Understanding Class Templates in C++

Class templates in C++ are a powerful feature that allows you to define a blueprint for creating classes that can work with any data type. This flexibility is achieved by using template parameters, which are placeholders for types that will be specified when the class is instantiated.

Key Concepts

1. Template Parameters

Template parameters are placeholders for types or values that will be specified when the class is used. These parameters are defined within angle brackets (<>) after the keyword template. For example:

template <typename T>
class MyClass {
    T data;
public:
    MyClass(T d) : data(d) {}
    T getData() { return data; }
};
    

2. Type Parameters

Type parameters are used to specify the type of data that the class will handle. The keyword typename or class is used to declare these parameters. For instance, in the example above, T is a type parameter.

3. Non-Type Parameters

Non-type parameters are used to specify constants or values that the class will use. These parameters can be integers, pointers, or other non-type entities. For example:

template <typename T, int size>
class Array {
    T elements[size];
public:
    T getElement(int index) { return elements[index]; }
};
    

4. Template Specialization

Template specialization allows you to define a specific implementation of a template for a particular type. This is useful when you need to handle a specific type differently. For example:

template <>
class MyClass<int> {
    int data;
public:
    MyClass(int d) : data(d) {}
    int getData() { return data * 2; }
};
    

5. Partial Specialization

Partial specialization allows you to specialize a template for a subset of its parameters. This is useful when you want to provide a different implementation for a specific combination of parameters. For example:

template <typename T>
class MyClass<T*> {
    T* data;
public:
    MyClass(T* d) : data(d) {}
    T* getData() { return data; }
};
    

6. Default Template Arguments

Default template arguments allow you to specify default values for template parameters. This makes the template easier to use, as you don't need to specify all parameters every time. For example:

template <typename T = int>
class MyClass {
    T data;
public:
    MyClass(T d) : data(d) {}
    T getData() { return data; }
};
    

7. Template Metaprogramming

Template metaprogramming involves using templates to perform computations at compile time. This can be used to optimize code and reduce runtime overhead. For example:

template <int N>
struct Factorial {
    enum { value = N * Factorial<N - 1>::value };
};

template <>
struct Factorial<0> {
    enum { value = 1 };
};
    

8. Using Class Templates

To use a class template, you specify the template arguments in angle brackets when you create an instance of the class. For example:

MyClass<double> obj(3.14);
std::cout << obj.getData(); // Outputs: 3.14
    

By understanding and utilizing class templates, you can create more flexible and reusable code in C++.