C virtual class template

The next two functions i. They simply do the arithmetic addition and subtraction and returning the result. Now let us proceed and understand the Generic.

Now, let us talk about generic classes. For understanding that first of all, let us look at the Arithmetic class. The arithmetic class is performing arithmetic operations i. What about float-type data? If we want to use a long integer or if we want to use double, then what? For them, this Arithmetic class will not work. We have to write a separate class for it. We should write a separate class for the floating-point types of data and performing the arithmetic operation.

The answer is a big No. You can use only one data type at a time and it works for any type of data. It is called a generic class and that is defined as a template. Now, let us convert this Arithmetic class into a generic class using a template so that it can operate on different data types and perform arithmetic operations.

We have written the Arithmetic class. Let us convert this Arithmetic class into generic. For understanding this please have a look at the following code. The left-hand side code is without a generic class and the right-hand side code with a generic class. By Priya Pedamkar. Template class, as the name suggests, is a Template for classes. This allows the class to be used on many different data types as per the requirements without the need of being re-written for each type.

If we consider the real-world example of template class for better understanding, then we can consider this as a blueprint. If a real-estate builder is designing a township, he prepares the layout of apartments that include the generic specifications like floor plan, placement of doors, windows, etc. This blueprint can be considered to be a template class that will give us a general idea of how an apartment is going to look like from a bigger picture.

The template class works on similar lines. If we are designing an enterprise application , it will have multiple entities that will represent the classes. Each class will have their specific properties and methods. However, a template can be designed that will manage to insert these entities into the database. We will be using this example in the coming sections of this article. However, by using the template class, we can get this work done by writing only a single class hence reducing a lot of time and removing the possibility of a lot of redundant duplicate code.

When working on an enterprise application, most of the times, the programmers face a scenario wherein the program structure becomes complicated as the number of model classes increases. Using template classes, one can reduce the code complexity by defining generic operations to be performed in a template class and use this template class with multiple data types to get the required results.

For Example, if we are writing a calculator program that will have methods that take 2 input parameters, can perform addition, subtraction, multiplication, and division and return us the output. InQusitive InQusitive 5, 2 2 gold badges 24 24 silver badges 58 58 bronze badges. I think it won't work for a very long time. Remember that your interface class with a template virtual function may be used not only in your own code, but also be included into multiple "client" binaries, possibly compiled to a dynamically-linked shared libraries.

Now, imagine that each of those libraries inherit from your class and introduce a new function instance. Imagine then that you open those shared libraries dynamically, e. The linking process when dlopen occurs would be troublesome, probably requiring recreating vtables for objects that are already in memory!

How would the compiler determine which instances of a theoretical virtual template member function to generate for code that doesn't even exist? As with templated functions in general it should be instantiated again in every compilation unit, right? Wouldn't that solve the problem? One possible solution would be to enable some kind of stable run-time type reflection and then creating a hash-map of type, function-ptr instead of vtable.

It is doable. But very complex and very different to what we have now. Show 2 more comments. Virtual Function Tables Let's begin with some background on virtual function tables and how they work source : [ My problem, or how I came here I'm attempting to use something like this now for a cubefile base class with templated optimized load functions which will be implemented differently for different types of cubes some stored by pixel, some by image, etc.

Mark Essel Mark Essel 4, 2 2 gold badges 26 26 silver badges 47 47 bronze badges. I met same situation, and the inheritance structure of mass classes.

It is easy for compiler to construct vtable It is impossible to define a class template member function as virtual, as you can see, it hard to determine function signature and allocate vtable entries. Brent81 Brent81 1, 1 1 gold badge 11 11 silver badges 19 19 bronze badges. A class template may have virtual member functions. A member function may not be both a member function template and a virtual member function.

On my system for sure Ubuntu This is totally different from what the question asked. Here the entire base class is templated. I've compiled this kind of thing before. This would compile on Visual Studio too — ds-bos-msk. No they can't. Tom Tom 6, 1 1 gold badge 32 32 silver badges 57 57 bronze badges.

This is known as CRTP if anyone is curious. But this doesn't help for the cases, where one has a class hierarchy and wants to be able to call virtual methods of pointers to the base classes.

KaiPetzke: You can't construct an unconstrained pointer, no. But you can template any code that doesn't need to know the concrete type, which has much the same effect conceptually at least - obviously completely different implementation.

No, template member functions cannot be virtual. Matteo Monti 7, 17 17 gold badges 56 56 silver badges bronze badges. My curiosity is: Why? What problems does the compiler faces in doing so? You need a declaration in scope at least, in order to get the types correct. It is required by the standard and the language to have a declaration in scope for identifiers you use.

Template functions are useful for writing code only once using different types. Virtual functions are useful for having a common interface for different classes. To answer the second part of the question: If they can be virtual, what is an example of a scenario in which one would use such a function? Java generics are syntactic sugar for casting. They are not the same as templates. Dempsey: You could say that casting is the way Java implements generics, rather than the other way around It's purely a macro.

For this example you could exclude the on the macro definition. It's not meant to be called but rather fill out the required functions via the pre-compiler. Otherwise you would have to redefine all functions. I came up with this solution while solving a problem at work. It seems similar to Mark Essel's one above, but, I hope, is better implemented and explained. I'd already qualify this as code obfuscation, and you still don't get around the fact that you have to modify the original Base class each time you need to call a template function with an argument type not compatible to the ones implemented so far.

Avoiding this necessity is the intention of templates Essels approach is totally different: Ordinary virtual functions accepting different template instantiations - and the final template function in the derived class only serves to avoid code duplication and does not even have a counter part in the base class Maxim Sinev Maxim Sinev 1 1 silver badge 3 3 bronze badges.

Bernd Bernd 1, 7 7 silver badges 17 17 bronze badges. How right function is called in case of virtual? Sign up or log in Sign up using Google.

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