| implement | clause |
| Syntax | An implementClause is:
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| Description | An implement clause is used to specify that the module, monitor or class containing the clause is to be the implementation of another module, monitor or class. This implementation is a special kind of expansion. The module, monitor or class containing the clause gains access to (inherits) all the declarations inside the target item. See inherit clause for rules about expansions, which are also rules for implementations. The implement clause can only be used in a unit. See unit for the definition of a unit.
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| Example | Here is a stack module which defers all of its exported subprograms. This module is an interface but not an implementation. Following stack is the stackBody module that implements the stack module, giving the bodies for stack's subprograms. Any call to stack's push or pop procedures, such as stack.push("Ed"), will actually call the procedures given in stackBody.
module stack % Interface
implement by stackBody
% stackBody has implementation
export push, pop
deferred procedure push ( s : string )
deferred procedure pop ( var s : string )
end stack
Next comes the expansion which gives the bodies for the deferred procedures push and pop. The stackBody body also adds declarations for the top and contents variables.
module stackBody % Implementation
implement stack % stack has interface
var top : int := 0
var contents : array 1 .. 100 of string
body procedure push % ( s : string )
top := top + 1
contents ( top ) := s
end push
body procedure pop % ( var s : string )
s := contents ( top )
top := top - 1
end pop
end stackBody
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| Details | Module, monitor or class D can be in C's implement-by clause if, and only if, C is in D's implement clause. In other words, an interface must apply to exactly one implementation and vice versa. A module can implement only a module, a monitor only a monitor, and a class only a class. Classes (but not modules and monitors) can contain inherit clauses. A class cannot contain both an inherit and an implement clause. An implementItem is one of:
The second form is used when the implement clause is for a separate unit and the imported item is in a file whose name is different from the item's name, as in:
implement ledger in "ledg.t"The fileName must be an explicit character string, e.g., "ledg.t". See also unit. Parentheses are allowed around the items in implement clauses, as in:
implement ( ledger in "ledg.t" )There is no restriction on the declarations that an interface may contain. In particular, an interface (any module, monitor or class containing an implement-by clause), can contain subprogram bodies and variable declarations, exactly as is the case in expansions. This is different from languages such as C++ in which there are strict rules limiting what you can put in an interface. Even though D contains an implement clause, D can also contain an implement-by clause, which implies further implementation by further automatic expansion. Suppose class D is in class C's implement-by clause and that p is a pointer to class C:
var p : ^ CEven though C is implemented by D, p remains a pointer to class C. Each creation of an object of class C actually creates an object of type D, for example:
new p % Creates object of class DClass D, which implements C, could also have an implement-by clause, which causes its implementation to be automatically created and so on. If another class E inherits C, this expansion does not include D. If the new statement contains an explicit class name E that is a descendant of C (but not actually C), as in
new E, pthe object of the explicit class is created. If E has an implement-by clause, the expansion is created.
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| See also | unit, module, monitor and class. See also implement by clause, inherit clause, export list, and import list. See also deferred subprograms.
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