As we continue our exploration of STL, let's look into function objects, also known as functors. A functor is an object that can be called as if it were a normal function, hence the blend of 'function' and 'object' into 'functor'. STL algorithms often use functors as arguments, allowing you to inject custom logic into their operations. To illustrate, let's take an example within a finance application.

Imagine we have a collection of financial transaction records and we want to filter them based on certain custom logic such as transactions in a specific date range or exceeding a certain amount. With STL algorithms and functors, we can easily accomplish this in an optimal way.

In the next code snippet, you'll see a simple functor that checks if a transaction amount exceeds a given limit. We also showcase the use of the copy_if() algorithm, which copies elements from one container to another based on a predicate provided as a functor. These concepts although might seem complex at first, greatly simplify your code and improve efficiency by leveraging STL's optimized implementations.

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#include <iostream>

#include <vector>

#include <algorithm>

using namespace std;

​

// Define the functor

struct ExceedsLimit {

  double limit;

  ExceedsLimit(double limit) : limit(limit) {}

  bool operator()(double amount) { return amount > limit; }

};

​

int main() {

  // Create a vector of financial transactions

  vector<double> transactions = {1500.0, 2200.0, 3500.0, 800.0, 1200.0};

​

  // Define the limit

  double limit = 2000.0; 

​

  // Create a vector to store transactions exceeding the limit

  vector<double> exceeding;

​

  // Use copy_if to find and copy transactions exceeding the limit

  copy_if(transactions.begin(), transactions.end(), back_inserter(exceeding), ExceedsLimit(limit));

​

  // Print exceeding transactions

  for(double amount : exceeding) {

    cout << amount << endl;

  }