Why can B::Func call A::Func using syntax that makes it look like a static method call? Shouldn't this fail because it is an instance method?
class A {
public:
void Func() {
printf( "test" );
}
};
class B : private A {
public:
void Func() {
A::Func(); // why does it work? (look below in main())
}
};
int main() {
B obj;
obj.Func();
// but we cannot write here, because it's not static
// A::Func();
return 0;
}
That isn't "called like static". That's merely the syntax used to explicitly specify which member function to call. Even if Func were virtual, that syntax could be used to call a base class version.
class B : public A {
public:
void Func() {
this->A::Func(); /* this-> can be omitted */
}
};
int main() {
B obj;
obj.A::Func();
return 0;
}
Edit: obj.A::Func() would be invalid actually, in your case, because the inheritance is private, so main cannot see that A is a base of B. I've changed B's inheritance to public to make the answer correct, but it would otherwise still work outside of the class, when in a friend function.
Related
May be my question is wrong. I am new to C++.
Is there any way call a base class member function using derived class object if that function is being overridden in derived class?
For example:
class A {
public:
void add() { cout<<"A"; }
};
class B: public A {
public:
void add() { cout<<"B"; }
};
int main() {
B bObj;
bObj.add(); // calls member function of class B
return 0;
}
First of all, you aren't really overriding the add function, merely hiding the name, since A::add is not declared virtual.
To call A::add, just be explicit about it:
bObj.A::add();
Typically, B::add() exists for a reason and you probably shouldn't be calling A::add() on a B. Which isn't to say you can't - you just have to turn it into an A first:
B bObj;
static_cast<A&>(b).add(); // calls A::add()
Or explicitly specify that it's A::add():
bObj.A::add();
From within B, you have to qualify the call:
class B: public A {
public:
void add() { cout<<"B"; }
void test() {
add(); // calls B::add
A::add(); // calls A::add
}
};
Let's consider the following code:
#include <iostream>
class Base
{
public:
void foo() //Here we have some method called foo.
{
std::cout << "Base::foo()\n";
}
};
class Derived : public Base
{
public:
void foo() //Here we override the Base::foo() with Derived::foo()
{
std::cout << "Derived::foo()\n";
}
};
int main()
{
Base *base1 = new Base;
Derived *der1 = new Derived;
base1->foo(); //Prints "Base::foo()"
der1->foo(); //Prints "Derived::foo()"
}
If I have the above stated classes, I can call the foo method from any of Base or Derived classes instances, depending on what ::foo() I need. But there is some kind of problem: what if I need the Derived class instance, but I do need to call the Base::foo() method from this instance?
The solve of this problem may be next:
I paste the next method to the class Derived
public:
void fooBase()
{
Base::foo();
}
and call Derived::fooBase() when I need Base::foo() method from Derived class instance.
The question is can I do this using using directive with something like this:
using Base::foo=fooBase; //I know this would not compile.
?
der1->Base::foo(); //Prints "Base::foo()"
You can call base class method using scope resolution to specify the function version and resolve the ambiguity which is useful when you don't want to use the default resolution.
Similar (Not exactly same case) example is mentioned # cppreference
struct B { virtual void foo(); };
struct D : B { void foo() override; };
int main()
{
D x;
B& b = x;
b.foo(); // calls D::foo (virtual dispatch)
b.B::foo(); // calls B::foo (static dispatch)
}
is there a solution for this problem? I just want to call a member function in another class while they two in the same class.
class ClassA {
public:
void func() { printf("Hello World\n"); }
};
class ClassB {
public:
void testfunc() {
// TODO: call func() in classa;
}
};
class ClassAB {
private:
ClassA classa;
ClassB classb;
public:
ClassA& getClassa() { return classa; }
ClassB& getClassb() { return classb; }
};
int main() {
ClassAB classab;
classab.getClassb().testfunc();
return 0;
}
You need an instance of ClassA to invoke the func.
Perhaps you might pass the (only) instance you have in to testfunc():
// ...
class ClassB
{
public:
void testfunc(ClassA classa)
{
classa.func();
}
};
// ...
int main()
{
ClassAB classab;
classab.getClassb().testfunc(classab.getClassa());
return 0;
}
In your code, ClassA::func() is public, but not static. You can only call a non-static public method attribute from an instance of ClassA.
Change func() to static and then yes, you can call it with ClassA::func(), from main, or anywhere the method is in scope.
Firstly make func() public, which you have done.
Then it invoke func() from outside ClassA use:
ClassA::func();
May be my question is wrong. I am new to C++.
Is there any way call a base class member function using derived class object if that function is being overridden in derived class?
For example:
class A {
public:
void add() { cout<<"A"; }
};
class B: public A {
public:
void add() { cout<<"B"; }
};
int main() {
B bObj;
bObj.add(); // calls member function of class B
return 0;
}
First of all, you aren't really overriding the add function, merely hiding the name, since A::add is not declared virtual.
To call A::add, just be explicit about it:
bObj.A::add();
Typically, B::add() exists for a reason and you probably shouldn't be calling A::add() on a B. Which isn't to say you can't - you just have to turn it into an A first:
B bObj;
static_cast<A&>(b).add(); // calls A::add()
Or explicitly specify that it's A::add():
bObj.A::add();
From within B, you have to qualify the call:
class B: public A {
public:
void add() { cout<<"B"; }
void test() {
add(); // calls B::add
A::add(); // calls A::add
}
};
How can I call a base class method which is overridden by the derived class, from a derived class object?
class Base{
public:
void foo(){cout<<"base";}
};
class Derived:public Base{
public:
void foo(){cout<<"derived";}
}
int main(){
Derived bar;
//call Base::foo() from bar here?
return 0;
}
You can always(*) refer to a base class's function by using a qualified-id:
#include <iostream>
class Base{
public:
void foo(){std::cout<<"base";}
};
class Derived : public Base
{
public:
void foo(){std::cout<<"derived";}
};
int main()
{
Derived bar;
//call Base::foo() from bar here?
bar.Base::foo(); // using a qualified-id
return 0;
}
[Also fixed some typos of the OP.]
(*) Access restrictions still apply, and base classes can be ambiguous.
If Base::foo is not virtual, then Derived::foo does not override Base::foo. Rather, Derived::foo hides Base::foo. The difference can be seen in the following example:
struct Base {
void foo() { std::cout << "Base::foo\n"; }
virtual void bar() { std::cout << "Base::bar\n"; }
};
struct Derived : Base {
void foo() { std::cout << "Derived::foo\n"; }
virtual void bar() { std::cout << "Derived::bar\n"; }
};
int main() {
Derived d;
Base* b = &d;
b->foo(); // calls Base::foo
b->bar(); // calls Derived::bar
}
(Derived::bar is implicitly virtual even if you don't use the virtual keyword, as long as it's signature is compatible to Base::bar.)
A qualified-id is either of the form X :: Y or just :: Y. The part before the :: specifies where we want to look up the identifier Y. In the first form, we look up X, then we look up Y from within X's context. In the second form, we look up Y in the global namespace.
An unqualified-id does not contain a ::, and therefore does not (itself) specify a context where to look up the name.
In an expression b->foo, both b and foo are unqualified-ids. b is looked up in the current context (which in the example above is the main function). We find the local variable Base* b. Because b->foo has the form of a class member access, we look up foo from the context of the type of b (or rather *b). So we look up foo from the context of Base. We will find the member function void foo() declared inside Base, which I'll refer to as Base::foo.
For foo, we're done now, and call Base::foo.
For b->bar, we first find Base::bar, but it is declared virtual. Because it is virtual, we perform a virtual dispatch. This will call the final function overrider in the class hierarchy of the type of the object b points to. Because b points to an object of type Derived, the final overrider is Derived::bar.
When looking up the name foo from Derived's context, we will find Derived::foo. This is why Derived::foo is said to hide Base::foo. Expressions such as d.foo() or, inside a member function of Derived, using simply foo() or this->foo(), will look up from the context of Derived.
When using a qualified-id, we explicitly state the context of where to look up a name. The expression Base::foo states that we want to look up the name foo from the context of Base (it can find functions that Base inherited, for example). Additionally, it disables virtual dispatch.
Therefore, d.Base::foo() will find Base::foo and call it; d.Base::bar() will find Base::bar and call it.
Fun fact: Pure virtual functions can have an implementation. They cannot be called via virtual dispatch, because they need to be overridden. However, you can still call their implementation (if they have one) by using a qualified-id.
#include <iostream>
struct Base {
virtual void foo() = 0;
};
void Base::foo() { std::cout << "look ma, I'm pure virtual!\n"; }
struct Derived : Base {
virtual void foo() { std::cout << "Derived::foo\n"; }
};
int main() {
Derived d;
d.foo(); // calls Derived::foo
d.Base::foo(); // calls Base::foo
}
Note that access-specifiers both of class members and base classes have an influence on whether or not you can use a qualified-id to call a base class's function on an object of a derived type.
For example:
#include <iostream>
struct Base {
public:
void public_fun() { std::cout << "Base::public_fun\n"; }
private:
void private_fun() { std::cout << "Base::private_fun\n"; }
};
struct Public_derived : public Base {
public:
void public_fun() { std::cout << "Public_derived::public_fun\n"; }
void private_fun() { std::cout << "Public_derived::private_fun\n"; }
};
struct Private_derived : private Base {
public:
void public_fun() { std::cout << "Private_derived::public_fun\n"; }
void private_fun() { std::cout << "Private_derived::private_fun\n"; }
};
int main() {
Public_derived p;
p.public_fun(); // allowed, calls Public_derived::public_fun
p.private_fun(); // allowed, calls Public_derived::public_fun
p.Base::public_fun(); // allowed, calls Base::public_fun
p.Base::private_fun(); // NOT allowed, tries to name Base::public_fun
Private_derived r;
r.Base::public_fun(); // NOT allowed, tries to call Base::public_fun
r.Base::private_fun(); // NOT allowed, tries to name Base::private_fun
}
Accessibility is orthogonal to name lookup. So name hiding does not have an influence on it (you can leave out public_fun and private_fun in the derived classes and get the same behaviour and errors for the qualified-id calls).
The error in p.Base::private_fun() is different from the error in r.Base::public_fun() by the way: The first one already fails to refer to the name Base::private_fun (because it's a private name). The second one fails to convert r from Private_derived& to Base& for the this-pointer (essentially). This is why the second one works from within Private_derived or a friend of Private_derived.
First of all Derived should inherit from Base.
class Derived : public Base{
That said
First of you can just not have foo in Derived
class Base{
public:
void foo(){cout<<"base";}
};
class Derived : public Base{
}
int main(){
Derived bar;
bar.foo() // calls Base::foo()
return 0;
}
Second you can make Derived::foo call Base::foo.
class Base{
public:
void foo(){cout<<"base";}
};
class Derived : public Base{
public:
void foo(){ Base::foo(); }
^^^^^^^^^^
}
int main(){
Derived bar;
bar.foo() // calls Base::foo()
return 0;
}
Third you can use qualified id of Base::foo
int main(){
Derived bar;
bar.Base::foo(); // calls Base::foo()
return 0;
}
Consider making foo() virtual in the first place.
class Base {
public:
virtual ~Base() = default;
virtual void foo() { … }
};
class Derived : public Base {
public:
virtual void foo() override { … }
};
However, this does the job:
int main() {
Derived bar;
bar.Base::foo();
return 0;
}
An important [additional] note: you will still have compilation errors if Name Hiding occurs.
In this case, either utilize the using keyword, or use the qualifer. Additionally, see this answer as well.
#include <iostream>
class Base{
public:
void foo(bool bOne, bool bTwo){std::cout<<"base"<<bOne<<bTwo;}
};
class Derived : public Base
{
public:
void foo(bool bOne){std::cout<<"derived"<<bOne;}
};
int main()
{
Derived bar;
//bar.foo(true,true); // error: derived func attempted
bar.foo(true); // no error: derived func
bar.Base::foo(true,true); // no error: base func, qualified
return 0;
}