开闭原则

开闭原则的命名被应用在两种方式上。这两种方式都使用了继承来解决明显的困境，但是它们的目的，技术以及结果是不同的。

梅耶开闭原则

伯特兰·迈耶一般被认为是最早提出开闭原则这一术语的人，在他1988年发行的《面向对象软件构造》中给出。这一想法认为一旦完成一个类的实现只应该因错误而修改，新的或者改变的特性应该通过新建不同的类实现。新建的类可以通过继承的方式来重用原类的代码。衍生的子类可以或不可以拥有和原类相同的接口。

梅耶的定义提倡实现继承。具体实现可以通过继承方式来重用，但是接口规格不必如此。已存在的实现对于修改是封闭的，但是新的实现不必实现原有的接口。

多态开闭原则

在20世纪90年代，由于抽象化接口的使用，开闭原则被广泛的重新定义。在这中间实现可以被改变，多种实现可以被创建，并且多态的替换不同的实现。

相比梅耶的使用方式，多态开闭原则的定义倡导对抽象基类的继承。接口规约可以通过继承来重用，但是实现不必重用。已存在的接口对于修改是封闭的，并且新的实现必须，至少，实现那个接口。

罗伯特·C·马丁1996年发表的文章《开闭原则》是使用这种方法的启发式著作。在2001年，Craig Larman把开闭原则关联到了Alistair Cockburn的名为“受护的变量”（Protected Variations）的模式以及David Parnas关于信息隐藏的讨论。

In object-oriented programming, the open–closed principle (OCP) states "software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification";

that is, such an entity can allow its behaviour to be extended without modifying its source code.

The name open–closed principle has been used in two ways. Both ways use generalizations (for instance, inheritance or delegate functions) to resolve the apparent dilemma, but the goals, techniques, and results are different.

The open–closed principle is one of the five SOLID principles of object-oriented design.

Meyer's open–closed principle

Bertrand Meyer is generally credited for having originated the term open–closed principle, which appeared in his 1988 book Object-Oriented Software Construction.

A module will be said to be open if it is still available for extension. For example, it should be possible to add fields to the data structures it contains, or new elements to the set of functions it performs.

A module will be said to be closed if [it] is available for use by other modules. This assumes that the module has been given a well-defined, stable description (the interface in the sense of information hiding).

At the time Meyer was writing, adding fields or functions to a library inevitably required changes to any programs depending on that library. Meyer's proposed solution to this problem relied on the notion of object-oriented inheritance (specifically implementation inheritance):

A class is closed, since it may be compiled, stored in a library, baselined, and used by client classes. But it is also open, since any new class may use it as parent, adding new features. When a descendant class is defined, there is no need to change the original or to disturb its clients.

Polymorphic open–closed principle

During the 1990s, the open–closed principle became popularly redefined to refer to the use of abstracted interfaces, where the implementations can be changed and multiple implementations could be created and polymorphically substituted for each other.

In contrast to Meyer's usage, this definition advocates inheritance from abstract base classes. Interface specifications can be reused through inheritance but implementation need not be. The existing interface is closed to modifications and new implementations must, at a minimum, implement that interface.

Robert C. Martin's 1996 article "The Open-Closed Principle" was one of the seminal writings to take this approach. In 2001, Craig Larman related the open–closed principle to the pattern by Alistair Cockburn called Protected Variations, and to the David Parnas discussion of information hiding.