What is operator promotion?

If an operation is specified with operands of two different types, they are converted to the smallest type that can hold both values. The result has the same type as the two operands wind up having. To interpret the rules, read the following table from the top down, and stop at the first rule that applies.

The following example code illustrates some cases of operator promotion. The variable f1 is set to 3/4. Because both 3 and 4 are integers, integer division is performed, and the result is the integer 0. The variable f2 is set to 3/4.0. Because 4.0 is a float, the number 3 is converted to a float as well, and the result is the float 0.75.

#include <stdio.h>
main()
{

float f1 = 3 / 4;
float f2 = 3 / 4.0;
printf(“3 / 4 == %g or %g depending on the type used.\n”, f1, f2);

}

When should a type cast be used?

There are two situations in which to use a type cast. The first use is to change the type of an operand to an arithmetic operation so that the operation will be performed properly. If you have read the above question, the following listing should look familiar. The variable f1 is set to the result of dividing the integer i by the integer
j. The result is 0, because integer division is used. The variable f2 is set to the result of dividing i by j as well. However, the (float) type cast causes i to be converted to a float. That in turn causes floating-point division to be used and gives the result 0.75.

#include <stdio.h>
main()
{

int i = 3;
int j = 4;
float f1 = i / j;
float f2 = (float) i / j;
printf(“3/4 == %g or %g depending on the type used.\n”, f1, f2);

}

The second case is to cast pointer types to and from void * in order to interface with functions that expect or return void pointers. For example, the following line type casts the return value of the call to malloc() to be a pointer to a foo structure.

struct foo *p = (struct foo *) malloc(sizeof(struct foo));

When should a type cast not be used?

A type cast should not be used to override a const or volatile declaration. Overriding these type modifiers can cause the program to fail to run correctly.

A type cast should not be used to turn a pointer to one type of structure or data type into another. In the rare events in which this action is beneficial, using a union to hold the values makes the programmer’s intentions clearer.