Editing Imperative programming

{{Short description|Type of programming paradigm in computer science}}
{{more citations needed|date=October 2011}}
In [[computer science]], '''imperative programming''' is a [[programming paradigm]] of [[software]] that uses [[Statement (computer science)|statement]]s that change a program's [[state (computer science)|state]]. In much the same way that the [[imperative mood]] in [[natural language]]s expresses commands, an imperative program consists of [[command (computing)|command]]s for the [[computer]] to perform. Imperative programming focuses on describing ''how'' a program operates step by step (with general order of the steps being determined in [[source code]] by the placement of statements one below the other),<ref>{{Cite web |last=Jain |first=Anisha |date=2022-12-10 |title=Javascript Promises— Is There a Better Approach? |url=https://medium.datadriveninvestor.com/javascript-promises-is-there-a-better-approach-dd6a0a329131 |access-date=2022-12-20 |website=Medium |language=en |archive-date=2022-12-20 |archive-url=https://web.archive.org/web/20221220020247/https://medium.datadriveninvestor.com/javascript-promises-is-there-a-better-approach-dd6a0a329131 |url-status=live }}</ref> rather than on high-level descriptions of its expected results.

The term is often used in contrast to [[declarative programming]], which focuses on ''what'' the program should accomplish without specifying all the details of ''how'' the program should achieve the result.<ref>{{Cite web |title=Imperative programming: Overview of the oldest programming paradigm |url=https://www.ionos.com/digitalguide/websites/web-development/imperative-programming/ |access-date=2022-05-03 |website=IONOS Digitalguide |date=21 May 2021 |language=en |archive-date=2022-05-03 |archive-url=https://web.archive.org/web/20220503083342/https://www.ionos.com/digitalguide/websites/web-development/imperative-programming/ |url-status=live }}</ref>

==Procedural programming==
[[Procedural programming]] is a type of imperative programming in which the program is built from one or more procedures (also termed [[subroutine]]s or functions). The terms are often used as synonyms, but the use of procedures has a dramatic effect on how imperative programs appear and how they are constructed. Heavy procedural programming, in which [[State (computer science)|state]] changes are localized to procedures or restricted to explicit arguments and returns from procedures, is a form of [[structured programming]]. Since the 1960s, structured programming and [[modular programming]] in general have been promoted as techniques to improve the [[maintainability]] and overall quality of imperative programs. The concepts behind [[object-oriented programming]] attempt to extend this approach.

Procedural programming could be considered a step toward declarative programming. A programmer can often tell, simply by looking at the names, arguments, and return types of procedures (and related comments), what a particular procedure is supposed to do, without necessarily looking at the details of how it achieves its result. At the same time, a complete program is still imperative since it ''fixes'' the statements to be executed and their order of execution to a large extent.

==Rationale and foundations of imperative programming==
The programming paradigm used to build programs for almost all computers typically follows an imperative model.<ref group=note>[[Reconfigurable computing]] is a notable exception.</ref> Digital computer hardware is designed to execute [[machine code]], which is native to the computer and is usually written in the imperative style, although low-level compilers and interpreters using other paradigms exist for some architectures such as [[lisp machine]]s.

From this low-level perspective, the program state is defined by the contents of memory, and the statements are instructions in the native machine language of the computer. Higher-level imperative languages use [[variable (programming)|variable]]s and more complex statements, but still follow the same paradigm. [[Recipe]]s and process [[checklist]]s, while not [[computer program]]s, are also familiar concepts that are similar in style to imperative programming; each step is an instruction, and the physical world holds the state. Since the basic ideas of imperative programming are both conceptually familiar and directly embodied in the hardware, most computer languages are in the imperative style.

[[Destructive assignment|Assignment statements]], in imperative paradigm, perform an operation on information located in memory and store the results in memory for later use. High-level imperative languages, in addition, permit the [[Evaluation (disambiguation)#Computer_science|evaluation]] of complex [[Expression (programming)|expressions]], which may consist of a combination of [[Arithmetic#Arithmetic operations|arithmetic operations]] and [[function (mathematics)|function]] evaluations, and the assignment of the resulting value to memory. Looping statements (as in [[while loop]]s, [[do while loop]]s, and [[for loop]]s) allow a sequence of statements to be executed multiple times. Loops can either execute the statements they contain a predefined number of times, or they can execute them repeatedly until some condition is met. [[Conditional (programming)|Conditional]] [[Branch (computer science)|branching]] statements allow a sequence of statements to be executed only if some condition is met. Otherwise, the statements are skipped and the execution sequence continues from the statement following them. Unconditional branching statements allow an execution sequence to be transferred to another part of a program. These include the jump (called ''[[goto]]'' in many languages), [[switch statement|switch]], and the subprogram, [[subroutine]], or procedure call (which usually returns to the next statement after the call).

Early in the development of [[high-level programming language]]s, the introduction of the [[block (programming)|block]] enabled the construction of programs in which a group of statements and declarations could be treated as if they were one statement. This, alongside the introduction of [[subroutine]]s, enabled complex structures to be expressed by hierarchical decomposition into simpler procedural structures.

Many imperative programming languages (such as [[Fortran]], [[BASIC]], and [[C (programming language)|C]]) are [[Abstraction (computer science)|abstraction]]s of [[assembly language]].<ref name=":0">{{cite book|author=Bruce Eckel|title=Thinking in Java|url=https://books.google.com/books?id=bQVvAQAAQBAJ&q=imperative&pg=PA24|publisher=[[Pearson Education]]|year=2006|isbn=978-0-13-187248-6|page=24}}</ref>

==History of imperative and object-oriented languages==
The earliest imperative languages were the machine languages of the original computers. In these languages, instructions were very simple, which made hardware implementation easier but hindered the creation of complex programs. [[FORTRAN]], developed by [[John Backus]] at [[International Business Machines]] (IBM) starting in 1954, was the first major programming language to remove the obstacles presented by machine code in the creation of complex programs. FORTRAN was a [[compiled language]] that allowed named variables, complex expressions, subprograms, and many other features now common in imperative languages. The next two decades saw the development of many other major high-level imperative programming languages. In the late 1950s and 1960s, [[ALGOL]] was developed in order to allow mathematical algorithms to be more easily expressed and even served as the [[operating system]]'s target language for some computers. [[MUMPS]] (1966) carried the imperative paradigm to a logical extreme, by not having any statements at all, relying purely on commands, even to the extent of making the IF and ELSE commands independent of each other, connected only by an intrinsic variable named $TEST. [[COBOL]] (1960) and [[BASIC]] (1964) were both attempts to make programming syntax look more like English. In the 1970s, [[Pascal (programming language)|Pascal]] was developed by [[Niklaus Wirth]], and [[C (programming language)|C]] was created by [[Dennis Ritchie]] while he was working at [[Bell Laboratories]]. Wirth went on to design [[Modula-2]] and [[Oberon (programming language)|Oberon]]. For the needs of the [[United States Department of Defense]], [[Jean Ichbiah]] and a team at [[Honeywell]] began designing [[Ada (programming language)|Ada]] in 1978, after a 4-year project to define the requirements for the language. The specification was first published in 1983, with revisions in 1995, 2005, and 2012.

The 1980s saw a rapid growth in interest in [[object-oriented programming]]. These languages were imperative in style, but added features to support [[object (computing)|objects]]. The last two decades of the 20th century saw the development of many such languages. [[Smalltalk]]-80, originally conceived by [[Alan Kay]] in 1969, was released in 1980, by the Xerox Palo Alto Research Center ([[PARC (company)|PARC]]). Drawing from concepts in another object-oriented language—[[Simula]] (which is considered the world's first [[object-oriented programming language]], developed in the 1960s)—[[Bjarne Stroustrup]] designed [[C++]], an object-oriented language based on [[C (programming language)|C]]. Design of [[C++]] began in 1979 and the first implementation was completed in 1983. In the late 1980s and 1990s, the notable imperative languages drawing on object-oriented concepts were [[Perl]], released by [[Larry Wall]] in 1987; [[Python (programming language)|Python]], released by [[Guido van Rossum]] in 1990; [[Visual Basic (classic)|Visual Basic]] and [[Visual C++]] (which included [[Microsoft Foundation Class Library]] (MFC) 2.0), released by [[Microsoft]] in 1991 and 1993 respectively; [[PHP]], released by [[Rasmus Lerdorf]] in 1994; [[Java (programming language)|Java]], by [[James Gosling]] ([[Sun Microsystems]]) in 1995, [[JavaScript]], by [[Brendan Eich]] ([[Netscape]]), and [[Ruby (programming language)|Ruby]], by Yukihiro "Matz" Matsumoto, both released in 1995. Microsoft's [[.NET Framework]] (2002) is imperative at its core, as are its main target languages, [[VB.NET]] and [[C Sharp (programming language)|C#]] that run on it; however Microsoft's [[F Sharp (programming language)|F#]], a functional language, also runs on it.

==Examples==

===Fortran===
[[FORTRAN]] (1958) was unveiled as "The IBM Mathematical FORmula TRANslating system." It was designed for scientific calculations, without [[String (computer science)|string]] handling facilities. Along with [[Declaration (computer programming)|declarations]], [[Expression (computer science)|expressions]], and [[Statement (computer science)|statements]], it supported:
* [[Array data structure|arrays]]
* [[subroutine]]s 
* [[For loop|"do" loops]]

It succeeded because:
* programming and debugging costs were below computer running costs
* it was supported by IBM
* applications at the time were scientific.<ref name="cpl_3rd-ch2-16">{{cite book
  | last = Wilson
  | first = Leslie B.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 16
  | isbn = 0-201-71012-9
}}</ref>

However, non IBM vendors also wrote Fortran compilers, but with a syntax that would likely fail IBM's compiler.<ref name="cpl_3rd-ch2-16"/> The [[American National Standards Institute]] (ANSI) developed the first Fortran standard in 1966. In 1978, Fortran 77 became the standard until 1991. Fortran 90 supports:
* [[Record (computer science)|records]]
* [[Pointer (computer programming)|pointers]] to arrays

===COBOL===
[[COBOL]] (1959) stands for "COmmon Business Oriented Language." Fortran manipulated symbols. It was soon realized that symbols did not need to be numbers, so strings were introduced.<ref name="cpl_3rd-ch2-24">{{cite book
  | last = Wilson
  | first = Leslie B.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 24
  | isbn = 0-201-71012-9
}}</ref> The [[US Department of Defense]] influenced COBOL's development, with [[Grace Hopper]] being a major contributor. The statements were English-like and verbose. The goal was to design a language so managers could read the programs. However, the lack of structured statements hindered this goal.<ref name="cpl_3rd-ch2-25">{{cite book
  | last = Wilson
  | first = Leslie B.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 25
  | isbn = 0-201-71012-9
}}</ref>

COBOL's development was tightly controlled, so dialects did not emerge to require ANSI standards. As a consequence, it was not changed for 15 years until 1974. The 1990s version did make consequential changes, like [[object-oriented programming]].<ref name="cpl_3rd-ch2-25"/>

===Algol===
[[ALGOL]] (1960) stands for "ALGOrithmic Language." It had a profound influence on programming language design.<ref name="cpl_3rd-ch2-19">{{cite book
  | last = Wilson
  | first = Leslie B.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 19
  | isbn = 0-201-71012-9
}}</ref> Emerging from a committee of European and American programming language experts, it used standard mathematical notation and had a readable structured design. Algol was first to define its [[Syntax (programming languages)|syntax]] using the [[Backus–Naur form]].<ref name="cpl_3rd-ch2-19"/> This led to [[Syntax-directed translation|syntax-directed]] compilers. It added features like:
* block structure, where variables were local to their block
* arrays with variable bounds
* [[For loop|"for" loops]]
* [[Subroutine#Functions|functions]]
* [[Recursion (computer science)|recursion]]<ref name="cpl_3rd-ch2-19"/>

Algol's direct descendants include [[Pascal (programming language)|Pascal]], [[Modula-2]], [[Ada (programming language)|Ada]], [[Delphi (software)|Delphi]] and [[Oberon (programming language)|Oberon]] on one branch. On another branch there's [[C (programming language)|C]], [[C++]] and [[Java (programming language)|Java]].<ref name="cpl_3rd-ch2-19"/>

===Basic===
[[BASIC]] (1964) stands for "Beginner's All Purpose Symbolic Instruction Code." It was developed at [[Dartmouth College]] for all of their students to learn.<ref name="cpl_3rd-ch2-30">{{cite book
  | last = Wilson
  | first = Leslie B.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 30
  | isbn = 0-201-71012-9
}}</ref> If a student did not go on to a more powerful language, the student would still remember Basic.<ref name="cpl_3rd-ch2-30"/> A Basic interpreter was installed in the [[microcomputers]] manufactured in the late 1970s. As the microcomputer industry grew, so did the language.<ref name="cpl_3rd-ch2-30"/>

Basic pioneered the [[Read–eval–print loop|interactive session]].<ref name="cpl_3rd-ch2-30"/> It offered [[operating system]] commands within its environment:
* The 'new' command created an empty slate
* Statements evaluated immediately
* Statements could be programmed by preceding them with a line number
* The 'list' command displayed the program
* The 'run' command executed the program

However, the Basic syntax was too simple for large programs.<ref name="cpl_3rd-ch2-30"/> Recent dialects added structure and object-oriented extensions. [[Microsoft|Microsoft's]] [[Visual Basic]] is still widely used and produces a [[graphical user interface]].<ref name="cpl_3rd-ch2-31">{{cite book
  | last = Wilson
  | first = Leslie B.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 31
  | isbn = 0-201-71012-9
}}</ref>

===C===
[[C (programming language)|C programming language]] (1973) got its name because the language [[BCPL]] was replaced with [[B (programming language)|B]], and [[Bell Labs|AT&T Bell Labs]] called the next version "C." Its purpose was to write the [[UNIX]] [[operating system]].<ref name="cpl_3rd-ch2-37">{{cite book
  | last = Wilson
  | first = Leslie B.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 37
  | isbn = 0-201-71012-9
}}</ref> C is a relatively small language -- making it easy to write compilers. Its growth mirrored the hardware growth in the 1980s.<ref name="cpl_3rd-ch2-37"/> Its growth also was because it has the facilities of [[assembly language]], but uses a [[High-level programming language|high-level syntax]]. It added advanced features like:
* [[inline assembler]]
* arithmetic on pointers
* pointers to functions
* bit operations
* freely combining complex [[Operators in C and C++|operators]]<ref name="cpl_3rd-ch2-37"/>

[[File:Computer-memory-map.png|thumb|right|Computer memory map]]
''C'' allows the programmer to control in which region of memory data is to be stored. ''Global variables'' and ''static variables'' require the fewest [[Clock signal|clock cycles]] to store. The [[call stack|stack]] is automatically used for the standard variable [[Declaration (computer programming)|declarations]]. [[Manual memory management|Heap]] memory is returned to a [[Pointer (computer programming)|pointer variable]] from the [[C dynamic memory allocation|<code>malloc()</code>]] function.

* The ''global and static data'' region is located just above the ''program'' region. (The program region is technically called the ''text'' region. It's where machine instructions are stored.)
:* The global and static data region is technically two regions.<ref name="geeksforgeeks">{{cite web
| url = https://www.geeksforgeeks.org/memory-layout-of-c-program/
| title = Memory Layout of C Programs
| date = 12 September 2011
| access-date = 25 May 2022
| archive-date = 6 November 2021
| archive-url = https://web.archive.org/web/20211106175644/https://www.geeksforgeeks.org/memory-layout-of-c-program/
| url-status = live
}}</ref> One region is called the ''initialized [[data segment]]'', where variables declared with default values are stored. The other region is called the ''[[.bss|block started by segment]]'', where variables declared without default values are stored.
:* Variables stored in the ''global and static data'' region have their [[Memory address|addresses]] set at compile-time. They retain their values throughout the life of the process.

:* The global and static region stores the ''global variables'' that are declared on top of (outside) the <code>main()</code> function.<ref name="cpl-ch1-p31">{{cite book
 |title=The C Programming Language Second Edition
 |last1=Kernighan
 |first1=Brian W.
 |last2=Ritchie
 |first2=Dennis M.
 |publisher=Prentice Hall
 |year=1988
 |isbn=0-13-110362-8
 |page=31}}</ref> Global variables are visible to <code>main()</code> and every other function in the source code.

: On the other hand, variable declarations inside of <code>main()</code>, other functions, or within <code>{</code> <code>}</code> [[Block (programming)|block delimiters]] are ''local variables''. Local variables also include ''[[Parameter (computer programming)#Parameters and arguments|formal parameter]] variables''. Parameter variables are enclosed within the parenthesis of function definitions.<ref name="cpl_3rd-ch6-128">{{cite book
  | last = Wilson
  | first = Leslie B.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 128
  | isbn = 0-201-71012-9
}}</ref> They provide an [[Interface (computing)|interface]] to the function.

:* ''Local variables'' declared using the <code>static</code> prefix are also stored in the ''global and static data'' region.<ref name="geeksforgeeks"/> Unlike global variables, static variables are only visible within the function or block. Static variables always retain their value. An example usage would be the function <code>int increment_counter(){ static int counter = 0; counter++; return counter;}</code>

* The [[call stack|stack]] region is a contiguous block of memory located near the top memory address.<ref name="lpi-ch6-p121">{{cite book
 |title=The Linux Programming Interface
 |last=Kerrisk
 |first=Michael
 |publisher=No Starch Press
 |year=2010
 |isbn=978-1-59327-220-3
 |page=121}}</ref> Variables placed in the stack are populated from top to bottom.<ref name="lpi-ch6-p121"/> A [[Call stack#STACK-POINTER|stack pointer]] is a special-purpose [[processor register|register]] that keeps track of the last memory address populated.<ref name="lpi-ch6-p121"/> Variables are placed into the stack via the ''assembly language'' PUSH instruction. Therefore, the addresses of these variables are set during [[Runtime (program lifecycle phase)|runtime]]. The method for stack variables to lose their [[Scope (computer science)|scope]] is via the POP instruction.

:* ''Local variables'' declared without the <code>static</code> prefix, including formal parameter variables,<ref name="lpi-ch6-p122">{{cite book
 |title=The Linux Programming Interface
 |last=Kerrisk
 |first=Michael
 |publisher=No Starch Press
 |year=2010
 |isbn=978-1-59327-220-3
 |page=122}}</ref> are called ''automatic variables''<ref name="cpl-ch1-p31"/> and are stored in the stack.<ref name="geeksforgeeks"/> They are visible inside the function or block and lose their scope upon exiting the function or block.

* The [[Manual memory management|heap]] region is located below the stack.<ref name="geeksforgeeks"/> It is populated from the bottom to the top. The [[operating system]] manages the heap using a ''heap pointer'' and a list of allocated memory blocks.<ref name="cpl-ch1-p185">{{cite book
 |title=The C Programming Language Second Edition
 |last1=Kernighan
 |first1=Brian W.
 |last2=Ritchie
 |first2=Dennis M.
 |publisher=Prentice Hall
 |year=1988
 |isbn=0-13-110362-8
 |page=185}}</ref> Like the stack, the addresses of heap variables are set during runtime. An [[out of memory]] error occurs when the heap pointer and the stack pointer meet.

:* ''C'' provides the <code>malloc()</code> library function to [[C dynamic memory allocation|allocate]] heap memory.<ref name="cpl-ch8-p187">{{cite book
 |title=The C Programming Language Second Edition
 |last1=Kernighan
 |first1=Brian W.
 |last2=Ritchie
 |first2=Dennis M.
 |publisher=Prentice Hall
 |year=1988
 |isbn=0-13-110362-8
 |page=187}}</ref> Populating the heap with data is an additional copy function. Variables stored in the heap are economically passed to functions using pointers. Without pointers, the entire block of data would have to be passed to the function via the stack.

===C++===
In the 1970s, [[software engineering|software engineers]] needed language support to break large projects down into [[Modular programming|modules]].<ref name="cpl_3rd-ch2-38">{{cite book
  | last = Wilson
  | first = Leslie B.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 38
  | isbn = 0-201-71012-9
}}</ref> One obvious feature was to decompose large projects ''physically'' into separate [[computer file|files]]. A less obvious feature was to decompose large projects ''logically'' into [[Abstract and concrete|abstract]] [[Data type|datatypes]].<ref name="cpl_3rd-ch2-38"/> At the time, languages supported concrete ([[Variable (computer science)|scalar]]) datatypes like [[integer]] numbers, [[Floating-point arithmetic|floating-point]] numbers, and [[String (computer science)|strings]] of [[Character (computing)|characters]]. Concrete datatypes have their representation as part of their name.<ref name="stroustrup-ch3-65">{{cite book
  | last = Stroustrup
  | first = Bjarne
  | title = The C++ Programming Language, Fourth Edition
  | publisher = Addison-Wesley
  | year = 2013
  | page = 65
  | isbn = 978-0-321-56384-2
}}</ref> Abstract datatypes are [[Record (computer science)|structures]] of concrete datatypes — with a new name assigned. For example, a [[List (abstract data type)|list]] of integers could be called <code>integer_list</code>.

In object-oriented jargon, abstract datatypes are called [[Class (computer programming)|classes]]. However, a ''class'' is only a definition; no memory is allocated. When memory is allocated to a class, it's called an [[Object (computer science)|object]].<ref name="cpl_3rd-ch8-193">{{cite book
  | last = Wilson
  | first = Leslie B.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 193
  | isbn = 0-201-71012-9
}}</ref>

''[[Object-oriented programming|Object-oriented imperative languages]]'' developed by combining the need for classes and the need for safe [[functional programming]].<ref name="cpl_3rd-ch2-39">{{cite book
  | last = Wilson
  | first = Leslie B.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 39
  | isbn = 0-201-71012-9
}}</ref> A function, in an object-oriented language, is assigned to a class. An assigned function is then referred to as a [[Method (computer programming)|method]], [[Method (computer programming)#Member functions in C++|member function]], or ''[[Operation (mathematics)|operation]]''. ''Object-oriented programming'' is executing ''operations'' on ''objects''.<ref name="cpl_3rd-ch2-35">{{cite book
  | last = Wilson
  | first = Leslie B.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 35
  | isbn = 0-201-71012-9
}}</ref>

''Object-oriented languages'' support a syntax to model [[subset|subset/superset]] relationships. In [[set theory]], an [[Element (mathematics)|element]] of a subset inherits all the attributes contained in the superset. For example, a student is a person. Therefore, the set of students is a subset of the set of persons. As a result, students inherit all the attributes common to all persons. Additionally, students have unique attributes that other persons don't have. ''Object-oriented languages'' model ''subset/superset'' relationships using [[Inheritance (object-oriented programming)|inheritance]].<ref name="cpl_3rd-ch8-192">{{cite book
  | last = Wilson
  | first = Leslie B.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 192
  | isbn = 0-201-71012-9
}}</ref> ''Object-oriented programming'' became the dominant language paradigm by the late 1990s.<ref name="cpl_3rd-ch2-38"/>

[[C++]] (1985) was originally called "C with Classes."<ref name="stroustrup-notes-22">{{cite book
  | last = Stroustrup
  | first = Bjarne
  | title = The C++ Programming Language, Fourth Edition
  | publisher = Addison-Wesley
  | year = 2013
  | page = 22
  | isbn = 978-0-321-56384-2
}}</ref> It was designed to expand [[C (programming language)|C's]] capabilities by adding the object-oriented facilities of the language [[Simula]].<ref name="stroustrup-notes-21">{{cite book
  | last = Stroustrup
  | first = Bjarne
  | title = The C++ Programming Language, Fourth Edition
  | publisher = Addison-Wesley
  | year = 2013
  | page = 21
  | isbn = 978-0-321-56384-2
}}</ref>

An object-oriented module is composed of two files. The definitions file is called the [[Include directive|header file]]. Here is a C++ ''header file'' for the ''GRADE class'' in a simple school application:

<syntaxhighlight lang="cpp">
// grade.h
// -------

// Used to allow multiple source files to include
// this header file without duplication errors.
// See: https://en.wikipedia.org/wiki/Include_guard
// ----------------------------------------------
#ifndef GRADE_H
#define GRADE_H

class GRADE {
public:
    // This is the constructor operation.
    // ----------------------------------
    GRADE ( const char letter );

    // This is a class variable.
    // -------------------------
    char letter;

    // This is a member operation.
    // ---------------------------
    int grade_numeric( const char letter );

    // This is a class variable.
    // -------------------------
    int numeric;
};
#endif
</syntaxhighlight>

A [[Constructor (object-oriented programming)|constructor]] operation is a function with the same name as the class name.<ref name="stroustrup-ch2-49">{{cite book
  | last = Stroustrup
  | first = Bjarne
  | title = The C++ Programming Language, Fourth Edition
  | publisher = Addison-Wesley
  | year = 2013
  | page = 49
  | isbn = 978-0-321-56384-2
}}</ref> It is executed when the calling operation executes the <code>new</code> statement.

A module's other file is the ''[[source code|source file]]''. Here is a C++ source file for the ''GRADE class'' in a simple school application:

<syntaxhighlight lang="cpp">
// grade.cpp
// ---------
#include "grade.h"

GRADE::GRADE( const char letter )
{
    // Reference the object using the keyword 'this'.
    // ----------------------------------------------
    this->letter = letter;

    // This is Temporal Cohesion
    // -------------------------
    this->numeric = grade_numeric( letter );
}

int GRADE::grade_numeric( const char letter )
{
    if ( ( letter == 'A' || letter == 'a' ) )
        return 4;
    else
    if ( ( letter == 'B' || letter == 'b' ) )
        return 3;
    else
    if ( ( letter == 'C' || letter == 'c' ) )
        return 2;
    else
    if ( ( letter == 'D' || letter == 'd' ) )
        return 1;
    else
    if ( ( letter == 'F' || letter == 'f' ) )
        return 0;
    else
        return -1;
}
</syntaxhighlight>

Here is a C++ ''header file'' for the ''PERSON class'' in a simple school application:

<syntaxhighlight lang="cpp">
// person.h
// --------
#ifndef PERSON_H
#define PERSON_H

class PERSON {
public:
    PERSON ( const char *name );
    const char *name;
};
#endif
</syntaxhighlight>

Here is a C++ ''source file'' for the ''PERSON class'' in a simple school application:

<syntaxhighlight lang="cpp">
// person.cpp
// ----------
#include "person.h"

PERSON::PERSON ( const char *name )
{
    this->name = name;
}
</syntaxhighlight>

Here is a C++ ''header file'' for the ''STUDENT class'' in a simple school application:

<syntaxhighlight lang="cpp">
// student.h
// ---------
#ifndef STUDENT_H
#define STUDENT_H

#include "person.h"
#include "grade.h"

// A STUDENT is a subset of PERSON.
// --------------------------------
class STUDENT : public PERSON{
public:
    STUDENT ( const char *name );
    ~STUDENT();
    GRADE *grade;
};
#endif
</syntaxhighlight>

Here is a C++ ''source file'' for the ''STUDENT class'' in a simple school application:

<syntaxhighlight lang="cpp">
// student.cpp
// -----------
#include "student.h"
#include "person.h"

STUDENT::STUDENT ( const char *name ):
    // Execute the constructor of the PERSON superclass.
    // -------------------------------------------------
    PERSON( name )
{
    // Nothing else to do.
    // -------------------
}

STUDENT::~STUDENT() 
{
    // deallocate grade's memory
    // to avoid memory leaks.
    // -------------------------------------------------
    delete this->grade;
}
</syntaxhighlight>

Here is a driver program for demonstration:

<syntaxhighlight lang="cpp">
// student_dvr.cpp
// ---------------
#include <iostream>
#include "student.h"

int main( void )
{
    STUDENT *student = new STUDENT( "The Student" );
    student->grade = new GRADE( 'a' );

    std::cout 
        // Notice student inherits PERSON's name
        << student->name
        << ": Numeric grade = "
        << student->grade->numeric
        << "\n";

    // deallocate student's memory
    // to avoid memory leaks.
    // -------------------------------------------------
    delete student;

	return 0;
}
</syntaxhighlight>

Here is a [[makefile]] to compile everything:

<syntaxhighlight lang="make">
# makefile
# --------
all: student_dvr

clean:
    rm student_dvr *.o

student_dvr: student_dvr.cpp grade.o student.o person.o
    c++ student_dvr.cpp grade.o student.o person.o -o student_dvr

grade.o: grade.cpp grade.h
    c++ -c grade.cpp

student.o: student.cpp student.h
    c++ -c student.cpp

person.o: person.cpp person.h
    c++ -c person.cpp
</syntaxhighlight>

==See also==
* [[Functional programming]]
* [[Reactive programming]]
* [[History of programming languages]]
* [[List of programming languages by category#Imperative languages|List of imperative programming languages]]

==Notes==
{{Reflist|group=note}}

==References==
{{Reflist}}

* Pratt, Terrence W. and [[Marvin Victor Zelkowitz|Marvin V. Zelkowitz]]. ''Programming Languages: Design and Implementation'', 3rd ed. Englewood Cliffs, N.J.: Prentice Hall, 1996.
* Sebesta, Robert W. ''Concepts of Programming Languages'', 3rd ed. Reading, Mass.: Addison-Wesley Publishing Company, 1996.

<!-- Reduced license notice by special permission of Stan Seibert, see Talk -->
: ''Originally based on the article 'Imperative programming' by Stan Seibert, from [[Nupedia]], licensed under the [[GNU Free Documentation License]].''

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