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Lambda Function

Explanation of the Lambda Function

A lambda function in C++ is an anonymous function that can be defined inline and used where a function object or a function pointer is required. whenever you have a function pointer you can use a lambda in C++ .

[](int value){ std::cout << "value: " << value << std::endl; }

Main Components

  1. Capture List []:

    • This part allows the lambda function to capture variables from the surrounding scope. In this case, laneID is captured by value.

  2. Parameter List ():

    • This specifies the parameters that the lambda function takes, just like in a regular function.
    • In this example, the lambda takes a single parameter lane of type const Lane &.

  3. Function Body {}:

    • This is the actual code that the lambda function executes when called.

Example

Here’s another simple example to reinforce the syntax:

auto add = [](int a, int b) { return a + b; };

Quick Syntax Reference

[ capture_list ] ( parameter_list ) -> return_type { function_body }

  • Capture List: [&, x, &y]

    • [] - No capture.
    • [x, &y] - Captures x by value, y by reference.
    • [=] - Captures all local variables by value.
    • [&] - Captures all local variables by reference.

  • Parameter List: (int a, int b)

    • Defines the input parameters like a regular function.

  • Return Type (Optional): -> int

    • Specifies the return type explicitly if needed. Often deduced automatically.

  • Function Body: { return a + b; }

    • Contains the logic to execute.

  • mutable:

    • you can change the values inside the lambda body that are captured with mutable; otherwise you are not allowed.
    auto add = [a]() mutable { a = 5; };

vs Raw Function Pointers

if you want to capture something to the lambda, you cannot declare it as a raw function pointer like void(*func)() for example as a parameter to a function. instead you need to use std::function which is defined in funcitonal header like this:

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

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

int main()
{
    std::vector<int> vector = {1, 3, 6, 2, 10};
    auto lambda = [](int value){ std::cout << "value: " << value << std::endl; };
    ForEach(vector, lambda);

    return 0;
}

How It Works in std::find_if

std::find_if is a function from the algorithm library that searches through a container, like a vector, to find the first element that meets a specified condition.

  • std::find_if takes a range and a predicate (the lambda function).
  • It iterates over the elements in the range and applies the predicate to each element.
  • The iteration stops when the predicate returns true, and std::find_if returns an iterator to that element. If no such element is found, it returns vector.end() iterator (this iterator is one place ahead of the last item in the memory which is an undefined value): more info in vector
#include <iostream>
#include <vector>
#include <algorithm>


int main()
{
    std::vector<int> vector = {1, 3, 6, 2, 10};
    auto it  = std::find_if(vector.begin(), vector.end(), [](int value){ return value > 3;});
    std::cout << *it << std::endl;

    return 0;
}