Virtual classes allow for multiple inheritance in object-oriented programming languages. The diamond problem arises when two parent classes have a common grandparent, causing ambiguity. C++ solves this by making the grandparent a virtual class. Proper class hierarchy structuring is important to avoid errors.
A virtual class is a class that is allocated only once in memory, so that children of that class only use a copy of that parent class that is established in memory. A class is a group of objects that share common traits or attributes. Making a class virtual gives object-oriented programming languages the ability to use multiple inheritance when second-generation or more parent classes are derived from common ancestral base classes. Depending on the computer programming language being referenced, the keyword “virtual”, the similar keyword or the syntax and semantics of a class are used to imply that a particular class is to be considered virtual.
Not all object-oriented programming languages allow for multiple inheritance, which is when a child class can be derived from multiple parent classes at the same time. In a programming language like C++, a class can inherit all member data and member functions from both parent classes at the same time and have access to both parent datasets and functions. Other languages that support multiple inheritance include Perl, Python, Tcl, and Eiffel.
A problem occurs in multiple inheritance hierarchies when two or more parent classes used to create a child class originally derive from one or the same grandparent class. This is called the diamond problem, the name comes from what the hierarchy would look like in this situation. When a compiler program attempts to instantiate a class, such as a grandchild class composed of two parent classes with a common grandparent, two copies of the grandparent are created in memory, one for each parent.
Due to the ambiguity caused by multiple copies of the same grandparent in memory, the compiler cannot ascertain which copy of the grandparent the grandchild should use to access grandparent data or functions. To remedy this situation, C++ transforms the grandparent into a virtual class when declaring the parents. This causes the compiler to only make a copy of the grandparent in memory shared by both parents. Once the grandparent has become a virtual class, the compiler has no problem deciding how the grandchild should access the grandparent because there is only one copy of the grandparent.
Great care must be taken when structuring class hierarchies, especially when they will be included in or become the basis for much larger hierarchies in the future. Proper use of a virtual class gives class hierarchies more freedom to develop; however, they can cause errors that are hard to spot. Studying sound computer programming engineering principles will help ensure that you do not experience unexpected side effects resulting from improper program organization.
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