问题
One of my friends asked me "How Runtime Polymorphism is achieved in C++?" I answered "By Inheritance"
He said "No, it can be achieved only using virtual functions".
So I gave him an example of the following code :-
#include<iostream>
using namespace std;
class A
{
public:
int i;
A(){i=100;}
};
class B : public A
{
public:
int j;
B(){i = -1; j = 99;}
};
void func(A& myA)
{
cout<<myA.i << endl;
}
int main()
{
B b;
A* a = new B();
func(*a);
func(b);
delete a;
return 0;
}
Here, function func() takes reference of A but we pass object of B and we can print the value of public member "i". He said it is compile time polymorphism.
My questions are :-
1) Is Runtime polymorphism achieved only with virtual functions?
2) Is the example above has runtime polymorphism or compile time?
3) If I have the following code :-
void func2(A& myA)
{
cout << myA.i << endl;
// dynamic/static cast myA to myB
cout<<myB.j << endl;
}
what kind of polymorphism is it? Or is it even polymorphism?
回答1:
The example does not show dynamic polymorphism. The method to be called is known at compile time. There is no runtime decision(based on actual object type) as to which method should be called. There is no different behavior for different types.
For the example to be example of dynamic polymorphism.
You need to provide a virtual member function in Base class and overide it in derived class. The actual method to be called is decided by the actual type of the object pointed by the Base class pointer.
Online sample:
#include<iostream>
using namespace std;
class A
{
public:
virtual void doSomething()
{
std::cout<<"\nIn A::doSomething()";
}
};
class B : public A
{
public:
virtual void doSomething()
{
std::cout<<"\nIn B::doSomething()";
}
};
int main()
{
B b;
A obj;
A* a = &b;
a->doSomething();
a = &obj;
a->doSomething();
return 0;
}
Output:
In B::doSomething()
In A::doSomething()
Is Runtime polymorphism acheived only with virutal functions?
No, but virtual functions is the most common and correct way to do so.
Polymorphism can be achieved through function pointers. Consider the following code example, the actual method to call is decided at run-time depending on user input. It is a form of polymorphism through not in strict sense C++ sense which mandates different behaviors for different types.
#include <iostream>
typedef void (*someFunction)(int, char*);
void FirstsomeFunction(int i, char *c)
{
std::cout<<"\n In FirstsomeFunction";
}
void SecondsomeFunction(int i, char *c)
{
std::cout<<"\n In SecondsomeFunction";
}
int main()
{
someFunction arr[1];
int x = 0;
std::cin >> x;
if(x ==0)
arr[0] = &FirstsomeFunction;
else
arr[0] = &SecondsomeFunction;
(arr[0])(10,"Hello");
return 0;
}
Is the example above has runtime polymorphism or compile time?
There is no polymorphism of any kind. The same method will be called in all cases. There is no different behavior for different types and hence it does not classify as polymorphism of any kind.
回答2:
The C language's fprintf is a polymorphic function.
You can pass it various handles and it can print to a file, stdout, a printer, a socket, anything which the system can represent as a stream.
FILE* file = fopen("output.txt", "w"); // a file
FILE* file = stdout; // standard output
FILE* file = fopen("/dev/usb/lp0", "w"); // a usb printer
FILE* file = popen("/usr/bin/lpr -P PDF", "w"); // a PDF file
FILE* file = fdopen(socket(AF_INET,SOCK_STREAM,0), "r+"); // network socket
fprintf(file, "Hello World.\n");
回答3:
what you wrote is not polymorphism.
This is how you do polymorphism in C++ :
#include<iostream>
using namespace std;
class A
{
public:
virtual void func(){
cout << "printing A" << endl;
}
virtual ~A(){}
};
class B : public A
{
public:
void func(){
cout << "printing B" << endl;
}
};
int main()
{
A* a = new A();
A* b = new B();
a->func(); // "printing A"
b->func(); // "printing B"
delete a;
delete b;
return 0;
}
If you were to remove the virtual keyword, the method func of A would be called twice.
回答4:
One of my friends asked me "How Runtime Polymorphism is achieved in C++?" I answered "By Inheritance" He said "No, it can be achieved only using virtual functions".
First, the term polymorphism is ambiguous: in the general computing science sense it refers to an ability to implicitly invoke type-specific code, whether at compile time or run-time. In the C++ Standard it is defined very narrowly are being virtual dispatch (that's the perogative of standards). Obviously for your friend's question to be meaningful, as he's asking how it's achieved in C++ his perspective must be from outside C++ - in the larger context of Computing Science terminology.
Certainly, virtual functions/dispatch are an answer, but are they the only answer...?
To attempt to answer that, it helps to have a clear conception of what behaviour qualifies as run-time polymorphic. Consider:
void f(X& x)
{
// the only situation where C++ doesn't know the run-time type of a variable is
// where it's an instance of a class/struct accepted by reference or pointer
...some operation involving "x"...
}
Any mechanism that could result in different machine code for the operation being invoked involving "x", where the reason relates specifically to the run-time type of "x", is getting pretty close to run-time polymorphic, but there's one final issue: was that branching decided implicitly by the language, or arranged explicitly by the programmer?
In the case of virtual dispatch, the compiler implicitly knows to create the virtual dispatch tables and lookups that branch to the type-appropriate code.
But, say we have a function pointer that was previously set to address type-appropriate code, or a type-specific number or enum that is used to control a switch to a type-specific case. These functionally achieve the same behaviour as run-time virtual dispatch, but the set up had to be done explicitly by the developer, and there's no compiler enforcement to make sure that the determination is done purely on run-time type. Whether they qualify or not is arguable. Because C++ has a fully implicit mechanism in virtual dispatch, and because in the C++ Standard polymorphism has a narrowed definition related specifically to virtual dispatch, I'd guess that most C++ programmers would say "no".
But in the world of C, describing say qsort or bsearch (two Standard libC functions that handle arbitrary types using run-time dispatch via function pointer arguments) as run-time polymorphic might aid quick understanding... it's more normal to say that they're generic implementations though.
Still, there's doubtless hundreds of Computing Science textbooks out there with functional definitions of run-time polymorphism, and I'd bet dispatch using function pointers or other programmer-initialised metadata satisfied a good percentage of them. So, it's pointless to be too insistent that there's a single unambiguous answer.
My questions are :-
1) Is Runtime polymorphism achieved only with virtual functions?
As above, I'd lean towards "yes" in the context of C++, but it's (endlessly) arguable.
2) Is the example above has runtime polymorphism or compile time?
Neither... there's not even two functions to choose between on the basis of type - you're always running the same machine code for func(): that picked by the compiler given an expectation that the type is A.
3) If I have the following code :-
void func2(A& myA)
{
cout << myA.i << endl;
// dynamic/static cast myA to myB
cout<<myB.j << endl;
}
what kind of polymorphism is it? Or is it even polymorphism?
Not polymorphic at all, as you have no branching based on type. A dynamic cast could consult the compiler-populated type meta-data in the run-time type of myA, and if you used that to only conditionally invoke the access to myB.j - which would be undefined behaviour unless myA was a B - then you're back at manually, explicitly developer coordinated type-specific behaviour, and whether that qualifies as "polymorphism" for you is discussed above.
回答5:
Polymorphism is achieved with virtual functions. But to have any effect, i.e. different behaviour depending on type, you need inheritance too
struct A {
virtual void f() = 0;
};
struct B : public A {
void f() {
// do B things
std::cout << "B::f() called\n";
}
};
struct C : public A {
void f() {
// do C things
std::cout << "C::f() called\n";
}
};
Now you can have pointers or references to A with different behaviour, depending on whether it's a B or C.
回答6:
[C++]
Polymorphism is defined as one interface to control access to a general class of actions. There are two types of polymorphism one is compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is functions and operators overloading. Runtime time polymorphism is done using inheritance and virtual functions.
Polymorphism means that functions assume different forms at different times. In case of compile time it is called function overloading. For example, a program can consist of two functions where one can perform integer addition and other can perform addition of floating point numbers but the name of the functions can be same such as add. The function add() is said to be overloaded. Two or more functions can have same name but their parameter list should be different either in terms of parameters or their data types. The functions which differ only in their return types cannot be overloaded. The compiler will select the right function depending on the type of parameters passed. In cases of classes constructors could be overloaded as there can be both initialized and uninitialized objects.
来源：https://stackoverflow.com/questions/14953222/are-virtual-functions-the-only-way-to-achieve-runtime-polymorphism-in-c