59 Lambdas in C++

1. What is Lambda Used For?

Lambda is essentially a way for us to define something called an anonymous function. This method allows us to create a quick, one-time function without actually defining a formal function. We tend to view it more as a variable rather than a formal function that exists as a symbol in our compiled code.

As long as you have a function pointer, you can use lambda in C++. This is how it works, so lambda is a method for us to define a function without needing to define it through a function declaration. The usage of lambda is that wherever we would set a function pointer to point to a function, we can set it to a lambda instead.

Lambda is a great way to specify code that we want to run in the future as a function.

What if we want to bring external variables into the instructions inside the lambda function? Just like when we create our own functions, there are two methods: pass by value and pass by reference. This is where capturing comes into play. The [] specifies how we intend to pass variables. ([=], pass all variables by value; [&] pass all variables by reference) You can also pass individual variables, [a] passes a by value, [&a] passes a by reference.

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

void ForEach(const std::vector<int>& values,const std::function<void(int)>& func)
{
    for (int value : values)
        func(value);
}

int main()
{
    std::vector<int> values = { 1,5,4,2,3 };
    int a = 5;
    auto lambda = [=](int value) {std::cout << a << std::endl; };
    ForEach(values,lambda);

    std::cin.get();
}

Even though a is passed by value and copied in the lambda here, it does not allow us to reassign a within it, unlike in a normal function. To fix this, simply add mutable in front (all of this can be learned on cppreference.com)

auto lambda = [=](int value) mutable { a = 6; std::cout << a << std::endl; };

// 6 6 6 6 6

2. Why You Might Want to Use Lambda

#include <algorithm>

auto it = std::find_if(values.begin(), values.end(), [](int value) {return value > 3; });

std::cout << *it << std::endl;

The std::find_if function searches within the values container for the first element that satisfies the condition specified by the lambda expression. values.begin() and values.end() represent the start and end iterators of the container, respectively. This function will search within the range of all elements in the container. [](int value) {return value > 3; } is an anonymous lambda expression that specifies the search condition: the element's value is greater than 3. std::find_if will return an iterator pointing to the first element that satisfies the condition. If no such element is found, it will return the iterator values.end().

Equivalent to:

void ForEach(const std::vector<int>& values,const std::function<void(int)>& func)
{
    for (int value : values)
        if(value > 3)    // Add boolean judgment
            func(value);
}

But we can use lambda to quickly implement this by specifying this boolean condition.