Is it function overloading or overriding or something else ? ( hello function )
class A {
public:
void hello(int x) { cout << "A" << endl; }
};
class B : public A {
public:
void hello() { cout << "B" << endl; }
};
void main() {
B obj;
obj.hello();
}
It's neither, it's function hiding.
Declaring a non-virtual function with the same name (even if the signature is different) in a derived class hides the base class implementation completely.
To still have access to A::hello, you can do the following:
class B : public A {
public:
using A::hello;
void hello() { cout << "B" << endl; }
};
Overriding:
struct A
{
virtual void foo() {}
};
struct B : public A
{
/*virtual*/ void foo() {}
};
Overloading:
struct A
{
void foo() {}
void foo(int) {}
};
Overriding:
struct Base {
virtual void Foo() { std::cout << "Base\n"; };
};
struct Derived : Base {
virtual void Foo() { std::cout << "Derived\n"; };
// same name and signature as a virtual function in a base class,
// therefore this overrides that function. 'virtual' is optional in
// the derived class (but you should use it).
};
C++11 adds a way to ensure your function overrides:
struct Derived : Base {
virtual void Foo() override { std::cout << "Derived\n"; };
};
Now if the method Foo is not overriding something then you will get an error.
struct Base {
void Foo();
};
struct Derived : Base {
virtual void Foo() override; // error, Derived::Foo is hiding Base::Foo, not overriding
};
This is neither.
Overriding is a function with the same signature (same name, parameters and return value) in a subclass. Overloading is a function with the same name but different parameters.
It is not Overloaded Function because in Function Overloading either number of parameters or type of parameters should differ.For example :
void show(int a,int b);
void show(int a);
void show(char a,char b);
And neither it is Function Overriding because in Overridden function prototype must be same.For example :
Base class and Derived class having function
void show();
//function prototype must be same
Related
I have two base classes and a class that inherits both base classes.
Both base classes have a virtual function with the same signature, and I want to provide different implementations in the derived class to each virtual function.
class A{
virtual void f() = 0;
}
class B{
virtual void f() = 0;
}
class Derived:public A, public B{
void A::f() override{ // Error
...
}
void B::f() override{ // Error
...
}
}
What is the correct way to do this? (I cannot rename the virtual function. Actually the two base classes are generated from the same template class.)
template <typename T>
class AShim : public A {
void f() override {
static_cast<T*>(this)->A_f();
}
};
template <typename T>
class BShim : public B {
void f() override {
static_cast<T*>(this)->B_f();
}
};
class Derived: public AShim<Derived>, public BShim<Derived> {
void A_f();
void B_f();
};
class A {
public:
virtual void f() = 0;
};
class B {
public:
virtual void f() = 0;
};
class Derived :public A, public B {
public:
void A::f() {
cout << "Inside A's version"<<endl;
}
void B::f() {
cout << "Inside B's version"<<endl;
}
};
int main()
{
Derived derived;
cout << "calling A" << endl;
A *a;
a = &derived;
a->f();
cout << "calling B" << endl;
B *b;
b = &derived;
b->f();
}
Works fine for me. No need to explicitly mention override keyword as pure virtual functions will be overridden by virtue of its default properties.Use base class's scope while defining the functions as you have already done. Use public access specifier to enable derived classes to override the pure virtual function. That's all.
class Base {
public:
virtual void f() {}
};
class Derived : private Base {
public:
void f() override {}
};
My question is there any use to such override? Private inheritance implies that you can not store Derived in Base pointer and thus it will never be needed to dynamically dispatch f to the correct type.
Just one example: A function of Derived::f1() can call a (public or protected) functions of Base::f2(), which in turn can call f(). In this case, dynamic dispatch is needed.
Here is an example code:
#include "iostream"
using namespace std;
class Base {
public:
virtual void f() {
cout << "Base::f() called.\n";
}
void f2() {
f(); // Here, a dynamic dispatch is done!
}
};
class Derived:private Base {
public:
void f() override {
cout << "Derived::f() called.\n";
}
void f1() {
Base::f2();
}
};
int main() {
Derived D;
D.f1();
Base B;
B.f2();
}
Output:
Derived::f() called
Base::f() called
Here is some sample code:
#include <iostream>
class A {
public:
virtual void foo() {
std::cout << "base" << std::endl;
}
A() {
foo();
}
};
class B : public A {
int a;
public:
void foo() {
std::cout << "derived" << std::endl;
}
B(int a) :
a(a) {}
};
int main() {
B o(1);
return 0;
}
I want foo() to get called every time some A derived object is constructed. I do not want to call foo() explicitly in every derived class constructor.
Is there a way to do this in some elegant way?
There is no way you can call an overridden foo() from a base class constructor, no matter what you do. When the base class constructor is called, the derived class object has not been constructed yet, so you cannot call any of its methods or access any of its members. This is true for virtual functions and regular functions as well. In a base class constructor, the this pointer is pointing at the base class, not the derived class.
A potential workaround is to delegate construction to a separate function that clients will have to call instead. Then have that function call foo after construction:
class A {
public:
virtual void foo() {
std::cout << "base" << std::endl;
}
template<typename T, typename ... Args>
static T construct(Args ... args)
{
T newT{ args... };
newT.foo();
return std::move(newT);
}
protected:
A() {
//Construct A
}
};
class B : public A {
int a;
public:
void foo() {
std::cout << "derived" << std::endl;
}
B(int a) :
a(a) {}
};
int main()
{
B o = A::construct<B>(1);
A a = A::construct<A>();
return 0;
}
Class foo act as interface, it has a pure virtual function and also provide some public overloaded function with same name for convenience so that derived class won't need to provide them in every implementation.
But since derived class must override the pure virtual function, it hides foo's public function.
I tried with "using foo::A", but it won't work because "using" will bring all function including the private one and result in compiler error "error C2876: 'foo' : not all overloads are accessible".
class foo
{
private:
virtual void A(int a)=0;
public:
void A()
{
A(1000);
}
void A(int a, int b)
{
A(a+b);
}
};
class bar : public foo
{
private:
virtual void A(int a)
{
cout << a << "\n";
}
};
int main()
{
bar x;
x.A();
return 0;
}
I know I can redefine every overloaded function in every class which derived from foo, but this defeat the purpose of convenience.
Or I can cast bar to foo before calling A(), but this approach will get confused since I need to remember which function is define in which class. For example:
class foo
{
private:
virtual void A(int a)=0;
public:
void A()
{
A(1000);
}
void A(int a, int b)
{
A(a+b);
}
};
class bar : public foo
{
private:
virtual void A(int a)
{
cout << "bar:" << a << "\n";
}
public:
void A(int a, int b, int c)
{
A(a+b+c);
}
};
class foobar : public bar
{
private:
virtual void A(int a)
{
cout << "foobar:" << a << "\n";
}
};
int main()
{
foobar x;
x.foo::A();
x.bar::A(1,2,3);
return 0;
}
I need to clearly remember A withouth argument come from foo, and A with three arguments come from bar. This is also not good for convenience.
When using the non virtual interface idiom, it's safer, and more explicit, to name the two functions (the public non-virtual and the private virtual) differently.
This shows well what the code does, and would solve your problem:
class foo
{
private:
virtual void doA(int a)=0;
public:
void A()
{
doA(1000);
}
void A(int a, int b)
{
doA(a+b);
}
};
And of course don't forget to change in the hierarchy.
using C++, I have
struct Base {
virtual void stuff(/*base stuff*/);
};
struct Derived : public Base {
void stuff(/*derived stuff*/);
};
void function1(Derived& obj){
obj.stuff();
}
In this scenario, function1 will use Derived's do() function. What if in function1, I want to call the Base class's do() function instead ? Will it work if I call function1 as
function1(dynamic_cast<Base*>(derived_obj_ptr))?
After correcting the multitude of errors in your code, this is indeed achievable:
#include <iostream>
class Base {
public:
virtual void foo() { std::cout << "Base\n"; }
};
class Derived : public Base {
public:
void foo() { std::cout << "Derived\n"; }
};
void function1(Derived *p) {
p->Base::foo(); // <<<<< Here is the magic >>>>>
}
int main() {
Derived d;
function1(&d);
}
Output:
Base
(see http://ideone.com/FKFj8)