td><td class="col1"><code>=</code><br/>
<code>+=</code><br/>
<code>-=</code><br/>
<code>*=</code><br/>
<code>/=</code><br/>
<code>%=</code><br/>
<code>&=</code><br/>
<code>and_eq</code><br/>
<code>^=</code><br/>
<code>xor_eq</code><br/>
<code>|=</code><br/>
<code>or_eq</code><br/>
<code><<=</code><br/>
<code>>>=</code></td><td class="col2">Assignment operator<br/>
Increment and assign<br/>
Decrement and assign<br/>
Multiply and assign<br/>
Divide and assign<br/>
Modulo and assign<br/>
Bitwise AND and assign<br/>
Alternate spelling for &= <br/>
Bitwise exclusive or (XOR) and assign<br/>
Alternate spelling for ^= <br/>
Bitwise normal OR and assign<br/>
Alternate spelling for |=<br/>
Bitwise shift left and assign<br/>
Bitwise shift right and assign</td><td class="col3">int a = b;<br/>
a += 3;<br/>
b -= 4;<br/>
a *= 5;<br/>
a /= 2;<br/>
a %= 3;<br/>
flags &= new_flags;<br/>
<br/>
flags ^= new_flags;<br/>
<br/>
flags |= new_flags;<br/>
<br/>
flags <<= 2;<br/>
flags >>= 2;</td><td class="col4">yes<br/>
yes<br/>
yes<br/>
yes<br/>
yes<br/>
yes<br/>
yes<br/>
<br/>
yes<br/>
<br/>
yes<br/>
<br/>
yes<br/>
yes</td><td class="col5">right to left</td>
</tr>
<tr class="row17">
<td class="col0">17</td><td class="col1"><code>throw</code></td><td class="col2">throw exception</td><td class="col3">throw EClass(“Message”);</td><td class="col4">no</td><td class="col5"> </td>
</tr>
<tr class="row18">
<td class="col0">18</td><td class="col1"><code>,</code></td><td class="col2">Sequential evaluation operator</td><td class="col3">for (i = 0, j = 0; i < 10; i++, j++) …</td><td class="col4">yes</td><td class="col5">left to right</td>
</tr>
</table>
</div>
<h2><a name="order_of_evaluation_and_of_side_effects" id="order_of_evaluation_and_of_side_effects">Order of Evaluation and of Side Effects</a></h2>
<div class="level2">
<p>
One important aspect of C++ that is related to operator precedence, is the order of evaluation and the order of side effects in expressions.
In most circumstances, the order in which things happen is not specified.
For example in <code>f() + g()</code> whether <code>f()</code> or <code>g()</code> is called first is not specified.
If at least one of the functions has side effects the results may differ across compilers, different versions of the same compiler or even between multiple runs of the same compiler.
</p>
<p>
Further, the effect of certain expressions is undefined.
For example, consider the following code:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="kw4">float</span> x <span class="sy0">=</span> <span class="nu0">1</span><span class="sy0">;</span>
x <span class="sy0">=</span> x <span class="sy0">/</span> <span class="sy0">++</span>x<span class="sy0">;</span></pre>
<p>
The value of x and the rest of the behaviour of the program after evaluating this expression is undefined.
The program is semantically ill-formed:
x is modified twice between two consecutive sequence points.
</p>
<p>
Expressions like the one above must be avoided.
When in doubt, break a large expression into multiple statements to ensure that the order of evaluation is
correct.
</p>
</div>
<h2><a name="overloading_of_operators" id="overloading_of_operators">Overloading of Operators</a></h2>
<div class="level2">
<p>
Overloading of operators can be very useful and very dangerous.
On one hand overloading operators for a class you have created can help with logistics and
readability of code.
On the other hand you can overload an operator in such a way that it can either obfuscate or just downright break your program.
Use carefully.
In particular never overload <code>&&</code>, <code>||</code> or <code>,</code>.
In the overloaded context they lose the guarantee that the left operand is evaluated before the second and that there is a sequence point inbetween.
</p>
<p>
There are two ways to over load an operator: global function or class member.
</p>
<p>
Example of overloading with a global function:
</p>
<pre class="c code c++" style="font-family:monospace;"> ostream<span class="sy0">&</span> operator <span class="sy0"><<</span><span class="br0">(</span>ostream<span class="sy0">&</span> os<span class="sy0">,</span> <span class="kw4">const</span> myClass<span class="sy0">&</span> rhs<span class="br0">)</span><span class="sy0">;</span></pre>
<p>
But to be able to reach any private data within a user defined class you have to
declare the global function as a friend within the definition of the class.
</p>
<p>
Example:
</p>
<pre class="c code c++" style="font-family:monospace;"> class myClass <span class="br0">{</span>
<span class="co1">// Gives the operator << function access to 'myData'</span>
<span class="co1">// (this declaration should not go in public, private or protected)</span>
friend ostream<span class="sy0">&</span> operator <span class="sy0"><<</span><span class="br0">(</span>ostream<span class="sy0">&</span> lhs<span class="sy0">,</span> <span class="kw4">const</span> myClass<span class="sy0">&</span> rhs<span class="br0">)</span><span class="sy0">;</span>
private<span class="sy0">:</span>
<span class="kw4">int</span> myData<span class="sy0">;</span>
<span class="br0">}</span></pre>
<p>
Overloading with a class member can be done as follows:
</p>
<pre class="c code c++" style="font-family:monospace;"> class myClass <span class="br0">{</span>
public<span class="sy0">:</span>
<span class="co1">// The left hand side of this operator becomes '*this'.</span>
<span class="kw4">int</span> operator <span class="sy0">+</span><span class="br0">(</span><span class="kw4">const</span> myClass<span class="sy0">&</span> rhs<span class="br0">)</span><span class="sy0">;</span>
private<span class="sy0">:</span>
<span class="kw4">int</span> myData<span class="sy0">;</span>
<span class="br0">}</span></pre>
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