How to use Lambdas In C++

Q: How to use Lambdas In C++

In C++ 11, there is a std::function<> template class that allows to pass functions as objects. An object of std::function<> can be created as follows.

2. Using std::function<>

Lambdas in C++ provide a way to define inline, one-time, anonymous function objects. These lambdas can be defined in a place where it is required to pass a function as an argument.

Example:

C++

// C++ program to pass the function as
// parameter as a lambda expression
#include <functional>
#include <iostream>
using namespace std;
 
// Function that takes a pointer
// to a function
int invoke(int x, int y,
           function<int(int, int)> func)
{
    return func(x, y);
}
 
// Driver Code
int main()
{
 
    // Define lambdas for addition and
    // multiplication operation where
    // we want to pass another function
    // as a parameter
 
    // Perform Addition
    cout << "Addition of 20 and 10 is ";
    int k = invoke(20, 10,
                   [](int x,
                      int y) -> int {
                       return x + y;
                   });
 
    cout << k << '\n';
 
    // Perform Multiplication
    cout << "Multiplication of 20"
         << " and 10 is ";
    int l = invoke(20, 10,
                   [](int x,
                      int y) -> int {
                       return x * y;
                   });
 
    cout << l << '\n';
 
    return 0;
}

Output:

Addition of 20 and 10 is 30
Multiplication of 20 and 10 is 200

Passing a Function as a Parameter in C++

A function is a set of statements that take inputs, perform some specific computation, and produce output. The idea to use functions is to perform some commonly or repeatedly done tasks together and make a function so that instead of writing the same code again and again for different inputs.

The general form of a function is in the below format:

return_type function_name([ arg1_type arg1_name, ... ]) 
{
     // Perform Operations
}

Passing a function as an argument is a useful concept in C++. This concept has already been used while passing a custom comparator function as an argument in std::sort() to sort a sequence of objects as per the need. In this article, we will discuss different ways to design functions that accept another function as an argument.

A function can be passed as a parameter with 3 approaches i.e.