Skip to main content

Advanced C++ - Mutable Class Field

Today I would like to present C++ class' feature called mutable class field. Mutable class field is class' field modifier which allows to change its value even if object of the class is declared as const.

Take a look at the example: Output of this example is: In point I of that example we are defining object of TestClass. Note that this object is const. As you can see in point Ia this class has three different member fields (constInt, mutableConstInt, nonConstInt). Those variables are public for this example, but do not worry about encapsulation here. It is just omitted for simplify this example. As you can see one of this member fields is marked as mutable class file using mutable keyword (mutableConstInt). Such variable can be modified even if object of class TestClass is const. It will be explained in next points of this example.

In point II we are printing default values of testObject object initialized in initialization list of TestClass' default constructor.

Point III is key point of our example. As you noted testObject is const therefore we cannot modify its constInt and nonConstInt members (you can try uncomment first two lines before point III and you will see compilation error, that you are trying to modify read-only object.). However modification of mutableConstInt is possible despite that testObject is const. We are assigning value 300 to that member field. It is possible thanks to mutable class field feature.

In point IV we are printing values of testObject after modification. As you can see mutableConstInt's value is really changed and is equal 300 now.

Code of this example you can find in our GitHub repository: https://github.com/xmementoit/CppAdventureExamples/tree/master/advancedCpp/mutableMemberField

If you have any questions, please ask in the comments.

Comments

Popular posts from this blog

Blog's new layout

As you noticed this blog has new layout from today. I hope you like it. I think new layout looks better and more modern than previous one. Please, write you opinion about new layout in comments. If you have some ideas how to make this blog better, all ideas are welcomed. Enjoy new layout and blog articles.

STL - count and count_if algorithms

One of the basic and most useful STL algorithms is algorithm which can be used to count number of elements within selected container according to specified criteria. In order to do that we can use std::count or std::count_if algorithm. std::count (firstElementIterator, lastElementIterator, elementForSearch) - is function which will go through container using firstElementIterator and lastElementIterator and return number of container elements which value is equal elementForSearch std::count_if (firstElementIterator, lastElementIterator, UnaryPredicateFunction) - is function which examine range from firstElementIterator to lastElementIterator and return number of container elements which fulfill UnaryPredicateFunction criteria. UnaryPredicateFunction is function having following signature: bool functionName(const Type& a) . So, count_if returns number of elements where UnaryPredicateFunction returns true for. For better understanding let's take a...

QT - foreach algoriithm with const references performance improvement

Today I would like to show you optimal way of using foreach QT algorithm . I will show you why we should pass elements of foreach algorithm by const reference instead of passing them by value. Let me explain it on the below example: Output of this example is: In point I we are creating 3 objects of MyClass class and push them to myClasses QList element. In point II we are using QT foreach algorithm to invoke getValue() method for each object from myClasses list. As you can see on output text for that part of code we are invoking copy constructor before and destructor after invoking getValue() function. It is because we are passing each myClasses list element to foreach algorithm by value. Therefore we are copying that element at the beginning of foreach loop step and removing them (destructing) at the end. This is inefficient solution, especially when class of object being copied is big. It decreases performance. of our application. Solution for that i...