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C++ Keywords
  • Last Updated : 17 May, 2021

C++ is a powerful language. In C++ we can write structured programs and object-oriented programs also. C++ is a superset of C and therefore most constructs of C are legal in C++ with their meaning unchanged. However, there are some exceptions and additions.

Token: When the compiler is processing the source code of a C++ program, each group of characters separated by white space is called a token. Tokens are the smallest individual units in a program. A C++ program is written using tokens. It has the following tokens:

Keywords (also known as reserved words) have special meaning to the C++ compiler and are always written or typed in short(lower) cases. Keywords are words that the language uses for a special purpose, such as void, int, public, etc. It can’t be used for a variable name or function name. Below is the table for the complete set of C++ keywords.

C++ Keyword



asmdoublenewswitch
autoelseoperatortemplate
breakenumprivatethis
caseexternprotectedthrow
catchfloatpublictry
charforregistertypedef
classfriendreturnunion
constgotoshortunsigned
continueifsignedvirtual
defaultinlinesizedofvoid
deleteintstaticvolatile 
dolongstructwhile 

Note: The keywords not found in ANSI C are shown here in boldface.

  • asm: To declare that a block of code is to be passed to the assembler.
  • auto: A storage class specifier that is used to define objects in a block.
  • break: Terminates a switch statement or a loop.
  • case: Used specifically within a switch statement to specify a match for the statement’s expression.
  • catch: Specifies actions taken when an exception occurs.
  • char: Fundamental data type that defines character objects.
  • class: To declare a user-defined type that encapsulates data members and operations or member functions.
  • const: To define objects whose value will not alter throughout the lifetime of program execution.
  • continue:- Transfers control to the start of a loop.
  • default:- Handles expression values in a switch statement that are not handled by case.
  • delete: Memory deallocation operator.
  • do: indicate the start of a do-while statement in which the sub-statement is executed repeatedly until the value of the expression is logical-false.
  • double:  Fundamental data type used to define a floating-point number.
  • else: Used specifically in an if-else statement.
  • enum: To declare a user-defined enumeration data type.
  • extern: An identifier specified as extern has external linkage to the block.
  • float:- Fundamental data type used to define a floating-point number.
  • for: Indicates the start of a statement to achieve repetitive control.
  • friend: A class or operation whose implementation can access the private data members of a class.
  • goto: Transfer control to a specified label.
  • if: Indicate the start of an if statement to achieve selective control.
  • inline: A function specifier that indicates to the compiler that inline substitution of the function body is to be preferred to the usual function call implementation.
  • int: Fundamental data type used to define integer objects.
  • long: A data type modifier that defines a 32-bit int or an extended double.
  • new: Memory allocation operator.
  • operator: Overloads a c++ operator with a new declaration.
  • private: Declares class members which are not visible outside the class.
  • protected: Declares class members which are private except to derived classes
  • public: Declares class members who are visible outside the class.
  • register: A storage class specifier that is an auto specifier, but which also indicates to the compiler that an object will be frequently used and should therefore be kept in a register.
  • return: Returns an object to a function’s caller.
  • short: A data type modifier that defines a 16-bit int number.
  • signed: A data type modifier that indicates an object’s sign is to be stored in the high-order bit.
  • sizeof: Returns the size of an object in bytes.
  • static: The lifetime of an object-defined static exists throughout the lifetime of program execution.
  • struct: To declare new types that encapsulate both data and member functions.
  • switch: This keyword used in the “Switch statement”.
  • template: parameterized or generic type.
  • this:  A class pointer points to an object or instance of the class.
  • throw: Generate an exception.
  • try: Indicates the start of a block of exception handlers.
  • typedef: Synonym for another integral or user-defined type.
  • union: Similar to a structure, struct, in that it can hold different types of data, but a union can hold only one of its members at a given time.
  • unsigned: A data type modifier that indicates the high-order bit is to be used for an object.
  • virtual: A function specifier that declares a member function of a class that will be redefined by a derived class.
  • void: Absent of a type or function parameter list.
  • volatile: Define an object which may vary in value in a way that is undetectable to the compiler.
  • while: Start of a while statement and end of a do-while statement.

What is identifier and how it is different from keywords:

Identifiers refer to the name of variables, functions, arrays, classes, etc. created by the programmer. They are the fundamental requirement of any language.

Rules for naming identifiers:

  • Identifier name can not start with a digit or any special character.
  • A keyword cannot be used as s identifier name.
  • Only alphabetic characters, digits, and underscores are permitted.
  • The upper case and lower case letters are distinct. i.e., A and a are different in C++.
  • The valid identifiers are GFG, gfg, geeks_for_geeks.

Program 1:

C++




// C++ program to illustrate the use
// of identifiers
 
#include <iostream>
using namespace std;
 
// Driver Code
int main()
{
    // Use of Underscore (_) symbol
    // in variable declaration
    int geeks_for_geeks = 1;
 
    cout << "Identifier result is: "
         << geeks_for_geeks;
 
    return 0;
}
Output: 
Identifier result is: 1

 

Now, the question arises how keywords are different from identifiers?

So there are some main properties of keywords that distinguish keywords from identifiers:

  • Keywords are predefined/reserved words and identifiers are the values used to define different programming items like a variable, integers, structures, unions.
  • Keywords always start with lowercase whereas identifier can start with the uppercase letter as well as a lowercase letter.
  • A keyword contains only alphabetical characters, but an identifier can consist of alphabetical characters, digits, and underscores.
  • No special symbol, punctuations used in keywords and identifiers. The only underscore can be used in an identifier.
  • Example of keywords and identifiers:
    • Keywords: int, char, while, do.
    • Identifiers: Geeks_for_Geeks, GFG, Gfg1.

Program 2:

Below is the program for how to use different keywords in the program:

C++




// C++ Program to demonstrate keywords
#include <iostream>
using namespace std;
 
// Driver Code
int main()
{
    // Variable declaration and
    // initialization
    int n = 2;
 
    // Switch Case Statement
    switch (n) {
    case 1:
        cout << "Computer Network"
             << endl;
        break;
    case 2:
        cout << "C++" << endl;
        break;
    case 3:
        cout << "DBMS" << endl;
        break;
    case 4:
        cout << "Data Struture"
             << endl;
        break;
    case 5:
        cout << "Operating System"
             << endl;
        break;
    default:
        cout << "Enter Valid number"
             << endl;
    }
 
    // Return keyword returns an object
    // to a function's caller
    return 0;
}
Output: 
C++

 

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