Class Templates

Class Templates

Just like functions, classes can be templated. One class template produces a different real class for every type you put in.

A template version of a class

template <typename T>
class Box {
public:
  T value;
  Box(T v) : value(v) {}
};

T stands for the type of whatever the box holds. When you supply a concrete type, the template becomes a real class you can build objects from.

Declaring an object

Box<int> intBox(5);      // a box that holds an int
Box<double> dBox(6.5);   // a box that holds a double

The type in angle brackets completes the template. intBox.value is an int; the other box's value gets its own type.

Member functions in the template

template <typename T>
class Box {
public:
  T value;
  Box(T v) : value(v) {}
  T get() { return value; }
};

Member functions defined inside the template can use T just like the data members. Every instantiation - Box<int>, Box<string>, and so on - gets a compiled get.

Same template, different classes

Box<int> and Box<string> are two different, unrelated classes, even though one template made both.

TL;DR

• A class template uses the placeholder T inside the class body.
• Instantiate with a concrete type: Box<int> b;.
• A separate class is generated for each type used.
• Member functions inside the template can use the placeholders.