Non-virtual interface pattern: Difference between revisions

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The '''non-virtual interface [[Design pattern (computer science)|design pattern]]''' ('''NVI''') controls how methods in a [[base class]] are overridden. Such methods are public, non-virtual members that may be called by clients and overridable methods with core functionality.<ref>{{cite web

| url = http://www.blackwasp.co.uk/

| title = Non-Virtual Interface Design Pattern

| last1 = Carr

| first1 = Richard

| authorlink1 = https://profiles.google.com/113882985360066691734| date = 2011-09-03

| date = 2011-09-03

| work =

| publisher = BlackWasp

| archiveurl = http://www.blackwasp.co.uk/NVI.aspx

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A design that adheres to this pattern results in a separation of a class interface into two distinct interfaces:

# Client interface: This is the public non-virtual interface

# Subclass interface: This is the protectedprivate interface, which can have any combination virtual and (non-)virtual interfacemethods.

With such a structure, the [[Fragile base class|fragile base class]] interface problem is mitigated. The only detriment is that the code is enlarged a little.<ref>{{cite web

| url = http://cpptruths.blogspot.co.at/

| title = Non-Virtual Interface (NVI) idiom and the design intent

| last1 = Tambe

| first1 = Sumant

| authorlink1 = https://plus.google.com/117220997561739301261| date = 2007-04-11

| date = 2007-04-11

| work =

| publisher = C++ truths

| archiveurl = http://cpptruths.blogspot.co.at/2007/04/non-virtual-interface-nvi-idiom-and.html

| archivedate = 2007-04-11

| quote =

| accessdate = 2012-09-12

}}</ref>

== C# example ==

[[File:Non-virtual interface pattern.svg]]

<syntaxhighlight lang="c#" line="1">

public abstract class Saveable

{

// The invariant processing for the method is defined in the non virtual interface.

// The behaviour so defined is inherited by all derived classes.

// For example, creating and committing a transaction.

public void Save()

{

Console.WriteLine("Creating transaction");

CoreSave();

Console.WriteLine("Committing transaction");

}

// The variant processing for the method is defined in the subclass interface.

// This behaviour can be customised as needed by subclasses.

// For example the specific implementation of saving data to the database.

protected abstract void CoreSave();

}

public class Customer : Saveable

{

public string Name { get; set; }

public decimal Credit { get; set; }

protected override void CoreSave()

{

Console.WriteLine($"Saved customer {Name} with credit limit {Credit}");

}

}

</syntaxhighlight><ref>{{Cite web|title=Non-Virtual Interface Design Pattern|url=http://www.blackwasp.co.uk/nvi.aspx|access-date=2021-09-19|website=www.blackwasp.co.uk}}</ref><ref>{{Cite web|title=Non-Virtual Interface Design Pattern (Page 2 of 2)|url=http://www.blackwasp.co.uk/nvi_2.aspx|access-date=2021-09-19|website=www.blackwasp.co.uk}}</ref>

== See also ==

<!-- Moved from the* [[Template method pattern]] to here -->

== References ==

{{Reflist|2}}

== External links ==

<!-- Moved from the [[Template method pattern]] to here -->

* [http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Non-Virtual_Interface Non-Virtual Interface]

{{software-stub}}

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