What is the “Diamond Problem” That Can Occur with C++ Support of Multiple Inheritance?

Reliable computing is a cornerstone of modern human experience. Every cellular phone, tablet, and “smart” device of whatever variety houses powerful computer hardware which itself serves as a platform for myriad software programs and applications.

The measure of effective software in everyday use tends to be 1) intuitive functionality and 2) seamless integration, though other factors are taken into account based on user need, consumer expectation, industry standard, et cetera.

What often goes ignored by those who most benefit from modern computer programs and applications is the underlying matrix of coding which allows such things to function in the first place.

This coding is achieved by way of the programming process, which is, of itself, a mystery to many.

C++ – The Programming World’s Common Tongue

Programming at its core is a technical language predicated upon algorithmic sequencing and/or organizing of processes. It provides software with the procedural instructions by which its functionality is rendered operational.

And while there is a good deal of creativity requisite in the work of quality programming, it is also highly logic-driven and requires agreed upon languages to be undertaken in a scalable, recognizable, consistent capacity.

One such language is C++, a gold standard in the programming realm.

For nearly forty years’ time, C++ has been a foundational language for computer programmers. Though not without its many detractors, the software-writing medium is ubiquitous and widely used for instructional purposes.

If you are a programmer, amateur or professional, you are almost certainly familiar with C++ and have either used it frequently or found reasons to avoid doing so. You may also have crossed paths with the language’s vulnerability to what many coders refer to as the “diamond problem.”

The Diamond Problem Understood

Oddly enough, this programming phenomenon, which is alternately known as the “deadly diamond of death” amongst industry insiders, is, in its name, exactly as misleading as it is clarifying.

This programming phenomenon has nothing at all to do with gemstones, nor with the word “diamond” itself.

What it refers to is a “parenting” vulnerability which, when correctly diagrammed, assumes a diamond-like shape, with a hierarchical superclass situated at the top, two subordinate classes branch off, respectively, to the lower right and left, and a third subordinate class situated at the diagram’s bottom, thereby completing the rough diamond form.

So, then, what exactly is happening in the case of a diamond problem?

Hierarchical Inheritance – A Programming Family Tree

It has everything to do with hierarchical inheritance and the classes organized within such hierarchies.

In object-oriented programming, “objects” (data, methods, functions) are organized into distinctive classes. This characteristic lends itself to increased pragmatism within the coding process, and allows for the inheriting of class properties, which on its face is admittedly quite beneficial from a routine coding standpoint, particularly where high-volume or potentially repetitive work is in play.

Objects are effectively categorized and housed within classes which are then subject to hierarchical inheritance principles. This model allows for programming architectures to more readily take shape, as programmers are able to build upon extant bodies of class-organized objects without having to generate said classes or their properties from the ground up.

Accounting for the Multiple Inheritance Dynamic

Where errors are prone to manifesting is in the multiple (or hybrid) inheritance aspect of C++, or the ability within that programming language for subordinate (or “child”) classes to inherit objects from multiple parent classes. Though a largely beneficial feature of C++, programmers nevertheless need to correct for the known diamond problem pitfall of multiple inheritance.

Examined in its simplest of incarnations, the C++ diamond problem occurs when at least two child classes inherit an object from a single superclass with certain overrides in place. If a fourth class – one subordinate to the two child classes – inherits the same object but does not possess an override directive, confusion will ensue:

Does the fourth class inherit the object in its superclass form, or does it inherit the version supplied by the two child classes, complete with overridden material?

The favorable aspects of multiple inheritance are impossible to dismiss, as software programming relies heavily on the ability to seamlessly incorporate existing objects into larger coding structures.

But the risk of inheritance uncertainty is made possible entirely because of this feature.

Benefiting From Programmer Foresight

There do exist certain precautionary measures which programmers implement to neutralize these risks from the outset. This requires an element of foresight on part of the programmer, as averting problems before they arise depends on what steps the programmer has or has not taken.

One solution frequently posited is that of virtual inheritance, a C++ programming measure which maintains the fidelity of superclass objects as inherited via intermediary subclasses.

In other words, it ensures that second-generation classes won’t disrupt the integrity of data on its way to third-generation inheritors. There do exist other workarounds, which indicate the process of preventing this problem depending on recognizing its existence.

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