Slots¶

In this chapter, we show how to call getters and setters with the function-call syntax, and how to define methods for getters and setters. We show techniques for initializing slots, including slot options and initialize methods. We describe the different allocations that slots can have. We find a need for symbols, so we describe and use symbols as well.

Dot-syntax abbreviation for simple function calls¶

The dot syntax that we have shown for getters in Getters and setters of slot values, is an abbreviation for a function call. The first expression is an abbreviation for the second expression:

object.function-name
function-name(object)

The dot syntax used with the assignment operator also is an abbreviation for a function call. The first two expressions are abbreviations for the third expression:

object.name := new-value;
name(object) := new-value;
name-setter(new-value, object);

You can use the dot syntax as an abbreviation for any function call that takes a single argument and returns a single value. For example, in Methods on <time-offset>, we defined the following method:

define method past? (time :: <time-offset>) => (past? :: <boolean>)
  time.total-seconds < 0;
end method past?;

The following two calls are equivalent:

past?(*my-time-offset*);
*my-time-offset*.past?;

In the remainder of this book, we use the dot syntax for function calls that return a property of an object (such as the past? property of a <time-offset> instance), and that take a single argument and return a single value.

Getters and setters for slots¶

As shown in Getters and setters of slot values, when you define a class, Dylan automatically defines a getter method to return the value of a slot, and defines a setter method to change the value of a slot.

The name of the getter is always the name of the slot. Thus, the getter for the total-seconds slot is total-seconds. Let’s look at an example of calling a getter. The first expression is an abbreviation for the second expression:

*my-time-of-day*.total-seconds;
total-seconds(*my-time-of-day*);

The preceding expressions are calls to the getter function named total-seconds. The choice of which syntax to use is purely a matter of personal style. The first syntax is provided for those people who prefer the slightly more concise dot syntax. The second syntax is provided for those people who prefer slot accesses to look like function calls. In this book, we use the dot syntax.

By default, the name of the setter is the slot’s name followed by -setter. Thus, the setter for the total-seconds slot is total-seconds-setter. You can use the setter: slot option to specify a different name for the setter.

The dot-syntax abbreviation for assignment enables you to invoke the setter by using assignment with the name of the getter. For example, the first two expressions are abbreviations for the third expression:

*my-time-of-day*.total-seconds := 180;
total-seconds(*my-time-of-day*) := 180;
total-seconds-setter(180, *my-time-of-day*);

Each of these expressions stores the value 180 in the slot named total-seconds of the object that is the value of the *my-time-of-day* variable.

Most Dylan programmers do not use the syntax of the third expression to call a setter, because it is more verbose than the first and second expressions. However, it is important to know the name of the setter, so that you can define setter methods. For example, to define a method on the setter for the total-seconds slot, you define it on total-seconds-setter. For an example of a setter method, see Setter methods.

If you do not want Dylan to define a setter method for a slot, you can define the slot to be constant, using the constant slot adjective, or you can give the setter: #f slot option.

For more information about accessing slots, see Slot references, and Assignment.

Advantages of accessing slots via generic functions¶

A slot is conceptually like a variable, in that it has a value. But the only way to access a slot’s value is to call a generic function. Using generic functions and methods to gain access to slot values has three important advantages:

Generic functions provide a public interface to the private implementation of a slot. By making the representation of the slot visible to only the methods of the generic functions, you can change the representation without changing any of the users of the information — the callers of the generic functions. In most cases, a compiler can optimize slot references to reduce or eliminate the cost of hiding the implementation.
A subclass can specialize, or filter, references to superclass slots. For example, the classes <latitude> and <longitude> inherit the direction slot from their superclass <directed-angle>. In Virtual slots, we show how to provide a setter method for the direction slot of <latitude> that ensures that the value is north or south, and a setter method for the direction slot of <longitude> that ensures that the value is east or west.
A slot access can involve arbitrary computation. For example, a slot can be virtual. See Virtual slots.

Setter methods¶

In most cases, the getter and setter methods that Dylan defines for each slot are perfectly adequate. In certain cases, however, you might want to change the way a getter or setter works.

For example, we can define a setter method to solve a problem in our time library. The class <time-of-day> inherits the total-seconds slot from the class <sixty-unit>. The type of the slot is <integer> . However, the semantics of <time-of-day> state that the total-seconds should not be less than 0. We can define a setter method for <time-of-day> to ensure that the new value for the total-seconds slot is 0 or greater.

In our setter method, we will use the type defined in Examples of types that are not classes, and repeated here:

// Define nonnegative integers as integers that are >= zero
define constant <nonnegative-integer> = limited(<integer>, min: 0);

The setter method is as follows:

define method total-seconds-setter
    (total-seconds :: <integer>, time :: <time-of-day>)
 => (total-seconds :: <nonnegative-integer>)
  if (total-seconds >= 0)
    next-method();
  else
    error("%d is invalid. total-seconds cannot be negative.",
          total-seconds);
  end if;
end method total-seconds-setter;

When the setter for the total-seconds slot is called with an instance of <time-of-day>, the preceding method will be invoked, because it is more specific than the method that Dylan generated on the <sixty-unit> class. If the new value for the total-seconds slot is valid (that is, is greater than or equal to 0), then this method calls next-method, which invokes the setter method on <sixty-unit>. If the new value is less than 0, an error is signaled.

The following example show what happens when you call total-seconds-setter with a negative value for total-seconds:

? begin
    let test-time-of-day = make(<time-of-day>);
    test-time-of-day.total-seconds := -15;
  end;
=> ERROR: -15 is invalid. total-seconds cannot be negative.

This setter method ensures that no one can assign an invalid value to the slot. For completeness, we must also ensure that no one can initialize the slot to an invalid value. The way to do that is to define an initialize method, as shown in Initialize methods.

Considerations for naming slots and other objects¶

A binding is an association between a name and an object. For example, there is a binding that associates the name of a constant and the value of the constant. The names of functions, module variables, local variables, and classes are also bindings. There is a potential problem that can occur if you use short names. If a client module uses other modules that also define and export bindings with short names, there is a significant chance that name clashes will occur, with different bindings with the same name being imported from different modules.

If you use the Dylan naming conventions, then a variable will not have the same name as a class, a function, or a constant. The naming conventions avoid name clashes between different kinds of objects.

A slot is identified by the name of its getter. The getter is visible to all client modules. There is no problem if two getters with the same name are defined by unrelated classes, because the appropriate getter is selected through method dispatch. There is a problem if a getter has the same name as a generic function with an incompatible parameter list or values declaration. (See Parameter-list congruence.) When such a problem occurs, the only way to resolve it is to use options to define module to exclude or rename some of the problem bindings. This solution is undesirable, because it requires work on the part of the author of the client module, who must spot and resolve such clashes, and then use an interface that no longer matches its documentation.

Therefore, for getters that you intend to export, it makes sense prevent clashes by considering the name of the slot carefully. One technique is to prefix the name of the property with the name of the class. For example, you might define a <person> class with a slot person-name, instead of the shorter possibility, name. One drawback of this technique is that it might expose too much information about the implementation — that is, the name betrays the class that happens to implement the slot at a particular time, and you have to remember which superclass introduces a property if you are to access that property.

There is a compromise between using short names and using the class name as a prefix — you can choose a prefix for a whole group of classes beneath a given class. For example, you might use the prefix person- for slots of many classes that inherit from the <person> class, including <employee>, <consultant>, and so on.

define class <person> (<object>)
  slot person-name;
  slot person-age;
end class <person>;

define class <employee> (<person>)
  slot person-number;
  slot person-salary;
end class <employee>;

define class <consultant> (<employee>)
  slot person-perks;
  slot person-parking-lot;
end class <consultant>;

Now, in a method on <consultant>, all accesses are consistent, and we do not have to remember where the slots actually originate:

// Method 1
define method person-status (p :: <consultant>) => (status :: <integer>)
  (p.person-perks.evaluation + p.person-salary.evaluation)
    / p.person-age;
end method person-status;

If we had defined the classes differently, such that we prefixed each getter with the name of the class that defined it, the method would look like this:

// Method 2
define method person-status (p :: <consultant>) => (status :: <integer>)
  (p.consultant-perks.evaluation + p.employee-salary.evaluation)
    / p.person-age;
end method person-status;

Method 2 is more difficult to write and read than is Method 1, and is more fragile. If, at some point, all employees are allocated perks, then the use of the consultant-perks getter becomes a problem.

Initialize methods¶

Every time you call make to create an instance of a class, make calls the initialize generic function. The purpose of the initialize generic function is to initialize the instance before it is returned by make. You can customize the initialization by defining a method on initialize. Methods for initialize receive the instance as the first argument, and receive all keyword arguments given in the call to make.

We define an initialize method:

define method initialize (time :: <time-of-day>, #key)
  next-method();
  if (time.total-seconds < 0)
    error("%d is invalid. total-seconds cannot be negative",
          time.total-seconds);
  end if;
end method initialize;

On line 2, we call next-method. All methods for initialize should call next-method as their first action, to allow any less specific initializations (that is, initialize methods defined on superclasses) to execute first. If you call next-method as the first action, then, in the rest of the method, you can operate on an instance that has been properly initialized by any initialize methods of superclasses. If you forget to include the call to next-method, your initialize method will be operating on an improperly initialized instance.

Lines 3 through 6 contain the real action of this method. We check that the value is valid. If it is invalid, we signal an error.

The following example shows what happens when total-seconds is not valid when we are creating an instance:

? make(<time-of-day>, total-seconds: -15);
=> ERROR: -15 is invalid. total-seconds cannot be negative.

Slot options for initialization of slots¶

Unlike variables and constants, slots can be uninitialized; that is, you can create an instance without initializing all the slots. If you call a getter for a slot that has not been initialized, Dylan signals an error. In the following sections, we describe a variety of techniques for avoiding the problem of accessing an uninitialized slot. The most general technique is to define an initialize method for a slot, as shown in Initialize methods.

A slot can be uninitialized. Once a slot receives a value, however, it will always have a value: There is no way to return a slot to the uninitialized state. Sometimes it is useful to store in a slot a value that means none. To make that possible, you need to define a new type for that slot, as shown in Examples of types that are not classes. In Sections The init-value: slot option through The init-function: slot option, we show techniques for initializing slots.

The `init-value:` slot option¶

We can use the init-value: slot option to give a default initial value to a slot:

define abstract class <sixty-unit> (<object>)
  slot total-seconds :: <integer>,
    init-keyword: total-seconds:, init-value: 0;
end class <sixty-unit>;

When we use make to create any subclass of <sixty-unit> (such as <time-of-day>), and we do not supply the total-seconds: keyword to make, the total-seconds slot is initialized to 0.

The init-value: slot option specifies an expression that is evaluated once, before the first instance of the class is made, to yield a value. Every time that an instance is made and the slot needs a default value, this same value is used as the default.

In general, a slot receives its default initial value when no init keyword is defined or when the caller does not supply the init-keyword argument to make.

The `required-init-keyword:` slot option¶

Instead of giving the slot a default initial value, we can require the caller of make to supply an init keyword for the slot. The required-init-keyword: slot option defines a required init keyword. If the caller of make does not supply the required init keyword, then an error is signaled.

define abstract class <sixty-unit> (<object>)
  slot total-seconds :: <integer>, required-init-keyword: total-seconds:;
end class <sixty-unit>;

The total-seconds slot is defined in the <sixty-unit> class. By making total-seconds: a required init keyword in this class, we make it required for every class that inherits from it, including <time>, <angle>, and all their subclasses.

Slot options for an inherited slot¶

You can define a slot in only one particular class in a set of classes related by inheritance. You can use the inherited slot specification to override the default initial value of an inherited slot, or the init function of an inherited slot. See The init-function: slot option.

In this example, assume that the <sixty-unit> class defines the total-seconds slot and the init keyword total-seconds:, and provides the default initial value of 0 for that slot, as shown:

define abstract class <sixty-unit> (<object>)
  slot total-seconds :: <integer>,
    init-keyword: total-seconds:, init-value: 0;
end class <sixty-unit>;

define abstract class <time> (<sixty-unit>)
end class <time>;

The <time-offset> class provides a different default initial value for the inherited slot total-seconds:

define class <time-offset> (<time>)
  inherited slot total-seconds, init-value: encode-total-seconds(1, 0, 0);
end class <time-offset>;

By using the inherited slot specification, we are not defining the slot, but rather are stating that this slot is defined by a superclass. We can then provide either a default initial value or an init function for the inherited slot.

The `init-function:` slot option¶

We can use the init-function: slot option to provide a function of no arguments to be called to return a default initial value for the slot. These functions are called init functions. They allow the initial value of a slot to be an arbitrary computation.

define class <time-of-day> (<time>)
  inherited slot total-seconds, init-function: get-current-time;
end class <time-of-day>;

Every time that we make an instance of the <time-of-day> class and we need a default value for the total-seconds slot, the get-current-time function is called to provide an initial value. Here, we assume that get-current-time is available as a library function; it is not part of the core Dylan language.

The init-function: slot option specifies an expression that is evaluated once, before the first instance of the class is made, to yield a function. The function must have no required arguments and must return at least one value. Every time that an instance is made and the slot needs a default value, this function is called with no arguments, and the value that it returns is used as the default. An init function is called during instance creation when no keyword argument is defined or when an optional keyword argument is not passed to make.

Init expressions¶

An init expression is another way of providing a default slot value. Here is an example:

define class <time-of-day> (<time>)
  inherited slot total-seconds = get-current-time();
end class <time-of-day>;

Every time that we make an instance of the <time-of-day> class and we need a default value for the total-seconds slot, the expression get-current-time(); is evaluated to provide an initial value.

An init expression specifies an expression. Every time that an instance is made and the slot needs a default value, this expression is evaluated and its value is used as the default.

Notice the similarity between the init-function: slot option and an init expression. In fact, the following slot specifications are equivalent:

inherited slot total-seconds, init-function: get-current-time;
inherited slot total-seconds = get-current-time();

That substitution works for functions that have no required arguments. More generally, the following slot specifications are equivalent:

slot slot = expression;
slot slot, init-function: method () expression end method;

The expression can be a call to a function that requires arguments. Here, we use method to define a method with no name.

The init-value: slot option, init-function: slot option, and init expression are mutually exclusive. A given slot specification can have only one of these.

Allocation of slots¶

Each slot has a particular kind of allocation. The allocation of a slot determines where the storage for the slot’s value is allocated, and it determines which instances share the value of the slot. There are four kinds of allocation:

Instance:: Each instance allocates storage for the slot, and each instance of the class that defines the slot has its own value for the slot. Changing a slot in one instance does not affect the value of the same slot in a different instance. Instance allocation is the default, and is the most commonly used kind of allocation.

Virtual:: No storage is allocated for the slot. You must provide a getter method that computes the value of the virtual slot. See Virtual slots.

Class:: The class that defines the slot allocates storage for the slot. Instances of the class that defines the slot and instances of all that class’s subclasses see the same value for the slot. That is, all general instances of the class share the value for the slot.

Each-subclass:: The class that defines the slot and each of its subclasses allocate storage for the slot. Thus, if the class that defines the slot has four subclasses, the slot is allocated in five places. All the direct instances of each class share a value for the slot.

We can give an example of an each-subclass slot by defining a <vehicle> class:

define class <vehicle> (<physical-object>)
  // Every vehicle has a unique identification code
  slot vehicle-id :: <string>, required-init-keyword: id:;
  // The normal operating speed of this class of vehicle
  each-subclass slot cruising-speed :: <integer>;
end class <vehicle>;

The slot cruising-speed is defined with the each-subclass slot allocation. We use each-subclass allocation to express that, for example, all instances of Boeing 747 aircraft share a particular cruising speed, and all instances of McDonnell Douglas MD-80 aircraft share a particular cruising speed, but the cruising speed of 747s does not need to be the same as the cruising speeds of MD-80s.

Virtual slots¶

Virtual slots are useful when there is information conceptually associated with an object that is better computed than stored in an ordinary slot. By using a virtual slot instead of writing a method, you make the information appear like a slot to the callers of the getter. The information appears like a slot because the caller cannot distinguish the getter of a virtual slot from a getter of an ordinary slot. In both cases, the getter takes a single required argument — the instance — and returns a single value.

A virtual slot does not occupy storage; instead, its value is computed. When you define a virtual slot, Dylan defines a generic function for the getter and setter. You must define a getter method to return the value of the virtual slot. Unlike those of other slots, the value of a virtual slot can change without a setter being called, because that value is computed, rather than stored. You can optionally define a setter method. If you want to initialize a virtual slot when you create an instance, you can define an initialize method.

We can use virtual slots to control the access to a slot. For example, we want to ensure that the value of the direction slot is north or south for <latitude>, and is east or west for <longitude>. (An alternative technique is to use enumeration types, as shown in Enumerations.) To enforce this restriction, we must

Check the value when the setter method is invoked. In this section, we show how to do this check using a virtual slot. We also show how to use symbols, instead of strings, to represent north, south, east, and west.
Check the value of the direction slot when an instance is created and initialized. We do that checking in Initialize method for a virtual slot.

We redefine the <directed-angle> class to include a virtual slot and an ordinary slot:

define abstract class <directed-angle> (<angle>)
  virtual slot direction :: <symbol>;
  slot internal-direction :: <symbol>;
end class <directed-angle>;

We define the slot direction with the virtual slot allocation. Notice that the slot’s allocation appears before the name of the slot (as contrasted with slot options, which appear after the name of the slot).

In the <directed-angle> class, we use the slot internal-direction to store the direction. We shall provide a setter method for the virtual slot direction that checks the validity of the value of the direction before storing the value in the internal-direction slot.

Symbols¶

Symbols are much like strings. A symbol is an instance of the built-in class <symbol>. The key difference between strings and symbols lies in the way similarity (as tested by = ) and identity (as tested by == ) are defined for each of them. Two string operands can be similar but not identical. However, two symbol operands that are similar are always identical — that is, they always refer to the same object.

There are two reasons to use symbols in certain cases where you might consider using strings. First, symbol comparison is not case sensitive. Second, comparison of two symbols is much faster than is comparison of two strings, because symbols are compared by identity, and strings are usually compared element by element.

In the <directed-angle> class, we define the type of the two slots as <symbol>, instead of <string>, which we used in previous versions of this class. If we use strings, then when we checked whether the direction slot of a latitude was "north" or "south", we would have to worry about uppercase versus lowercase. For example, we would have to decide whether each of these were valid values: "north", "NORTH", "North", "NOrth", and so on. We simplify that decision by using the <symbol> type instead of <string>.

There are two equivalent syntaxes for specifying symbols:

Examples of use of the keyword syntax are: north: and south:.
Examples of use of the hash syntax are:#"north" and #"south".

Here, we show that symbol comparison is not case sensitive:

? #"NORTH" == #"North";
=> #t

Here, we show that the two syntaxes are equivalent:

? north: == #"norTH";
=> #t

It is our convention in this book to reserve the keyword syntax for keyword parameters, and otherwise to use the hash syntax. For example, we would give the call:

make(<latitude>, direction: #"north")

instead of the call:

make(<latitude>, direction: north:)

Getter and setter methods for a virtual slot¶

Here is the getter method for the virtual slot direction:

// Method 1
define method direction (angle :: <directed-angle>) => (dir :: <symbol>)
  angle.internal-direction;
end method direction;

Here are the setter methods for the virtual slot direction:

// Method 2
define method direction-setter
    (dir :: <symbol>, angle :: <directed-angle>) => (new-dir :: <symbol>)
  angle.internal-direction := dir;
end method direction-setter;

// Method 3
define method direction-setter
    (dir :: <symbol>, latitude :: <latitude>) => (new-dir :: <symbol>)
  if (dir == #"north" | dir == #"south")
    next-method();
  else
    error("%= is not north or south", dir);
  end if;
end method direction-setter;

// Method 4
define method direction-setter
    (dir :: <symbol>, longitude :: <longitude>) => (new-dir :: <symbol>)
  if (dir == #"east" | dir == #"west")
    next-method();
  else
    error("%= is not east or west", dir);
  end if;
end method direction-setter;

The preceding methods work as follows:

When you call direction on an instance of <directed-angle> or any of its subclasses, method 1 is invoked. Method 1 calls the getter internal-direction, and returns the value of the internal-direction slot.
When you call direction-setter on a direct instance of <latitude>, method 3 is invoked. Method 3 checks that the direction is valid for latitude; if it finds that the direction is valid, it calls next-method, which invokes method 2. Method 2 stores the direction in the internal-direction slot.
When you call direction-setter on a direct instance of <longitude>, method 4 is called. Method 4 checks that the direction is valid for longitude; if it finds that the direction is valid, it calls next-method, which invokes method 2. Method 2 stores the direction in the internal-direction slot.
When you call direction-setter on a direct instance of <directed-angle>, method 2 is invoked. Method 2 stores the direction in the internal-direction slot.

In these methods, we use dir, rather than direction, as the name of the parameter that represents direction. Recall that direction is the name of a getter. Although we technically could use direction as the parameter name in these methods (because we do not call the direction getter in the bodies), direction as a parameter name might be confusing to other people reading the code.

The error function signals an error. For more information about signaling and handling errors, see Exceptions.

The direction-setter methods check the direction when the setter is called. In Initialize method for a virtual slot, we check the direction when an instance is made.

Initialize method for a virtual slot¶

We define the initialize method:

 define method initialize (angle :: <directed-angle>, #key direction: dir)
   next-method();
   angle.direction := dir;
 end method initialize;

For keyword parameters, the name of the keyword that you supply to make is normally the same name as the parameter that is initialized within the body. In this case, we want to avoid confusion between the getter direction and the keyword parameter direction:, so we use dir as the name of the keyword parameter for the initialize method. When you call make, you use the direction: keyword. However, within this method, the parameter is named dir.

Line 3 calls the setter for the direction slot. We defined the methods for direction-setter in Getter and setter methods for a virtual slot. If the argument is a latitude, then method 3 is invoked to check the value. If the argument is a longitude, then method 4 is invoked to check the value.

We can create a new instance of <absolute-position>.

? define variable *my-absolute-position* =
    make(<absolute-position>,
         latitude:
           make(<latitude>,
                total-seconds: encode-total-seconds(42, 19, 34),
                direction: #"north"),
         longitude:
           make(<longitude>,
                total-seconds: encode-total-seconds(70, 56, 26),
                direction: #"west"));

The preceding example works, because the values for direction are appropriate for latitude and longitude. The following example shows what happens when the direction is not valid when an instance is created:

? make(<latitude>, direction: #"nooth");
=> ERROR: nooth is not north or south

The following example shows what happens when the direction is not valid when the direction setter is used:

? begin
    let my-longitude = make(<longitude>, direction: #"east");
    my-longitude.direction := #"north";
  end;
=> ERROR: north is not east or west

Summary¶

In this chapter, we covered the following:

We described techniques for initializing slots; see Summary of slot-initialization techniques.
We discussed the syntax of calling getters and setters; see Syntax of calling getters and setters.
We showed how to define methods for getters and setters.
We showed how and why you can use symbols instead of strings.
We described the different kinds of slot allocation; see Summary of slot allocations.

Summary of slot-initialization techniques¶
Technique	Summary
`initialize` method	You can define a method for `initialize` for a class to perform any actions to initialize the instance. The `make` function calls the `initialize` generic function after `make` creates an instance and supplies those initial slot values that it can. If you need to do any complex computation to determine and set the value of a slot, you can do it in an `initialize` method.
Init keyword	You can use the `init-keyword:` slot option to declare an optional keyword argument, or the `required-init-keyword:` slot option to declare a required keyword argument for `make` when you create an instance of the class. The value of the keyword argument becomes the value of the slot.
Init value	You can use the `init-value:` slot option to give a default initial value for the slot. This option specifies an expression that is evaluated once, before the first instance of the class is made, to yield a value. Every time an instance is made and the slot needs a default value, this same value is used as the default. The slot receives its default initial value when no init keyword is defined, or when the caller does not supply the init-keyword argument to `make`.
Init function	You can use the `init-function:` slot option to provide a function that returns a default value. This option specifies an expression that is evaluated once, before the first instance of the class is made, to yield a function. The function must have no required arguments and must return at least one value. Every time that an instance is made and the slot needs a default value, this function is called with no arguments, and the value that it returns is used as the default. The slot receives its default initial value when no init keyword is defined or when the caller does not supply the init-keyword argument to `make`.
Init expression	You can use an init expression to provide an expression that yields a default value. Every time that an instance is made and the slot needs a default value, this expression is evaluated, and its value is used as the default. The slot receives its default initial value when no init keyword is defined, or when the caller does not supply the init-keyword argument to `make`.

Syntax of calling getters and setters¶
Call	Translation
`object.function-name`	`function-name(object)`
`my-time-of-day.total-seconds;`	`total-seconds(my-time-of-day);`
`object.name := new-value;`	`name-setter(new-value, object);`
`name(object) := new-value;`	`name-setter(new-value, object);`
`my-time-of-day.total-seconds := 0;`	`total-seconds-setter (0, my-time-of-day);`
`total-seconds(my-time-of-day) := 0;`	`total-seconds-setter(0, my-time-of-day);`

Summary of slot allocations¶
Allocation	Summary
Instance	Each instance allocates storage for the slot, and each instance of the class that defines the slot has its own value of the slot. Instance allocation is the default.
Virtual	No storage is allocated for the slot. You must provide a getter method that computes the value of the virtual slot.
Class	The class that defines the slot allocates storage for the slot. All general instances of the class share the value of the slot.
Each-subclass	The class that defines the slot and each of its subclasses allocate storage for the slot. All the direct instances of each class share the value of the slot.