sorting a vector of custom objects

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## Sorting a Vector of Custom Objects in C++: A Detailed Tutorial

This tutorial will guide you through the process of sorting a `std::vector` of custom objects in C++. We'll cover several approaches, including using the default comparison operator, defining custom comparison functions, and leveraging lambda expressions. We'll also discuss performance considerations and best practices.

**1. Defining the Custom Object**

First, we need to define our custom object (class or struct). Let's create a `Student` class with attributes like name, age, and GPA:
This defines a `Student` class with a name (string), age (integer), and GPA (double). We also include a helper `operator` to make printing `Student` objects easier.

**2. Creating a Vector of Custom Objects**

Now, let's create a `std::vector` containing several `Student` objects:
We use `emplace_back` to efficiently add `Student` objects to the vector, avoiding unnecessary temporary object creation. We also print the initial unsorted vector for comparison.

**3. Sorting Using the Default Comparison Operator (`operator`)**

The simplest way to sort is to use `std::sort` with the default comparison operator (`operator`). To make this work, you need to overload the `` operator for your custom class. This operator defines how two objects of your class should be compared.
* **Explanation:**
* We overloaded the `operator` to compare `Student` objects based on their `name`. You can change the logic inside the `operator` to sort by age or GPA, as shown in the comments.
* `std::sort(students.begin(), students.end())` calls the `operator` to determine the order of elements. It uses an efficient sorting algorithm (usually introsort, a hybrid of quicksort, heapsort, and insertion sort).
* **Important**: `std::sort` expects a **strict weak ordering**. This means that the comparison operator must satisfy the following properties:
* **Irreflexivity:** `a a` must be false.
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