Modern (Effective) C++ - Applications of std::unique_ptr - Part 1

The article is hugely inspired by Item 18 of Effective Modern C++ by Scott Meyers. Here I will talk about the applications of std::unique_ptr .

The two main applications of std::unique_ptr are -
1. To return derived class objects in a factory method
2. To implement PImpl ( Pointer to Implementation ) idiom.

We will see first use case here.

Let's take a slight detour and revisit a key concept in inheritance OOPS feature. Inheritance is designed based on IS-A model. For example, we have a class Car which is derived from a more general, class Vehicle. Hence any object instantiated from the car class is also a vehicle ( as in the real world). This concept is used in many of the GoF design patterns and is used in the factory design pattern as well.

A factory design pattern is one of the most widely used design pattern.
 
In this pattern,

• We have a primary base class. 
• There are two or more classes derived from the base class. 
• We have a factory method, which returns the base object created on the heap.
• This factory method takes one or more parameters, of which the first parameter is used to ascertain the derived class to be instantiated.
• The other parameters of the factory method is passed to the constructor of the derived class.
• Inside a factory method, we create and return a derived class object by using a switch-case suite or if-else ladder.

Here is the test code in modern C++

This factory method uses variadic templates. If you are not well versed with it, read this blog. The concept is explained quite elegantly here