Copyright 2005-2007 Curt Hill

Casts

What are they?Why do we need them?

Copyright 2005-2007 Curt Hill

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 2005-2007 Curt Hill

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 2005-2007 Curt Hill

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 2005-2007 Curt Hill

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 2005-2007 Curt Hill

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 2005-2007 Curt Hill

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 2005-2007 Curt Hill

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

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 2005-2007 Curt Hill

Copyright 2005-2007 Curt Hill

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 2005-2007 Curt Hill

The C++ Cast• C++ introduced a cast that looks

like a function call or constructordouble(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/bfor multiple names

Copyright 2005-2007 Curt Hill

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 2005-2007 Curt Hill

Rounding• How is a double converted into an

int?• Truncation!

– (int)0.99999 is zero!

• How do we round?• Simple: add 0.5 prior to the cast• (int)(0.9999 + 0.5) is• (int)(1.4999) is• 1