Lambda functions (closure)

• C++ 11 introduced lambda expressions
• allow inline functions which can be used for short snippets of code that are not going to be reused and therefore do not require a name.

[ capture clause ] (parameters) -> return-type  
{   
   definition of method   
} 

• Generally, the return-type in lambda expressions is evaluated by the compiler itself and we don’t need to specify it explicitly. Also the -> return-type part can be ignored.

• However, in some complex cases e.g. conditional statements, the compiler can’t determine the return type and explicit specification is required.

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Capture clause

• A lambda expression can have more power than an ordinary function by having access to variables from the enclosing scope.

Capture variables for lambda functions

• Capture by reference
• Capture by value
• Capture by both (mixed capture)

Syntax used for capturing variables

• [&] : capture all external variables by reference
• [=] : capture all external variables by value
• [a, &b] : capture a by value and b by reference
• [=, &b] : capture all variables by value, and b by reference
• [] : don't capture any variable
• [this, ...] : Capture raw pointer this (DANGEROUS!). Can't prevent use after deletion bug
• [self = shared_from_this(), ...] : Capture shared_ptr to this (SAFE!)

this v/s shared_from_this()

TL;DR:

• Use [this] only if you're sure the object won't be deleted.
• Use [self = shared_from_this()] to prevent dangling pointers in async code. 🚀

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Code

• map function: lambda function to square all numbers of the array and return new array.

#include <functional>
#include <iostream>
#include <vector>

int sum(int a, int b) { return a + b; }

std::vector<int> forEach(std::vector<int> v, std::function<int(int)> fn) {
  std::vector<int> v_sq;
  for (auto &e : v) {
    v_sq.push_back(fn(e));
  }
  return v_sq;
}

void printVec(std::vector<int> &v) {
  for (auto &e : v) {
    std::cout << e << ", ";
  }
  std::cout << "\n";
}
int main() {
  std::vector<int> v = {1, 2, 3, 4, 5};
  std::vector<int> v_sq = forEach(v, [](int i) { return i * i; });
  printVec(v);
  printVec(v_sq);
}

---

Code for capturing value

#include <iostream>
#include <vector>

void printVec(std::vector<int> &v) {
  for (auto &e : v) {
    std::cout << e << ", ";
  }
  std::cout << "\n";
}

int main() {
  std::vector<int> v = {1, 2, 3, 4, 5};
  auto fn = [&v]() {
    // captured vector by reference
    for (auto &e : v) {
      e = e * e;
    }
  };

  printVec(v);
  std::cout << "calling lambda fn\n";
  fn();
  std::cout << "updated vector: \n";
  printVec(v);
}