C++ classes


A class in C++ is a user-defined type or data structure declared with keyword class that has data and functions as its members whose access is governed by the three access specifiers private, protected or public. By default access to members of a C++ class is private. The private members are not accessible outside the class; they can be accessed only through methods of the class. The public members form an interface to the class and are accessible outside the class.
Instances of a class data type are known as objects and can contain member variables, constants, member functions, and overloaded operators defined by the programmer.

Differences between a structure and a class in C++

In C++, a class defined with the class keyword has private members and base classes by default. A structure is a class defined with the struct keyword.ISO/IEC. ISO/IEC 14882:2003: Programming Languages - C++ §9 Classes para. 4 Its members and base classes are public by default. In practice, structs are typically reserved for data without functions. When deriving a struct from a class/struct, default access-specifier for a base class/struct is public. And when deriving a class, default access specifier is private.

Aggregate classes

An aggregate class is a class with no user-declared constructors, no private or protected
non-static data members, no base classes, and no virtual functions.ISO/IEC. ISO/IEC 14882:2003: Programming Languages - C++ §8.5.1 Aggregates para. 1 Such a class can be initialized with a brace-enclosed comma-separated list of initializer-clauses.ISO/IEC. ISO/IEC 14882:2003: Programming Languages - C++ §8.5.1 Aggregates para. 2 The following code has the same semantics in both C and C++.

struct C ;
struct D ;
// initialize an object of type C with an initializer-list
C c = ;
// D has a sub-aggregate of type C. In such cases initializer-clauses can be nested
D d = ;

POD-structs

A POD-struct is an aggregate class that has no non-static data members of type non-POD-struct, non-POD-union or reference, and has no user-defined assignment operator and no user-defined destructor. A POD-struct could be said to be the C++ equivalent of a C struct. In most cases, a POD-struct will have the same memory layout as a corresponding struct declared in C. For this reason, POD-structs are sometimes colloquially referred to as "C-style structs".

Properties shared between structs in C and POD-structs in C++

  • Data members are allocated so that later members have higher addresses within an object, except where separated by an access-specifier.ISO/IEC. ISO/IEC 14882:2003: Programming Languages - C++ §9.2 Class members para. 12
  • Two POD-struct types are layout-compatible if they have the same number of nonstatic data members, and corresponding nonstatic data members have layout-compatible types.ISO/IEC. ISO/IEC 14882:2003: Programming Languages - C++ §9.2 Class members para. 14
  • A POD-struct may contain unnamed padding.ISO/IEC. ISO/IEC 14882:2003: Programming Languages - C++ §9.2 Class members para. 17
  • A pointer to a POD-struct object, suitably converted using a reinterpret cast, points to its initial member and vice versa, implying that there is no padding at the beginning of a POD-struct.
  • A POD-struct may be used with the offsetof macro.ISO/IEC. ISO/IEC 14882:2003: Programming Languages - C++ §18.1 Types para. 5

    Declaration and usage

C++ classes have their own members. These members include variables, functions known as methods, constructors and destructors. Members are declared to be either publicly or privately accessible using the public: and private: access specifiers respectively. Any member encountered after a specifier will have the associated access until another specifier is encountered. There is also inheritance between classes which can make use of the protected: specifier.

Global and local class

A class defined outside all methods is a global class because its objects can be created from anywhere in the program. If it is defined within a function body then it's a local class because objects of such a class are local to the function scope.

Basic declaration and member variables

Classes are declared with the class or struct keyword. Declaration of members are placed within this declaration.
The above definitions are functionally equivalent. Either code will define objects of type Person as having two public data members, name and age. The semicolons after the closing braces are mandatory.
After one of these declarations, Person can be used as follows to create newly defined variables of the Person datatype:

  1. include
  2. include
struct Person ;
int main

Executing the above code will output
Calvin: 30
Hobbes: 20

Member functions

An important feature of the C++ class and structure are member functions. Each datatype can have its own built-in functions that have access to all members of the datatype. In the body of these non-static member functions, the keyword this can be used to refer to the object for which the function is called. This is commonly implemented by passing the address of the object as an implicit first argument to the function. Take the above Person type as an example again:

  1. include
class Person ;
void Person::Print const

In the above example the Print function is declared in the body of the class and defined by qualifying it with the name of the class followed by ::. Both name_ and age_ are private and Print is declared as public which is necessary if it is to be used from outside the class.
With the member function Print, printing can be simplified into:

a.Print;
b.Print;

where a and b above are called senders, and each of them will refer to their own member variables when the Print function is executed.
It is common practice to separate the class or structure declaration and the definition into separate units. The interface, needed by the user, is kept in a header and the implementation is kept separately in either source or compiled form.

Inheritance

The layout of non-POD classes in memory is not specified by the C++ standard. For example, many popular C++ compilers implement single inheritance by concatenation of the parent class fields with the child class fields, but this is not required by the standard. This choice of layout makes referring to a derived class via a pointer to the parent class type a trivial operation.
For example, consider

struct P ;


struct C : P ;

An instance of P with a P* p pointing to it might look like this in memory:
+----+
|P::x|
+----+
↑
p
An instance of C with a P* p pointing to it might look like this:
+----+----+
|P::x|C::y|
+----+----+
↑
p
Therefore, any code that manipulates the fields of a P object can manipulate the P fields inside the C object without having to consider anything about the definition of C's fields. A properly written C++ program shouldn't make any assumptions about the layout of inherited fields, in any case. Using the static_cast or dynamic_cast type conversion operators will ensure that pointers are properly converted from one type to another.
Multiple inheritance is not as simple. If a class D inherits P and C, then the fields of both parents need to be stored in some order, but only one of the parent classes can be located at the front of the derived class. Whenever the compiler needs to convert a pointer from the D type to either P or C, the compiler will provide an automatic conversion from the address of the derived class to the address of the base class fields.
For more on multiple inheritance, see virtual inheritance.

Overloaded operators

In C++, operators, such as + - * /, can be overloaded to suit the needs of programmers. These operators are called overloadable operators.
By convention, overloaded operators should behave nearly the same as they do in built-in datatypes, but this is not required. One can declare a structure called Integer in which the variable really stores an integer, but by calling Integer * Integer the sum, instead of the product, of the integers might be returned:

struct Integer ;

The code above made use of a constructor to "construct" the return value. For clearer presentation, the above code can be rewritten as:

Integer operator* const

Programmers can also put a prototype of the operator in the struct declaration and define the function of the operator in the global scope:

struct Integer ;
Integer Integer::operator* const

i above represents the sender's own member variable, while k.i represents the member variable from the argument variable k.
The const keyword appears twice in the above code. The first occurrence, the argument const integer& k, indicated that the argument variable will not be changed by the function. The second incidence at the end of the declaration promises the compiler that the sender would not be changed by the function run.
In const integer& k, the ampersand means "pass by reference". When the function is called, a pointer to the variable will be passed to the function, rather than the value of the variable.
The same overloading properties above apply also to classes.
Note that arity, associativity and precedence of operators cannot be changed.

Binary overloadable operators

Binary operators are overloaded by declaring a function with an "identifier" operator which calls one single argument. The variable on the left of the operator is the sender while that on the right is the argument.

Integer i = 1;
/* we can initialize a structure variable this way as
if calling a constructor with only the first
argument specified. */
Integer j = 3;
/* variable names are independent of the names of the
member variables of the structure. */
Integer k = i * j;
std::cout << k.i << std::endl;

'3' would be printed.
The following is a list of binary overloadable operators:
OperatorGeneral usage
+ - * / %Arithmetic calculation
^ & | << >>Bitwise calculation
< > != <= >=Logical comparison
&&Logical conjunction
||Logical disjunction
+= -= *= /= %=
^= &= |= <<= >>=		Compound assignment
,

The '=' operator between two variables of the same structure type is overloaded by default to copy the entire content of the variables from one to another. It can be overwritten with something else, if necessary.
Operators must be overloaded one by one, in other words, no overloading is associated with one another. For example, < is not necessarily the opposite of >.

Unary overloadable operators

While some operators, as specified above, takes two terms, sender on the left and the argument on the right, some operators have only one argument - the sender, and they are said to be "unary". Examples are the negative sign and the "logical NOT".
Sender of unary operators may be on the left or on the right of the operator. The following is a list of unary overloadable operators:
OperatorGeneral usagePosition of sender
+ -Positive / negative signright
* &Dereferenceright
! ~Logical / bitwise NOTright
++ --Pre-increment / decrementright
++ --Post-increment / decrementleft

The syntax of an overloading of a unary operator, where the sender is on the right, is as follows:
When the sender is on the left, the declaration is:
@ above stands for the operator to be overloaded. Replace return_type with the datatype of the return value
The int parameter essentially means nothing but a convention to show that the sender is on the left of the operator.
const arguments can be added to the end of the declaration if applicable.

Overloading brackets

The square bracket and the round bracket can be overloaded in C++ structures. The square bracket must contain exactly one argument, while the round bracket can contain any specific number of arguments, or no arguments.
The following declaration overloads the square bracket.
The content inside the bracket is specified in the argument part.
Round bracket is overloaded a similar way.
Contents of the bracket in the operator call are specified in the second bracket.
In addition to the operators specified above, the arrow operator, the starred arrow, the new keyword and the delete keyword can also be overloaded. These memory-or-pointer-related operators must process memory-allocating functions after overloading. Like the assignment operator, they are also overloaded by default if no specific declaration is made.

Constructors

Sometimes programmers may want their variables to take a default or specific value upon declaration. This can be done by declaring constructors.

Person::Person

Member variables can be initialized in an initializer list, with utilization of a colon, as in the example below. This differs from the above in that it initializes, rather than using the assignment operator. This is more efficient for class types, since it just needs to be constructed directly; whereas with assignment, they must be first initialized using the default constructor, and then assigned a different value. Also some types cannot be assigned to and therefore must be initialized in the initializer list.

Person : name_, age_

Note that the curly braces cannot be omitted, even if empty.
Default values can be given to the last arguments to help initializing default values.

Person : name_, age_

When no arguments are given to the constructor in the example above, it is equivalent to calling the following constructor with no arguments :

Person : name_, age_

The declaration of a constructor looks like a function with the same name as the datatype. In fact, a call to a constructor can take the form of a function call. In that case an initialized Person type variable can be thought of as the return value:

int main

An alternate syntax that does the same thing as the above example is

int main

Specific program actions, which may or may not relate to the variable, can be added as part of the constructor.

Person

With the above constructor, a "Hello!" will be printed when the default Person constructor is invoked.

Default constructor

Default constructors are called when constructors are not defined for the classes.

struct A ;
// Object created using parentheses.
A* a = new A; // Calls default constructor, and b will be initialized with '0'.
// Object created using no parentheses.
A* a = new A; // Allocate memory, then call default constructor, and b will have value '0'.
// Object creation without new.
A a; // Reserve space for a on the stack, and b will have an unknown garbage value.

However, if a user defined constructor was defined for the class, both of the above declarations will call this user defined constructor, whose defined code will be executed, but no default values will be assigned to the variable b.

Destructors

A destructor is the inverse of a constructor. It is called when an instance of a class is destroyed, e.g. when an object of a class created in a block is deleted after the closing brace, then the destructor is called automatically. It will be called upon emptying of the memory location storing the variables. Destructors can be used to release resources, such as heap-allocated memory and opened files when an instance of that class is destroyed.
The syntax for declaring a destructor is similar to that of a constructor. There is no return value and the name of the method is the same as the name of the class with a tilde in front.

~Person

Similarities between constructors and destructors

  • Both have same name as the class in which they are declared.
  • If not declared by user both are available in a class by default but they now can only allocate and deallocate memory from the objects of a class when an object is declared or deleted.
  • For a derived class: During the runtime of the base class constructor, the derived class constructor has not yet been called; during the runtime of the base class destructor, the derived class destructor has already been called. In both cases, the derived class member variables are in an invalid state.

    Class templates

In C++, class declarations can be generated from class templates. Such class templates represent a family of classes. An actual class declaration is obtained by instantiating the template with one or more template arguments. A template instantiated with a particular set of arguments is called a template specialization.

Properties

The syntax of C++ tries to make every aspect of a structure look like that of the basic datatypes. Therefore, overloaded operators allow structures to be manipulated just like integers and floating-point numbers, arrays of structures can be declared with the square-bracket syntax, and pointers to structures can be dereferenced in the same way as pointers to built-in datatypes.

Memory consumption

The memory consumption of a structure is at least the sum of the memory sizes of constituent variables. Take the TwoNums structure below as an example.

struct TwoNums ;

The structure consists of two integers. In many current C++ compilers, integers are 32-bit integers by default, so each of the member variables consume four bytes of memory. The entire structure, therefore, consumes at least eight bytes of memory, as follows.
+----+----+
| a | b |
+----+----+
However, the compiler may add padding between the variables or at the end of the structure to ensure proper data alignment for a given computer architecture, often padding variables to be 32-bit aligned. For example, the structure

struct BytesAndSuch ;

could look like
+-+-+-+-+--+--+----+--------+
|c|C|D|X|s |XX| i | d |
+-+-+-+-+--+--+----+--------+
in memory, where X represents padded bytes based on 4 bytes alignment.
As structures may make use of pointers and arrays to declare and initialize its member variables, memory consumption of structures is not necessarily constant. Another example of non-constant memory size is template structures.

Bit fields

Bit fields are used to define the class members that can occupy less storage than an integral type. This field is applicable only for integral types and excludes float or double.

struct A ;

  • Memory structure

 4 byte int 4 byte int

Bit fields are not allowed in a union. It is applicable only for the classes defined using the keyword struct or class.

Pass by reference

Many programmers prefer to use the ampersand to declare the arguments of a function involving structures. This is because by using the dereferencing ampersand only one word is required to be passed into the function, namely the memory location to the variable. Otherwise, if pass-by-value is used, the argument needs to be copied every time the function is called, which is costly with large structures.
Since pass-by-reference exposes the original structure to be modified by the function, the const keyword should be used to guarantee that the function does not modify the parameter, when this is not intended.

The ''this'' keyword

To facilitate structures' ability to reference themselves, C++ implements the this keyword for all member functions. The this keyword acts as a pointer to the current object. Its type is that of a pointer to the current object.
The this keyword is especially important for member functions with the structure itself as the return value:

Complex& operator+=

As stated above, this is a pointer, so the use of the asterisk is necessary to convert it into a reference to be returned.