C++ Program Structure
Let us look at a simple code that would print the words Hello World.
#include <iostream>
using namespace std;
// main() is where program execution begins.
int main()
The line using namespace std; tells the compiler to use the std namespace. Namespaces are a relatively recent addition to C++.The next line '// main() is where program execution begins.' is a single-line comment available in C++. Single-line comments begin with // and stop at the end of the line.
The line int main() is the main function where program execution begins.
The next line cout << "Hello World"; causes the message "Hello World" to be displayed on the screen.
The next line return 0; terminates main( )function and causes it to return the value 0 to the calling process.
Compile and Execute C++ Program
Let's look at how to save the file, compile and run the program. Please follow the steps given below −
Open a text editor and add the code as above.
Save the file as: hello.cpp
Open a command prompt and go to the directory where you saved the file.
Type 'g++ hello.cpp' and press enter to compile your code. If there are no errors in your code the command prompt will take you to the next line and would generate a.out executable file.
Now, type 'a.out' to run your program.
You will be able to see ' Hello World ' printed on the window.
$ g++ hello.cpp
$ ./a.out
Hello World
Make sure that g++ is in your path and that you are running it in the directory containing file hello.cpp.
You can compile C/C++ programs using makefile. For more details, you can check our 'Makefile Tutorial'.
Semicolons and Blocks in C++
In C++, the semicolon is a statement terminator. That is, each individual statement must be ended with a semicolon. It indicates the end of one logical entity.
For example, following are three different statements −
x = y;
y = y + 1;
add(x, y);
A block is a set of logically connected statements that are surrounded by opening and closing braces. For example −
{
cout << "Hello World"; // prints Hello World
return 0;
}
C++ does not recognize the end of the line as a terminator. For this reason, it does not matter where you put a statement in a line. For example −
x = y;
y = y + 1;
add(x, y);
is the same as
x = y; y = y + 1; add(x, y);
C++ Identifiers
A C++ identifier is a name used to identify a variable, function, class, module, or any other user-defined item. An identifier starts with a letter A to Z or a to z or an underscore (_) followed by zero or more letters, underscores, and digits (0 to 9).
C++ does not allow punctuation characters such as @, $, and % within identifiers. C++ is a case-sensitive programming language. Thus, Manpower and manpower are two different identifiers in C++.
Here are some examples of acceptable identifiers −
mohd zara abc move_name a_123
myname50 _temp j a23b9 retVal
C++ Keywords
The following list shows the reserved words in C++. These reserved words may not be used as constant or variable or any other identifier names.
Trigraphs
A few characters have an alternative representation, called a trigraph sequence. A trigraph is a three-character sequence that represents a single character and the sequence always starts with two question marks.
Trigraphs are expanded anywhere they appear, including within string literals and character literals, in comments, and in preprocessor directives.
Following are most frequently used trigraph sequences −
All the compilers do not support trigraphs and they are not advised to be used because of their confusing nature.
Whitespace in C++
A line containing only whitespace, possibly with a comment, is known as a blank line, and C++ compiler totally ignores it.
Whitespace is the term used in C++ to describe blanks, tabs, newline characters and comments. Whitespace separates one part of a statement from another and enables the compiler to identify where one element in a statement, such as int, ends and the next element begins.
Statement 1
int age;
In the above statement there must be at least one whitespace character (usually a space) between int and age for the compiler to be able to distinguish them.
Statement 2
fruit = apples + oranges; // Get the total fruit
In the above statement 2, no whitespace characters are necessary between fruit and =, or between = and apples, although you are free to include some if you wish for readability purpose.
Comments in C++
Program comments are explanatory statements that you can include in the C++ code. These comments help anyone reading the source code. All programming languages allow for some form of comments.
C++ supports single-line and multi-line comments. All characters available inside any comment are ignored by C++ compiler.
C++ comments start with /* and end with */. For example −
/* This is a comment */
/* C++ comments can also
* span multiple lines
*/
A comment can also start with //, extending to the end of the line. For example −
#include <iostream>
using namespace std;
main() {
cout << "Hello World"; // prints Hello World
return 0;
}
When the above code is compiled, it will ignore // prints Hello World and final executable will produce the following result −
Hello World
Within a /* and */ comment, // characters have no special meaning. Within a // comment, /* and */ have no special meaning. Thus, you can "nest" one kind of comment within the other kind. For example −
/* Comment out printing of Hello World:
cout << "Hello World"; // prints Hello World
*/
C++ Data Types
While writing program in any language, you need to use various variables to store various information. Variables are nothing but reserved memory locations to store values. This means that when you create a variable you reserve some space in memory.
You may like to store information of various data types like character, wide character, integer, floating point, double floating point, boolean etc. Based on the data type of a variable, the operating system allocates memory and decides what can be stored in the reserved memory.
Primitive Built-in Types
C++ offers the programmer a rich assortment of built-in as well as user defined data types. Following table lists down seven basic C++ data types −
Several of the basic types can be modified using one or more of these type modifiers −
- signed
- unsigned
- short
- long
The following table shows the variable type, how much memory it takes to store the value in memory, and what is maximum and minimum value which can be stored in such type of variables.
IThe size of variables might be different from those shown in the above table, depending on the compiler and the computer you are using.
Following is the example, which will produce correct size of various data types on your computer.
Live Demo
#include <iostream>
using namespace std;
int main()
{
cout << "Size of char : " << sizeof(char) << endl;
cout << "Size of int : " << sizeof(int) << endl;
cout << "Size of short int : " << sizeof(short int) << endl;
cout << "Size of long int : " << sizeof(long int) << endl;
cout << "Size of float : " << sizeof(float) << endl;
cout << "Size of double : " << sizeof(double) << endl;
cout << "Size of wchar_t : " << sizeof(wchar_t) << endl;
return 0;
}
This example uses endl, which inserts a new-line character after every line and << operator is being used to pass multiple values out to the screen. We are also using sizeof() operator to get size of various data types.
When the above code is compiled and executed, it produces the following result which can vary from machine to machine −
Size of char : 1
Size of int : 4
Size of short int : 2
Size of long int : 4
Size of float : 4
Size of double : 8
Size of wchar_t : 4
typedef Declarations
You can create a new name for an existing type using typedef. Following is the simple syntax to define a new type using typedef −
typedef type newname;
For example, the following tells the compiler that feet is another name for int −
typedef int feet;
Now, the following declaration is perfectly legal and creates an integer variable called distance −
feet distance;
Enumerated Types
An enumerated type declares an optional type name and a set of zero or more identifiers that can be used as values of the type. Each enumerator is a constant whose type is the enumeration.
Creating an enumeration requires the use of the keyword enum. The general form of an enumeration type is −
enum enum-name { list of names } var-list;
Here, the enum-name is the enumeration's type name. The list of names is comma separated.
For example, the following code defines an enumeration of colors called colors and the variable c of type color. Finally, c is assigned the value "blue".
enum color { red, green, blue } c;
c = blue;
By default, the value of the first name is 0, the second name has the value 1, and the third has the value 2, and so on. But you can give a name, a specific value by adding an initializer. For example, in the following enumeration, green will have the value 5.
enum color { red, green = 5, blue };
Here, blue will have a value of 6 because each name will be one greater than the one that precedes it.
C++ Variable Types
A variable provides us with named storage that our programs can manipulate. Each variable in C++ has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.
The name of a variable can be composed of letters, digits, and the underscore character. It must begin with either a letter or an underscore. Upper and lowercase letters are distinct because C++ is case-sensitive −
There are following basic types of variable in C++ as explained in last chapter −
C++ also allows to define various other types of variables, which we will cover in subsequent chapters like Enumeration, Pointer, Array, Reference, Data structures, and Classes.
Following section will cover how to define, declare and use various types of variables.
Variable Definition in C++
A variable definition tells the compiler where and how much storage to create for the variable. A variable definition specifies a data type, and contains a list of one or more variables of that type as follows −
type variable_list;
Here, type must be a valid C++ data type including char, w_char, int, float, double, bool or any user-defined object, etc., and variable_list may consist of one or more identifier names separated by commas. Some valid declarations are shown here −
int i, j, k;
char c, ch;
float f, salary;
double d;
The line int i, j, k; both declares and defines the variables i, j and k; which instructs the compiler to create variables named i, j and k of type int.
Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of an equal sign followed by a constant expression as follows −
type variable_name = value;
Some examples are −
extern int d = 3, f = 5; // declaration of d and f.
int d = 3, f = 5; // definition and initializing d and f.
byte z = 22; // definition and initializes z.
char x = 'x'; // the variable x has the value 'x'.
For definition without an initializer: variables with static storage duration are implicitly initialized with NULL (all bytes have the value 0); the initial value of all other variables is undefined.
Variable Declaration in C++
A variable declaration provides assurance to the compiler that there is one variable existing with the given type and name so that compiler proceed for further compilation without needing complete detail about the variable. A variable declaration has its meaning at the time of compilation only, compiler needs actual variable definition at the time of linking of the program.
A variable declaration is useful when you are using multiple files and you define your variable in one of the files which will be available at the time of linking of the program. You will use extern keyword to declare a variable at any place. Though you can declare a variable multiple times in your C++ program, but it can be defined only once in a file, a function or a block of code.
Example
Try the following example where a variable has been declared at the top, but it has been defined inside the main function −
Live Demo
#include <iostream>
using namespace std;
// Variable declaration:
extern int a, b;
extern int c;
extern float f;
int main () {
// Variable definition:
int a, b;
int c;
float f;
// actual initialization
a = 10;
b = 20;
c = a + b;
cout << c << endl ;
f = 70.0/3.0;
cout << f << endl ;
return 0;
}
When the above code is compiled and executed, it produces the following result −
30
23.3333
Same concept applies on function declaration where you provide a function name at the time of its declaration and its actual definition can be given anywhere else. For example −
// function declaration
int func();
int main() {
// function call
int i = func();
}
// function definition
int func() {
return 0;
}
Lvalues and Rvalues
There are two kinds of expressions in C++ −
lvalue − Expressions that refer to a memory location is called "lvalue" expression. An lvalue may appear as either the left-hand or right-hand side of an assignment.
rvalue − The term rvalue refers to a data value that is stored at some address in memory. An rvalue is an expression that cannot have a value assigned to it which means an rvalue may appear on the right- but not left-hand side of an assignment.
Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric literals are rvalues and so may not be assigned and can not appear on the left-hand side. Following is a valid statement −
int g = 20;
But the following is not a valid statement and would generate compile-time error −
10 = 20;
Variable Scope in C++
A scope is a region of the program and broadly speaking there are three places, where variables can be declared −
Inside a function or a block which is called local variables,
In the definition of function parameters which is called formal parameters.
Outside of all functions which is called global variables.
We will learn what is a function and it's parameter in subsequent chapters. Here let us explain what are local and global variables.
Local Variables
Variables that are declared inside a function or block are local variables. They can be used only by statements that are inside that function or block of code. Local variables are not known to functions outside their own. Following is the example using local variables −
Live Demo
#include <iostream>
using namespace std;
int main () {
// Local variable declaration:
int a, b;
int c;
// actual initialization
a = 10;
b = 20;
c = a + b;
cout << c;
return 0;
}
Global Variables
Global variables are defined outside of all the functions, usually on top of the program. The global variables will hold their value throughout the life-time of your program.
A global variable can be accessed by any function. That is, a global variable is available for use throughout your entire program after its declaration. Following is the example using global and local variables −
Live Demo
#include <iostream>
using namespace std;
// Global variable declaration:
int g;
int main () {
// Local variable declaration:
int a, b;
// actual initialization
a = 10;
b = 20;
g = a + b;
cout << g;
return 0;
}
A program can have same name for local and global variables but value of local variable inside a function will take preference. For example −
Live Demo
#include <iostream>
using namespace std;
// Global variable declaration:
int g = 20;
int main () {
// Local variable declaration:
int g = 10;
cout << g;
return 0;
}
When the above code is compiled and executed, it produces the following result −
10
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