Declaration of variables

C ++ Tutorial
o	INTRODUCTION
	Instructions for use
o	BASICS OF C++
	Structure of a program
	Variables Data Types
	Declaration of variables
	Initialization of variables
	Constants
	Operators
	Basic Input / Output
o	CONTROL STRUCTURES
	Control Structures
	Functions (I)
	Functions (II)
	Overloaded functions
o	COMPOUND DATA TYPES
	Arrays
	Character Sequences
	Pointers
	Dynamic Memory
	Data structures
	Other Data Types
o	OBJECT ORIENTED PROGRAMMING
	Classes (I)
	Classes (II)
	Friendship and inheritance
	Polymorphism
o	ADVANCED CONCEPTS
	Templates
	Namespaces
	Exceptions
	Type Casting
	Preprocessor directives
o	C++ STANDARD LIBRARY
	Input/Output with files

C++ Language Tutorial

Basics of C++

In order to use a variable in C++, we must first declare it specifying which data type we want it to be. The syntax to declare a new variable is to write the specifier of the desired data type (like int, bool, float...) followed by a valid variable identifier. For example:

int a;
float mynumber;

These are two valid declarations of variables. The first one declares a variable of type int with the identifier a. The second one declares a variable of type float with the identifier mynumber. Once declared, the variables a and mynumber can be used within the rest of their scope in the program.

If you are going to declare more than one variable of the same type, you can declare all of them in a single statement by separating their identifiers with commas. For example:

int a, b, c;

This declares three variables (a, b and c), all of them of type int, and has exactly the same meaning as:

int a;
int b;
int c;

The integer data types char, short, long and int can be either signed or unsigned depending on the range of numbers needed to be represented. Signed types can represent both positive and negative values, whereas unsigned types can only represent positive values (and zero). This can be specified by using either the specifier signed or the specifier unsigned before the type name. For example:

unsigned short int NumberOfSisters;
signed int MyAccountBalance;

By default, if we do not specify either signed or unsigned most compiler settings will assume the type to be signed, therefore instead of the second declaration above we could have written:

int MyAccountBalance;

with exactly the same meaning (with or without the keyword signed)

An exception to this general rule is the char type, which exists by itself and is considered a different fundamental data type from signed char and unsigned char, thought to store characters. You should use either signed or unsigned if you intend to store numerical values in a char-sized variable.

short and long can be used alone as type specifiers. In this case, they refer to their respective integer
fundamental types: short is equivalent to short int and long is equivalent to long int. The following two
variable declarations are equivalent:

short Year;
short int Year;

Finally, signed and unsigned may also be used as standalone type specifiers, meaning the same as signed int and unsigned int respectively. The following two declarations are equivalent:

unsigned NextYear;
unsigned int NextYear;

To see what variable declarations look like in action within a program, we are going to see the C++ code of the example about your mental memory proposed at the beginning of this section:

Example:	Output:
// operating with variables #include <iostream> using namespace std; int main () { // declaring variables: int a, b; int result; // process: a = 5; b = 2; a = a + 1; result = a - b; // print out the result: cout << result; // terminate the program: return 0; }	4

Do not worry if something else than the variable declarations themselves looks a bit strange to you. You will see the rest in detail in coming sections.

Scope of variables

All the variables that we intend to use in a program must have been declared with its type specifier in an earlier point in the code, like we did in the previous code at the beginning of the body of the function main when we declared that a, b, and result were of type int.

A variable can be either of global or local scope. A global variable is a variable declared in the main body of the source code, outside all functions, while a local variable is one declared within the body of a function or a block.

Global variables can be referred from anywhere in the code, even inside functions, whenever it is after its declaration.

The scope of local variables is limited to the block enclosed in braces ({}) where they are declared. For example, if they are declared at the beginning of the body of a function (like in function main) their scope is between its declaration point and the end of that function. In the example above, this means that if another function existed in addition to main, the local variables declared in main could not be accessed from the other function and vice versa.

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