We have see already in the initial example the usage of arithmetic and many other \emph{operators}. Operators are special functions with typically prefix-notation (for unary operators) and infix notation (for binary operators) and are written with the classical mathematical symbols. In the initial example, we had \texttt{+, -, *, ::, ., <<, = , ",", ()}.
\begin{defn}
We call operators acting on one operand \emph{unary operators} (like \texttt{-, *, \&}), acting on two operands \emph{binary operator} (like \texttt{+, -, *, /, =, +=, ...}) and even acting on three operands a \emph{ternary operators} (there is only \texttt{?:}).
\end{defn}
Operators are characterized by its properties: \emph{associativity}, \emph{precedence}, and whether they can be \emph{overloaded}.
Operator associativity means the property of an operator determining the order in which to evaluate multiple occurrences of this operator in an expression without changing the result of the evaluation.
\end{defn}
Only in \emph{associative compositions} the result depends on the associativity. In order to fix the meaning of expressions with multiple
similar operators without brackets we introduce the convention
\begin{itemize}
\item A \emph{left-associative operator} is evaluated from left to right.
\item A \emph{right-associative operator} is evaluated from right to left.
\end{itemize}
All (binary) operators are classified as left or right associative
Operators are ordered by its precedence. This defines a \emph{partial ordering} of the operators and specified which operators is evaluated first. The precedence can be overridden using brackets.
From basic arithmetics you know the precedence of some arithmetic operators, especially \texttt{+, -, *, /}. This precedence is known in some countries as mnemonics:
\begin{itemize}
\item Germany: \textit{Punktrechnung geht vor Strichrechnung}, meaning Multiplication/Division before Addition/Subtraction
\item United States: \textbf{PEMDAS}: Parentheses, Exponents, Multiplication/Division, Addition/Subtraction. PEMDAS is often expanded to the mnemonic ``\textit{Please Excuse My Dear Aunt Sally}''.
\item Canada and New Zealand: \textbf{BEDMAS}: Brackets, Exponents, Division/Multiplication, Addition/Subtraction.
\item UK, Pakistan, India, Bangladesh and Australia and some other English-speaking countries: \textbf{BODMAS}: Brackets, Order, Division/Multiplication, Addition/Subtraction or Brackets, Of/Division/Multiplication, Addition/Subtraction.
\end{itemize}
\begin{example}
An example with classical mathematical operators on integers:
\[\begin{split}
x &=2*2+2\,/\,2-2\quad\Rightarrow\quad x =\left(\left(\left(2*2\right)+\left(2/2\right)\right)-2\right)=3\\
y &=8/2*(2+2)\quad\Rightarrow\quad y =(8/2)*(2+2)=16
\end{split}\]
\end{example}
\begin{rem}
The operator \cpp{^} does not mean exponentiation or power, but the logical X-Or. In Matlab/Octave this operator has the expected meaning, but in C++ the operator has a different precedence than the power operator would have from a mathematical perspective.
This means:
\cppline{a = b^2 + c}
is equivalent to
\cppline{a = b^(2 + c)}
BUT NOT TO
\cppline{a = (b^2) + c}
There is no power operator in C++! You have to call the function \cpp{std::pow} instead.
\end{rem}
\begin{guideline}{Principle}
When in doubt or to clarify the expression: use parentheses.
In the table below, the operators are ordered by precedence and information about associativity is given. Here I give you a summary of most of the operators and its meaning. Most of the operators are defined for arithmetic types (integers or floating-point numbers), but some are also defined for library types, like \cpp{std::string}, \cpp{std::vector} and others.
\subsubsection*{Arithmetic operators}
\begin{tabular}{l|l}
Operator & Action \\
\hline
\cpp{-}& Subtraction (unary minus) \\
\cpp{+}& Addition (unary plus) \\
\cpp{*}& Multiplication \\
\cpp{/}& Division \\
\cpp{%} & Modulo = Reminder of division, \ie for integers: if \cpp{r = a % b}, then there exists \cpp{c}, such that \cpp{a=b*c + r} \\
\cpp{--}& Decrement (Pre- and Postfix), \ie\cpp{--a} is equivalent to \cpp{a = a - 1}\\
\cpp{++}& Increment (Pre- and Postfix), \ie\cpp{++a} is equivalent to \cpp{a = a + 1}\\
\end{tabular}
\begin{minted}{c++}
int operator++(int& a, int) { // a++
int r = a;
a += 1;
return r;
}
int& operator++(int& a) { // ++a
a += 1
return a;
}
\end{minted}
\subsubsection*{Boolean operators}
Logical operators and comparison operators
\begin{tabular}{l|l}
Operator & Action \\
\hline
\cpp{>}& greater than \\
\cpp{>=}& greater than or equal to \\
\cpp{<}& less than \\
\cpp{<=}& less than or equal to \\
\cpp{==}& equal to \\
\cpp{!=}& unequal to \\
\cpp{&&}& AND \\
\cpp{||}& OR \\
\cpp{!}& NOT \\
\end{tabular}
\begin{rem}
The result of a boolean expression is a \cpp{bool} value, \eg
\cppline{bool out_of_bound = x < min_x || x > max_x}
\end{rem}
\begin{rem}
With \cxx{20}\marginpar{[\cxx{20}]} a new comparison operator is introduced, called \emph{three-way comparison operator} or sometimes also \emph{space-ship operator}. It is written as \cpp{<=>} and returns an object such that
\begin{itemize}
\item\cpp{(a <=> b) < 0} if \cpp{lhs < rhs}
\item\cpp{(a <=> b) > 0} if \cpp{lhs > rhs}
\item\cpp{(a <=> b) == 0} if \cpp{lhs} and \cpp{rhs} are equal/equivalent.
\end{itemize}
and the returned object indicates the type of ordering (strong ordering, partial ordering, weak equality, ...).
\end{rem}
\subsubsection*{Bitwise operators}
Modify or test for the bits of integers
\begin{tabular}{l|l}
Operator & Action \\
\hline
\cpp{&}& AND \\
\cpp{|}& OR \\
\cpp{^}& exclusive OR \\
\cpp{~}& 1-complement \\
\cpp{>>}& right-shift of the bits \\
\cpp{<<}& left-shift of the bits \\
\end{tabular}
\begin{rem}
The logical operators \cpp{<<} and \cpp{>>} are used in C++ often in a different context, namely to \emph{shift} something into a \emph{stream}. Streams are abstractions devices allowing input and output operations. The operator \cpp{<<} is therefore called \emph{insertion operator} and is used with output streams, whereas the operator \cpp{>>} is called \emph{extraction operator} and is used with input streams.
\end{rem}
\begin{example}
While a modern CPU efficiently implements arithmetic and logical operators on integers, one could implement those manually, by using bitwise operations and comparison with 0 only. The following pseudo code implements multiplication of two integers \texttt{a} and \texttt{b} with bit shifts, comparison and addition:
\begin{minted}{pascal}
c := 0
solange b <> 0
falls (b und 1) <> 0
c := c + a
schiebe a um 1 nach links
schiebe b um 1 nach rechts
return c
\end{minted}
It implements kind of a manual multiplication in the binary base but in the uncommon order from right to left.
As an exercise, you could implement this algorithm and compare the result with the classical multiplication.
\end{example}
\subsubsection*{Assignment operators}
Compound-assignment operators like \cpp{+=, -=, *=, /=, %= >>=, <<=, &=, ^=, |=} apply an operator to the left and right-side of an assignment, and store the result in the left operand.
Example:
\cppline{a += b // corresponds to a = a + b}
\begin{rem}
Compared to the simple assignment, a compound-assignment does not need to create a temporary and maybe copy it to the left-hand side operand, but works with the operand directly.
\begin{minted}{c++}
struct A { double value; };
A operator+(A const& a, A const& b) {
A c(a); // create local copy of a
c += b;
return c;
}
\end{minted}
but
\begin{minted}{c++}
A& operator+=(A& a, A const& b) {
a.value += b.value;
return a;
}
\end{minted}
\end{rem}
\subsubsection*{Bracket-Access operators}
The round brackets \cpp{()} and the square brackets \cpp{[]} also represent operators, namely \emph{bracket-access operators}. While the square bracket is typically used as vector access and allows only one argument, \eg the vector element index, the round brackets represent a function call and thus allow any number of arguments \texttt{>= 0}.
\subsection*{Sequence of evaluation and side effects}
A sequence point defines any point in a computer program's execution at which it is guaranteed that all side effects of previous evaluations will have been performed, and no side effects from subsequent evaluations have yet been performed.
A side effect is, for example, the change of a variable not directly involved in the computation.
Logical and-then/or-else (\cpp{&&} and \cpp{||}) operators, ternary \cpp{?:} question mark operators, and commas \cpp{,} create sequence points; \cpp{+, -, <<} and so on do not!
With \cxx{17}\marginpar{[\cxx{17}]} several more operations are now sequenced and are safe to include side effects. But in general, you should know that
\begin{guideline}{Attention}
When you use an expression with side effects multiple times in the absence of sequence points, the resulting behavior is \textbf{undefined} in C++. Any result is possible, including one that does not make logical sense.
\end{guideline}
See also: \href{http://en.wikipedia.org/wiki/Sequence_point}{Wikipedia}, \href{http://en.cppreference.com/w/cpp/language/eval_order}{CppReference.com}
For logical operators, at first the left operand is evaluated and based on its value it is decided whether to evaluate the right operand at all or not, \ie
\begin{minted}{c++}
A && B // first evaluate A, if A == true evaluate B and return its value
A || B // first evaluate A, if A == false, evaluate B and return its value
A ? B : C // first evaluate A, if true, evaluate B, otherwise evaluate C
(A,B,C, ...) // first evaluate A, then evaluate B, then evaluate C, ...
f() + g() // it is not specified whether f is evaluated first or g
\end{minted}
\begin{guideline}{Principle}
Never rely on a function call in a logical expression.
\end{guideline}
\begin{rem}
The precedence property is not related to the order of evaluation, but to the rank of the operator. For an arbitrary function \cpp{f(a,b,c,d)} it
is not specified in which order the expressions for a, b, c, d are evaluated.
\end{rem}
\begin{example}
The following expressions have different behavior:
In C++ nearly all operator can be written (and called) as a regular function. Let \texttt{o} be the symbol of the operator, \eg\texttt{o$~\in$\{+,*,(),+=,<,...\}}, then there zero, one or both of the following implementations are available:
\begin{minted}[frame=none]{c++}
Result operator o(Arg1 a, Arg2 b, ...) // a o b
Result Arg1::operator o(Arg2 b, ...) // Arg1 a; a o b
\end{minted}
The variant 1 implement the operator as a free function, whereas the variant 2 implements the operator as a member function of a class, where \texttt{Arg1} is the name of that class. Whether there is a function representation of the operator and whether it is allowed to \emph{overload} that function is specified in the table below.
In the column \textit{Overload} means (C), the operator can only be implemented as member function of a class, whereas $\checkmark$ allows to write the operator as member function or free function.
A more complete table can also be found at \url{http://en.wikipedia.org/w/index.php?title=C++\_operators}, or \url{https://en.cppreference.com/w/cpp/language/operator_precedence}.
