Standard Conversion and Promotions:

Standard Conversion and Promotions:

           When there are two operands of different types in an expression, the lower-type variable is converted to the type of the higher-type variable. For example, if one operand is of type int and other is of float, both are converted into float type. Thus, if data types are mixed in an expression, C++ performs the conversions automatically. This process is called implicit or automatic conversion. An example:

 

void main()

   {   int a=10;

    float b=40.5, result;

    result=a+b;

    cout<<"The sum is "<<result;    }

Output:

   The sum is 50.5.

                  In this case, variable a is of type int while b is of type float. While solving the expression a+b, the temporary variable of type float is created to store value of a and then two float values are added. The result is of type float and stored in variable result. Here, data type int is promoted to float. Thus, it is also known as standard data type promotion.

          The other implicit conversions are

      Int and float      -> float

      Int and long       -> long

      Int and double    -> double

      Long and float    -> float

      Float and double -> double

But, sometime programmer needs to convert a value from one type to another explicitly in a situation where the compiler will not do it automatically. This type of conversion is called explicit conversion or type casting. An example:

  void main()

   {    int a=4000;

    long  result;

    result=a*100;

    cout<<"The product is "<<result;      }      

Output:

         The product is 6784  // Error!!!!!

The corrected version is:

 void main()

   {    int a=4000;

    long  result;

    result=long(a)*100;  //type casting

    cout<<"The product is "<<result;      }     

Output:

  The result is 400000  // correct

             Here, while solving the expression long(a)*100 in above program, integer type variable a is converted into long type. For this, a temporary variable of type long is created to store value of a and this long value is multiplied by 100 and result is also of type long. The long result is stored in variable result.

                 Without casting, the result is not correct. This is because multiplication of two integers (i.e. a and 100) results integer value (i.e. 4000000). But this integer value is beyond the range of integer. Thus, garbage value is stored in result variable.