C++11 Lambda : How to Capture Member Variables Inside Lambda Function

In the previous article, we have discussed about CPP 11 Multithreading – Part 2: Joining and Detaching Threads. Let us learn how to Capture Member Variables Inside Lambda Function in C++ Program.

Capturing Member Variables Inside Lambda Function

In this article we are going to see how to capture a member variable inside a lambda function.

We will be using a class Odd to record the odd numbers read by the object. There we will be using the lambda function that would capture the member variable.

We can’t catch a member variable by passing it as a value or by reference. The below example will demonstrate how to not capture member variable.

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

class Odd
{
    // Couns the variable
    int variableCounter = 0;

public:
    int Count()
    {
        return variableCounter;
    }
    void update(std::vector<int> &vec)
    {
        // Capturing member variable by value
        std::for_each(vec.begin(), vec.end(), [variableCounter](int element)
                      {
                          if (element % 2)
                              variableCounter++; // Accessing member variable from outer scope
                      });
    }
};

int main()
{
    std::vector<int> val = {55, 78, 49, 2323, 11, 1, 25, 4};
    Odd obj;

    obj.update(val);
    int counterVariable = obj.Count();
    std::cout << "Counter is = " << counterVariable;
    return 0;
}

The above code where we pass by value will show error during compilation.

Capturing Member variables inside Lambda Function :

To capture the member variables into lambda function we have to use this .

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

class Odd
{
    // Counts the variable
    int variableCounter = 0;

public:
    int Count()
    {
        return variableCounter;
    }
    void update(std::vector<int> &vec)
    {
        // Capturing member variable by value using THIS
        std::for_each(vec.begin(), vec.end(), [this](int element)
                      {
                          if (element % 2)
                              variableCounter++;
                      });
    }
};

int main()
{
    std::vector<int> val = {55, 78, 49, 2323, 11, 1, 25, 4};
    Odd obj;

    obj.update(val);
    int counterVariable = obj.Count();
    std::cout << "Counter is = " << counterVariable;
    return 0;
}

Output :
Counter is = 6

In the above program we saw that “this” which is copied by value inside lambda, so all member variables from outer scope can be accessed directly.