Learning about templates

Another technique to add to your toolbox of programming concepts that we will use in the next section is the idea of templates. Templates are a way for you to be able to create generic classes that can be extended to have the same functionality for different datatypes. It's another form of abstraction, letting you define a base set of behavior for a class without knowing what type of data will be used on it. If you've used the STL before, you've already been using templates, perhaps without knowing it. That's why the list class can contain any kind of object.

Here's an example of a simple templated class:

#include <iostream> // std::cout 
 
template <class T> 
class TemplateExample 
{ 
public: 
  // Constructor 
  TemplateExample(); 
  // Destructor 
  ~TemplateExample(); 
  // Function 
  T TemplatedFunction(T); 
};

In this case, we created our TemplateExample class and it has three functions. The constructor and deconstructor look normal, but then I have this TemplateFunction function which takes in an object of type T, and returns an object of type T. This T comes from the first line of our example code with the template <class T> section of our code. Anywhere that there is a T it will be replaced with whatever class we want to use this template with.

Now, unlike regular functions, we have to define templated functions within our .h file, so that, when we need to create an object using this template, it will know what the functions will do. In addition to this, the syntax is also a bit different:

template <class T> TemplateExample<T>::TemplateExample() 
{ 
  printf("\nConstructor!"); 
} 
 
template <class T> TemplateExample<T>::~TemplateExample() 
{ 
  printf("\nDeconstructor!"); 
} 
 
template <class T> T TemplateExample<T>::TemplatedFunction(T obj) 
{ 
  std::cout << "\nValue: " << obj; 
  return obj; 
}

In this example, I'm just printing out text to display when a certain functionality is called, but I also want to point out the usage of std::cout and that using it will require you to add #include <iostream> to the top of your file.

We are using the standard library's cout function in this instance, instead of the printf that we have been using, because cout allows us to feed in obj--no matter what its type is--to display something, which isn't possible with printf by default.

Once that's finished, we can go ahead and use this inside of our project:

  TemplateExample<int> teInt; 
  teInt.TemplatedFunction(5); 
 
   
  TemplateExample<float> teFloat; 
  teFloat.TemplatedFunction(2.5); 
   
  TemplateExample<std::string> teString; 
  teString.TemplatedFunction("Testing");

As you can see, this will create three different kinds of TemplateExample class objects using different types. When we call the TemplatedFunction function, it will print out exactly the way we were hoping:

Later on, when we learn about abstract types, we can use templates with them to handle any kind of data. In our case right now, we are going to use this functionality to allow us to make as many Singletons as we'd like!