Template classes

Learn to write generic classes with template<typename T>.

Template class

A template class is a class that takes a type parameter. Declaration: template <typename T> class Box { T value; };. The T value member depends on the class's type parameter.

Instantiation: Box<int> b;. The type goes in angle brackets. The compiler generates a separate class for each type. Box<int> and Box<double> are two distinct classes. Member functions also use T; for example T getValue().

Template class members

Member functions of a template class use T. Like T getValue() { return value; }. The member function is called on an instance: Box<int> b(42); b.getValue().

The constructor can also take T: Box(T v) : value(v) {}. Here : value(v) is the initializer list that constructs the member. Box<int> b(42) creates an instance with value=42. b.getValue() returns 42. Template classes provide type-safe encapsulation.

Exercise Write the Box template class

Write template <typename T> class Box. Private member T value, constructor Box(T v) : value(v) {}, T getValue() returns the value. In main write Box<int> b(42); and std::cout << b.getValue() << std::endl;.

Show example solution

#include <iostream>

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

int main() {
    Box<int> b(42);
    std::cout << b.getValue() << std::endl;
    return 0;
}

Test yourself

Complete the class declaration

template <typename T> class ____ { T value; };

Show answer

template <typename T> class Box { T value; };

Match member to role

Box<int>

Instance of type int
T value

Type-parametered member
getValue()

Member function
b.getValue()

Call on instance
How is Box<double> declared?
```
template <typename T> class Box;
```
- Box double b;
- Box<b> double;
- Box<double> b;
- double<Box> b;
Show answer

Box<double> b; — Box<double> b; puts the type in angle brackets.

Next lesson: Multiple type parameters

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