1. What are they? Why do we need them?
Casts
What are they? Why do we need them?
Copyright Curt Hill

2. Mixed Mode Arithmetic Multiple types in one arithmetic expression
3.5 * 2
A real and integer constant
Most programming languages allow some form of mixed mode
C and C++ are quite liberal
Java somewhat more restrictive
Copyright Curt Hill

3. Better Example
Suppose: int a=2, b; double d; cin >> b >> d; d = d*b – a; a = b*d – a;
What is the difference?
One loses information and the other does not!
Copyright Curt Hill

4. Conversions C++ will perform certain conversions automatically
Converting an integer value to a double value is automatic
Doubles are stronger
This conversion does not lose anything
The conversion from double to int will also be done automatically
Even though information is lost
Copyright Curt Hill

5. When is the conversion done?
Consider the following: int a=2, b=3; double d = 3; d = a/b;
What do we get?
From an algebra view: 2/3 should be .6666…
Actually we get zero
Copyright Curt Hill

6. Why zero?
C++ only looks at the immediate pair of operands and refuses to look ahead
So it first does a/b, which is 2/3
Since both are integers, we do integer division
This gives a zero
Copyright Curt Hill

7. Observations C++ is very short sighted
Even though the resulting expression of: d = a/b*d; is double, it does no conversion until needed
It avoids some thornier problems by doing so
Copyright Curt Hill

8. So what is the fix? How do we get 2/3 instead of zero? A cast
A cast is a conversion of type from one type to another
The variable type is not changed!
We extract the value and change the type of that value
Copyright Curt Hill

9. Styles of Casts
The compiler will do a cast automatically for mixed mode arithmetic
For operators convert weaker to stronger
For assignments convert to receiving type
C style cast
C++ style cast
Copyright Curt Hill

10. The C Cast The form of a cast is the type in parenthesis
This is a unary operator, it takes the operand on its right and converts the value into the appropriate type: (double)a/b
It has very high precedence so it occurs before the division
Copyright Curt Hill

11. The C++ Cast
C++ introduced a cast that looks like a function call or constructor double(a)/b
Use the type name like a function name
Precedence is not an issue
This can only be used for types with a single name
Use (long double)a/b for multiple names
Copyright Curt Hill

12. The 2/3 problem solved
We have two ways to accomplish what we wanted: d = double(a)/b; d = (double)a/b; d = a/double(b);
These work, but these do not: d = (double)(a/b); d = double(a/b);
The damage is already done
Copyright Curt Hill

13. Rounding How is a double converted into an int? Truncation!
(int) is zero!
How do we round?
Simple: add 0.5 prior to the cast
(int)( ) is
(int)(1.4999) is 1
Copyright Curt Hill