Welcome to C++ Program Design!
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Basics of C++
Program Structure
Compound DataTypes
Classes of C++
C++ Programming and ObjectOriented Design
1.4 Operators
Once introduced to variables and constants, we can begin to operate with them by using operators. What follows is a complete list of operators. At this point, it is likely not necessary to know all of them, but they are all listed here to also serve as reference.
Assignment operator (=)
The assignment operator assigns a value to a variable.


This statement assigns the integer value
5
to the variable x
. The assignment operation always takes place from right to left, and never the other way around:


This statement assigns to variable
x
the value contained in variable y
. The value of x
at the moment this statement is executed is lost and replaced by the value of y
.Consider also that we are only assigning the value of
y
to x
at the moment of the assignment operation. Therefore, if y
changes at a later moment, it will not affect the new value taken by x
.For example, let's have a look at the following code  I have included the evolution of the content stored in the variables as comments:


a:4 b:7 
This program prints on screen the final values of
a
and b
(4 and 7, respectively). Notice how a
was not affected by the final modification of b
, even though we declared a = b
earlier.Assignment operations are expressions that can be evaluated(赋值操作是可以计算的表达式). That means that the assignment itself has a value, and for fundamental types this value is the one assigned in the operation. For example:


In this expression,
y
is assigned the result of adding 2 and the value of another assignment expression (which has itself a value of 5). It is roughly equivalent to:


With the final result of assigning 7 to
y
.The following expression is also valid in C++:


It assigns 5 to the all three variables:
x
, y
and z
; always from righttoleft.Arithmetic operators ( +, , *, /, % )
The five arithmetical operations supported by C++ are:operator  description 

+ 
addition 
 
subtraction 
* 
multiplication 
/ 
division 
% 
modulo 
Operations of addition, subtraction, multiplication and division correspond literally to their respective mathematical operators. The last one, modulo operator, represented by a percentage sign (
%
), gives the remainder of a division of two values. For example:


results in variable
x
containing the value 2, since dividing 11 by 3 results in 3, with a remainder of 2.Compound assignment (+=, =, *=, /=, %=, >>=, <<=, &=, ^=, =)
Compound assignment operators modify the current value of a variable by performing an operation on it. They are equivalent to assigning the result of an operation to the first operand:expression  equivalent to... 

y += x; 
y = y + x; 
x = 5; 
x = x  5; 
x /= y; 
x = x / y; 
price *= units + 1; 
price = price * (units+1); 
and the same for all other compound assignment operators. For example:


5 
Increment and decrement (++, )
Some expression can be shortened even more: the increase operator (++
) and the decrease operator (
) increase or reduce by one the value stored in a variable. They are equivalent to +=1
and to =1
, respectively. Thus:


are all equivalent in its functionality; the three of them increase by one the value of
x
.In the early C compilers, the three previous expressions may have produced different executable code depending on which one was used. Nowadays, this type of code optimization is generally performed automatically by the compiler, thus the three expressions should produce exactly the same executable code.
A peculiarity (特性) of this operator is that it can be used both as a prefix and as a suffix. That means that it can be written either before the variable name (
++x
) or after it (x++
). Although in simple expressions like x++
or ++x
, both have exactly the same meaning; in other expressions in which the result of the increment or decrement operation is evaluated, they may have an important difference in their meaning: In the case that the increase operator is used as a prefix (++x
) of the value, the expression evaluates to the final value of x
, once it is already increased. On the other hand, in case that it is used as a suffix (x++
), the value is also increased, but the expression evaluates to the value that x had before being increased. Notice the difference:Example 1  Example 2 

x = 3; 
x = 3; 
In Example 1, the value assigned to
y
is the value of x
after being increased. While in Example 2, it is the value x
had before being increased.Relational and comparison operators ( ==, !=, >, <, >=, <= )
Two expressions can be compared using relational and equality operators. For example, to know if two values are equal or if one is greater than the other.The result of such an operation is either true or false (i.e., a Boolean value).
The relational operators in C++ are:
operator  description 

== 
Equal to 
!= 
Not equal to 
< 
Less than 
> 
Greater than 
<= 
Less than or equal to 
>= 
Greater than or equal to 
Here there are some examples:


Of course, it's not just numeric constants that can be compared, but just any value, including, of course, variables. Suppose that
a=2
, b=3
and c=6
, then:


Be careful! The assignment operator (operator
=
, with one equal sign) is not the same as the equality comparison operator (operator ==
, with two equal signs); the first one (=
) assigns the value on the righthand to the variable on its left, while the other (==
) compares whether the values on both sides of the operator are equal. Therefore, in the last expression ((b=2) == a
), we first assigned the value 2
to b
and then we compared it to a
(that also stores the value 2), yielding true
.Logical operators ( !, &&,  )
The operator!
is the C++ operator for the Boolean operation NOT. It has only one operand, to its right, and inverts it, producing false
if its operand is true
, and true
if its operand is false
. Basically, it returns the opposite Boolean value of evaluating its operand. For example:


The logical operators
&&
and 
are used when evaluating two expressions to obtain a single relational result. The operator &&
corresponds to the Boolean logical operation AND, which yields true
if both its operands are true
, and false
otherwise. The following panel shows the result of operator &&
evaluating the expression a&&b
:&& OPERATOR (and)  

a 
b 
a && b 
true 
true 
true 
true 
false 
false 
false 
true 
false 
false 
false 
false 
The operator

corresponds to the Boolean logical operation OR, which yields true
if either of its operands is true
, thus being false only when both operands are false. Here are the possible results of ab
: OPERATOR (or)  

a 
b 
a  b 
true 
true 
true 
true 
false 
true 
false 
true 
true 
false 
false 
false 
For example:


When using the logical operators, C++ only evaluates what is necessary from left to right to come up with the combined relational result, ignoring the rest. Therefore, in the last example (
(5==5)(3>6)
), C++ evaluates first whether 5==5
is true
, and if so, it never checks whether 3>6
is true
or not. This is known as shortcircuit evaluation(短路评估), and works like this for these operators:operator  shortcircuit 

&& 
if the lefthand side expression is false , the combined result is false (the righthand side expression is never evaluated). 
 
if the lefthand side expression is true , the combined result is true (the righthand side expression is never evaluated). 
This is mostly important when the righthand expression has side effects, such as altering values:


Here, the combined conditional expression would increase
i
by one, but only if the condition on the left of &&
is true
, because otherwise, the condition on the righthand side (++i<n
) is never evaluated.Conditional ternary (三元的) operator ( ? )
The conditional operator evaluates an expression, returning one value if that expression evaluates totrue
, and a different one if the expression evaluates as false
. Its syntax is:condition ? result1 : result2
If
condition
is true
, the entire expression evaluates to result1
, and otherwise to result2
.


For example:


7 
In this example,
a
was 2, and b
was 7, so the expression being evaluated (a>b
) was not true
, thus the first value specified after the question mark was discarded in favor of the second value (the one after the colon) which was b
(with a value of 7).Comma operator ( , )
The comma operator (,
) is used to separate two or more expressions that are included where only one expression is expected. When the set of expressions has to be evaluated for a value, only the rightmost expression is considered.For example, the following code:


would first assign the value 3 to
b
, and then assign b+2
to variable a
. So, at the end, variable a
would contain the value 5 while variable b
would contain value 3.Bitwise (逐位的) operators ( &, , ^, ~, <<, >> )
Bitwise operators modify variables considering the bit patterns that represent the values they store.operator  asm equivalent  description 

& 
AND 
Bitwise AND 
 
OR 
Bitwise inclusive OR 
^ 
XOR 
Bitwise exclusive OR 
~ 
NOT 
Unary complement (bit inversion) 
<< 
SHL 
Shift bits left 
>> 
SHR 
Shift bits right 
Explicit type casting operator
Type casting operators allow to convert a value of a given type to another type. There are several ways to do this in C++. The simplest one, which has been inherited from the C language, is to precede the expression to be converted by the new type enclosed between parentheses (()):


The previous code converts the floatingpoint number
3.14
to an integer value (3
); the remainder is lost. Here, the typecasting operator was (int)
. Another way to do the same thing in C++ is to use the functional notation preceding the expression to be converted by the type and enclosing the expression between parentheses:


Both ways of casting types are valid in C++.
sizeof
This operator accepts one parameter, which can be either a type or a variable, and returns the size in bytes of that type or object:


Here,
x
is assigned the value 1
, because char
is a type with a size of one byte.The value returned by
sizeof
is a compiletime constant, so it is always determined before program execution.Other operators
Later in these tutorials, we will see a few more operators, like the ones referring to pointers or the specifics for objectoriented programming.Precedence of operators
A single expression may have multiple operators. For example:


In C++, the above expression always assigns 6 to variable
x
, because the %
operator has a higher precedence than the +
operator, and is always evaluated before. Parts of the expressions can be enclosed in parenthesis to override this precedence order, or to make explicitly clear the intended effect. Notice the difference:


From greatest to smallest priority, C++ operators are evaluated in the following order:
Level  Precedence group  Operator  Description  Grouping 

1  Scope  :: 
scope qualifier  Lefttoright 
2  Postfix (unary)  ++  
postfix increment / decrement  Lefttoright 
() 
functional forms  
[] 
subscript  
. > 
member access  
3  Prefix (unary)  ++  
prefix increment / decrement  Righttoleft 
~ ! 
bitwise NOT / logical NOT  
+  
unary prefix  
& * 
reference / dereference  
new delete 
allocation / deallocation  
sizeof 
parameter pack  
(type) 
Cstyle typecasting  
4  Pointertomember  .* >* 
access pointer  Lefttoright 
5  Arithmetic: scaling  * / % 
multiply, divide, modulo  Lefttoright 
6  Arithmetic: addition  +  
addition, subtraction  Lefttoright 
7  Bitwise shift  << >> 
shift left, shift right  Lefttoright 
8  Relational  < > <= >= 
comparison operators  Lefttoright 
9  Equality  == != 
equality / inequality  Lefttoright 
10  And  & 
bitwise AND  Lefttoright 
11  Exclusive or  ^ 
bitwise XOR  Lefttoright 
12  Inclusive or   
bitwise OR  Lefttoright 
13  Conjunction  && 
logical AND  Lefttoright 
14  Disjunction   
logical OR  Lefttoright 
15  Assignmentlevel expressions  = *= /= %= += = 
assignment / compound assignment  Righttoleft 
?: 
conditional operator  
16  Sequencing  , 
comma separator  Lefttoright 
When an expression has two operators with the same precedence level, grouping determines which one is evaluated first: either lefttoright or righttoleft.
Enclosing all substatements in parentheses (even those unnecessary because of their precedence) improves code readability.
