Introduction

One important new feature of C++11 was lambda expressions. In C++14 and C++17, the lambda expressions got some new additions, which have made them even more powerful. But first, what is a lambda expression?

Lambda expressions or lambda functions construct closures. A closure is a very generic term for unnamed objects that can be called like functions. In order to provide such a capability in C++, such an object must implement the () function calling operator, with or without parameters. Constructing such an object without lambda expressions before C++11 could still look like the following:

#include <iostream>
#include <string>

int main() {
    struct name_greeter {
        std::string name;

        void operator()() {
            std::cout << "Hello, " << name << '\n'; 
        }
    };

    name_greeter greet_john_doe {"John Doe"};
    greet_john_doe();
}

Instances of the name_greeter struct obviously carry a string with them. Note that both this...