Virtual base class in C++ (original) (raw)

Last Updated : 7 Aug, 2025

Virtual base classes are used in virtual inheritance in a way of preventing multiple "instances" of a given class appearing in an inheritance hierarchy when using multiple inheritances.

**Need for Virtual Base Classes: Consider the situation where we have one class **A . This class **A is inherited by two other classes **B and **C. Both these class are inherited into another in a new class **D as shown in figure below.

As we can see from the figure that data members/function of class **A are inherited twice to class **D. One through class **B and second through class **C. When any data / function member of class **A is accessed by an object of class **D, ambiguity arises as to which data/function member would be called? One inherited through **B or the other inherited through **C. This confuses compiler and it displays error. The **C++ Course covers this topic thoroughly, ensuring you grasp how to implement virtual inheritance correctly in your applications.

**Example: To show the need of Virtual Base Class in C++

CPP14 `

#include using namespace std;

class A { public: void show() { cout << "Hello from A \n"; } };

class B : public A { };

class C : public A { };

class D : public B, public C { };

int main() { D object; object.show(); }

`

**Compile Errors:

prog.cpp: In function 'int main()':
prog.cpp:29:9: error: request for member 'show' is ambiguous
object.show();
^
prog.cpp:8:8: note: candidates are: void A::show() void show()
^
prog.cpp:8:8: note: void A::show()

**How to resolve this issue?
To resolve this ambiguity when class **A is inherited in both class **B and class **C, it is declared as **virtual base class by placing a keyword **virtual as :

**Syntax for Virtual Base Classes:

**Syntax 1:
class B : virtual public A
{
};
**Syntax 2:
class C : public virtual A
{
};

**Note:
**virtual can be written before or after the **public. Now only one copy of data/function member will be copied to class **C and class **B and class **A becomes the virtual base class. Virtual base classes offer a way to save space and avoid ambiguities in class hierarchies that use multiple inheritances. When a base class is specified as a virtual base, it can act as an indirect base more than once without duplication of its data members. A single copy of its data members is shared by all the base classes that use virtual base.

**Example 1

CPP14 `

#include using namespace std;

class A { public: int a; A() // constructor { a = 10; } };

class B : public virtual A { };

class C : public virtual A { };

class D : public B, public C { };

int main() { D object; // object creation of class d cout << "a = " << object.a << endl;

return 0; 

}

`

**Explanation :
The class **A has just one data member **a which is **public. This class is virtually inherited in class **B and class **C. Now class **B and class **C use the virtual base class **A and no duplication of data member **a is done; Classes **B and **C share a single copy of the members in the virtual base class **A.

**Example 2:

CPP14 `

#include using namespace std;

class A { public: void show() { cout << "Hello from A \n"; } };

class B : public virtual A { };

class C : public virtual A { };

class D : public B, public C { };

int main() { D object; object.show(); }

`