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Note that oil_tanks[7] refers to the eighth cylinder object
in the oil_tanks array (the first being indexed by 0). Similarly,
oil_tank_pointers[7] refers to the eighth pointer in the
oil_tank_pointers array.
To refer to the cylinder object indexed by the value of a variable,
limit, in the oil_tanks array, you write
oil_tanks[limit]; to refer to the radius of that cylinder
object, you write oil_tanks[limit].radius.
When you work with an array of pointers to cylinder objects,
you need to dereference a pointer when you want to refer to a
cylinder object. Hence, *oil_tank_pointers[7] refers to the
cylinder object pointed to by the eighth pointer in an array. To
refer to the eighth cylinder object's radius, you can add
.radius, producing the following expression, which is baroque
looking, inasmuch as it has parentheses, an asterisk, brackets, and a
period:
(*oil_tank_pointers[7]).radius
Alternatively, and preferably, you can combine dereferencing and
member selection using the class-pointer operator, ->:
oil_tank_pointers[7] -> radius