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C++ Lambda Expressions

A lambda expression is a small unnamed function written where it is used. A lambda can be stored in a variable and called like a function.

A lambda stored and called

The greet lambda has no parameters. It is stored in a variable and called twice.

#include <iostream>

int main() {
  auto greet = [] {
    std::cout << "Hello from a lambda\n";
  };
  greet();
  greet();
  return 0;
}

Parameters and a return value

The add lambda takes two ints and returns their sum. The returned value prints like any function result.

#include <iostream>

int main() {
  auto add = [](int a, int b) {
    return a + b;
  };
  std::cout << "Sum: " << add(3, 4) << "\n";
  std::cout << "Sum: " << add(10, 25) << "\n";
  return 0;
}

A lambda inside sort

The sort algorithm orders words by length here. The comparison comes from a lambda passed as an argument.

#include <algorithm>
#include <iostream>
#include <string>
#include <vector>

int main() {
  std::vector<std::string> words = {"strawberry", "fig", "banana", "pear"};
  std::sort(words.begin(), words.end(), [](const std::string& a, const std::string& b) {
    return a.size() < b.size();
  });
  for (const std::string& word : words) {
    std::cout << word << "\n";
  }
  return 0;
}

Two lambdas chosen by an if

Two lambdas wait in two variables. An if statement picks which one runs.

#include <iostream>

int main() {
  auto greet = [] {
    std::cout << "Welcome in\n";
  };
  auto farewell = [] {
    std::cout << "See you soon\n";
  };
  bool arriving = true;
  if (arriving) {
    greet();
  } else {
    farewell();
  }
  arriving = false;
  if (arriving) {
    greet();
  } else {
    farewell();
  }
  return 0;
}

Example

#include <algorithm>
#include <iostream>
#include <vector>

int main() {
  std::vector<int> scores = {42, 87, 65, 91, 58};
  int passed = std::count_if(scores.begin(), scores.end(), [](int score) {
    return score >= 60;
  });
  std::cout << "Passed: " << passed << "\n";
  return 0;
}

Key points

  • A lambda starts with square brackets called the capture list.
  • Every lambda in this lesson has an empty capture list.
  • Parameters go in round brackets after the capture list.
  • The lambda body sits in braces like a function body.
  • A stored lambda is called through its variable name.
  • A lambda can be passed straight into an algorithm as an argument.

In this example, a lambda tests each score inside count_if. Three scores reach 60 or more. The count prints as 3.

Note

The std::function type can hold a lambda in a named variable of a fixed type.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

What does an empty capture list at the start of a lambda mean?