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My last article (Mar, 96) talked about the Liskov Substitution Principle (LSP). This principle,
when applied to C++, provides guidance for the use of public inheritance. It states that
every function which operates upon a reference or pointer to a base class, should be able to
operate upon derivatives of that base class without knowing it. This means that the virtual
member functions of derived classes must expect no more than the corresponding member
functions of the base class; and should promise no less. It also means that virtual member
functions that are present in base classes must also be present in the derived classes; and
they must do useful work. When this principle is violated, the functions that operate upon
pointers or references to base classes will need to check the type of the actual object to
make sure that they can operate upon it properly. This need to check the type violates the
Open-Closed Principle (OCP) that we discussed last January.
In this column, we discuss the structural implications of the OCP and the LSP. The
structure that results from rigorous use of these principles can be generalized into a principle
all by itself. I call it “The Dependency Inversion Principle” (DIP).附件：dip.pdf(29K) |
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