Variables and Assignment

In algebra, variables are used to represent numbers. The same is true in C++, except C++ variables also can represent values other than numbers. variable.cpp uses a variable to store an integer value and then prints the value of the variable.

[code type="Variable.cpp"]#include <iostream>
using namespace std;
int main() {
int x;
x = 10;
cout << x << endl;
}
[/code]

• int x;

This is a declaration statement. All variables in a C++ program must be declared. A declaration specifies the type of a variable. The word int indicates that the variable is an integer. The name of the integer variable is x. We say that variable x has type int. C++ supports types other than integers, and some types require more or less space in the computer’s memory. The compiler uses the declaration to reserve the proper amount of memory to store the variable’s value. The declaration enables the compiler to verify the programmer is using the variable properly within the program; for example, we will see that integers can be added together just like in mathematics. For some other data types, however, addition is not possible and so is not allowed. The compiler can ensure that a variable involved in an addition operation is compatible with addition. It can report an error if it is not. The compiler will issue an error if a programmer attempts to use an undeclared variable. The compiler cannot deduce the storage requirements and cannot verify the variable’s proper usage if it not declared. Once declared, a particular variable cannot be redeclared in the same context. A variable
may not change its type during its lifetime.

• x = 10;

This is an assignment statement. An assignment statement associates a value with a variable. The key to an assignment statement is the symbol = which is known as the assignment operator. Here the value 10 is being assigned to the variable x. This means the value 10 will be stored in the memory
location the compiler has reserved for the variable named x. We need not be concerned about where
the variable is stored in memory; the compiler takes care of that detail. After we declare a variable we may assign and reassign it as often as necessary. 

• cout << x << endl;

This statement prints the variable x’s current value.

The meaning of the assignment operator (=) is different from equality in mathematics. In mathematics, = asserts that the expression on its left is equal to the expression on its right. In C++, = makes the variable on its left take on the value of the expression on its right. It is best to read x = 5 as “x is assigned the value 5,” or “x gets the value 5.” This distinction is important since in mathematics equality is symmetric: if x = 5, we know 5 = x. In C++, this symmetry does not exist; the statement

5 = x;

attempts to reassign the value of the literal integer value 5, but this cannot be done, because 5 is always 5 and cannot be changed. Such a statement will produce a compiler error.