Discuss the importance of virtual base class with the help of an example.

A virtual base class in C++ is a class that is declared as virtual when used as a base class. One of its main uses is to prevent multiple "instances" of a given class appearing in an inheritance hierarchy when using multiple inheritances.

Consider the following example:

class A {
public:
    int x;
};

class B: public virtual A {
};

class C: public virtual A {
};

class D: public B, public C {
};

In this example, classes B and C are derived from class A. Class D is derived from both B and C. Without the keyword 'virtual', class D would have two separate sets of class A members - one from class B and one from class C. This could lead to ambiguity in the code.

However, by declaring class A as a virtual base class of both B and C, we ensure that only one set of class A members exist in class D. This is because the virtual keyword ensures that only one copy of the base class is inherited, even when multiple paths exist in the inheritance hierarchy.

This is the main importance of a virtual base class - it helps to prevent duplication and ambiguity in complex inheritance hierarchies. It also helps to reduce the potential for errors and confusion in the code.