của nó để thực hiện đa hình. Trừ khi bạn có con trỏ cơ sở lớp trỏ đến đối tượng có nguồn gốc, bạn không thể có đa hình ở đây.
One of the key features of derived classes is that a pointer to a derived class is type-compatible with a pointer to its base class. Polymorphism is the art of taking advantage of this simple but powerful and versatile feature, that brings Object Oriented Methodologies to its full potential.
In C++, a special type/subtype relationship exists in which a base class pointer or a reference can address any of its derived class subtypes without programmer intervention. This ability to manipulate more than one type with a pointer or a reference to a base class is spoken of as polymorphism.
Subtype polymorphism allows us to write the kernel of our application independent of the individual types we wish to manipulate. Rather, we program the public interface of the base class of our abstraction through base class pointers and references. At run-time, the actual type being referenced is resolved and the appropriate instance of the public interface is invoked. The run-time resolution of the appropriate function to invoke is termed dynamic binding (by default, functions are resolved statically at compile-time). In C++, dynamic binding is supported through a mechanism referred to as class virtual functions. Subtype polymorphism through inheritance and dynamic binding provide the foundation for objectoriented programming
The primary benefit of an inheritance hierarchy is that we can program to the public interface of the abstract base class rather than to the individual types that form its inheritance hierarchy, in this way shielding our code from changes in that hierarchy. We define eval(), for example, as a public virtual function of the abstract Query base class. By writing code such as _rop->eval(); user code is shielded from the variety and volatility of our query language. This not only allows for the addition, revision, or removal of types without requiring changes to user programs, but frees the provider of a new query type from having to recode behavior or actions common to all types in the hierarchy itself. This is supported by two special characteristics of inheritance: polymorphism and dynamic binding. When we speak of polymorphism within C++, we primarily mean the ability of a pointer or a reference of a base class to address any of its derived classes. For example, if we define a nonmember function eval() as follows, // pquery can address any of the classes derived from Query void eval(const Query *pquery) { pquery->eval(); } we can invoke it legally, passing in the address of an object of any of the four query types:
int main()
{
AndQuery aq;
NotQuery notq;
OrQuery *oq = new OrQuery;
NameQuery nq("Botticelli"); // ok: each is derived from Query
// compiler converts to base class automatically
eval(&aq);
eval(¬q);
eval(oq);
eval(&nq);
}
trong khi một nỗ lực để gọi eval() với địa chỉ của một đối tượng không bắt nguồn từ Query kết quả trong một lỗi thời gian biên dịch:
int main()
{ string name("Scooby-Doo"); // error: string is not derived from Query
eval(&name);
}
Within eval(), the execution of pquery->eval(); must invoke the appropriate eval() virtual member function based on the actual class object pquery addresses. In the previous example, pquery in turn addresses an AndQuery object, a NotQuery object, an OrQuery object, and a NameQuery object. At each invocation point during the execution of our program, the actual class type addressed by pquery is determined, and the appropriate eval() instance is called. Dynamic binding is the mechanism through which this is accomplished. In the object-oriented paradigm, the programmer manipulates an unknown instance of a bound but infinite set of types. (The set of types is bound by its inheritance hierarchy. In theory, however, there is no limit to the depth and breadth of that hierarchy.) In C++ this is achieved through the manipulation of objects through base class pointers and references only. In the object-based paradigm, the programmer manipulates an instance of a fixed, singular type that is completely defined at the point of compilation. Although the polymorphic manipulation of an object requires that the object be accessed either through a pointer or a reference, the manipulation of a pointer or a reference in C++ does not in itself necessarily result in polymorphism. For example, consider
// no polymorphism
int *pi;
// no language-supported polymorphism
void *pvi;
// ok: pquery may address any Query derivation
Query *pquery;
In C++, polymorphism exists only within individual class hierarchies. Pointers of type void* can be described as polymorphic, but they are without explicit language support — that is, they must be managed by the programmer through explicit casts and some form of discriminant that keeps track of the actual type being addressed.
Đây có thể là một trong những ví dụ tốt nhất cho việc sử dụng các chức năng ảo. Cảm ơn! – Garfield
Câu trả lời rõ ràng, nhưng khi tôi đã biết tôi phải thêm phát thải cho tất cả các đối tượng lớp này, tại sao tôi không thể tự tạo các đối tượng cho cả 'xe hơi' và 'xe buýt' và thêm chúng bình thường? Tại sao tôi cần con trỏ kiểu lớp cơ sở. – Yankee
Giả sử nếu chúng ta không sử dụng hàm ảo, lợi ích của việc sử dụng con trỏ lớp cơ sở cho lớp dẫn xuất là gì? – Rajesh