C++ Lambda Expressions with parameters

Introduction

Lambdas can have optional parameters inside the parenthesis: [](param1, param2) {}.

Example: 

#include <iostream> 

int main() //  w  w  w . ja va 2  s . c  o m
{ 
    auto mylambda = [](int x, int y) 
    { 
        std::cout << "The value of x is: " << x << ", y is: " << y; 
    }; 
    mylambda(123, 456); 
}

Lambdas are most often used as predicates inside the standard-library algorithm functions.

For example, if we want to count the number of even elements in the container, we would supply a lambda to a std::count_if function.

Example: 

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

int main() /*from w w  w.j a va  2s .c om*/
{ 
    std::vector<int> v = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30 }; 
    auto counteven = std::count_if(std::begin(v), std::end(v), [](int x) {return x % 2 == 0; }); 
    std::cout << "The number of even vector elements is: " << counteven; 
}

Here we have a lambda function that checks if an argument is an even number and returns true if it is.

This lambda is then used as a predicate inside the std::count_if function.

This function only counts the numbers for which the predicate (our lambda expression) returns true.

The std::count_if function iterates through all the vector elements, and each element becomes a lambda argument.

We can use lambdas in other standard-library algorithm functions accepting expressions named callables.

Examples of callables are lambdas and function objects.

By using lambdas, we can more clearly express ourselves, and we do not have to write the verbose class function objects.

Lambdas were introduced in the C++11 standard.




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