Functors vs Lambda Expressions

Both functors and lambda expressions allow you to define callable objects in C++. However, there are situations where one may be preferred over the other:

Use a functor when:

• You need to maintain state across multiple calls
• You require the callable to have a specific type (e.g., for template specialization)
• You need to define multiple overloads of the operator()
• You want to take advantage of class features like inheritance or member functions

Use a lambda expression when:

• You need a simple, one-off callable without extra overhead
• You don't need to maintain state across calls
• You want to capture variables from the surrounding scope

For example, a functor is a good choice for a custom comparator in a sorting algorithm, as it can maintain state and provide a specific type:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5class CustomCompare {
6 public:
7  CustomCompare(int t) : threshold(t) {}
8
9  bool operator()(int a, int b) const {
10    return (a > threshold) && (a < b);  
11  }
12
13 private:
14  int threshold;
15};
16
17int main() {
18  std::vector<int> vec{1, 5, 2, 4, 3};
19
20  std::sort(
21    vec.begin(), vec.end(), CustomCompare(3));  
22
23  for (int i : vec) {
24    std::cout << i << " ";
25  }
26  std::cout << "\n";
27}

11 5 2 4 3

On the other hand, a lambda is better suited for a simple, one-time operation:

1#include <iostream>
2#include <algorithm>
3#include <vector>
4
5int main() {
6  std::vector<int> vec {1, 2, 3, 4, 5};
7
8  auto it = std::find_if(vec.begin(), vec.end(),
9    [](int i){ return i > 3; });
10
11  std::cout << *it << "\n";
12}

14