Capture class method in a lambda

Question

I have several methods in my class. I want a lambda in one method to call another method of the class. But I don't want to capture this so that I can limit the scope of what the lambda has access to. Is this possible?

I've tried to capture the method as [&method = Class::my_method] or variations of that but there's something I'm missing.

#include <iostream>

class Hello
{
public:

  double test_lambda(const double in)
  {
      // If I capture `this` instead, then this works fine
      const auto a_lambda = [add_number = this->add_number](const double in)
      {
        return add_number(in);  
      };
      return a_lambda(in);
  }

private:

  double add_number(const double in)
  {
      return in + 2.0;
  }

};

int main()
{
    Hello h;
    std::cout << "Hello World: " << h.test_lambda(4.0); // Expect 6
    return 0;
}

I would hope that I could capture a method like this but I'm not sure if it's possible.

The reason I want to do this based on a coding preference to limit what the lambda has access to. For example, if I wanted the lambda to mutate members of a class, I could do

another_lambda = [this](){
m_value_one = 1.0;
m_value_two = 2.0;
// This has access to other members
}

Or to limit what the lambda can mutate, I can do:

another_lambda = [&value_one = this->m_value_one, &value_two = this->m_value_two](){
value_one = 1.0;
value_two = 2.0;
// This does not have access to other members
}

The post is asking whether the same can be done with class methods, but it seems that it cannot.

Capture a method that gets called on what object? What is the this object upon which you want return add_number(in); to operate? — Nicol Bolas
– Nicol Bolas, Commented Jun 21, 2019 at 22:02
Possible duplicate of Pass a class member function as a function parameter — JaMiT
– JaMiT, Commented Jun 21, 2019 at 22:11
@NicolBolas I think I see the problem here, which is why it works if add_number was static — Otto Nahmee
– Otto Nahmee, Commented Jun 24, 2019 at 15:10

Alecto · Accepted Answer · 2019-06-21 22:25:20Z

Capturing and using a pointer to member function

A member function isn't a first-class object, and that means that you can't assign in to a variable. The closest thing to this is assigning a pointer to the member function to a variable, but in order to call a pointer to a member function, you need to have a pointer to the original class as well:

#include <iostream>

class Hello
{
   public:
    double test_lambda(double in)
    {
        auto a_lambda = [add_number = &Hello::add_number, this](double in) {
            // This is how we call a pointer to a member function
            return (this->*add_number)(in);
        };
        return a_lambda(in);
    }

   private:
    double add_number(double in) { return in + 2.0; }
};

int main()
{
    Hello h;
    std::cout << "Hello World: " << h.test_lambda(4.0); // Expect 6
    return 0;
}

Simpler solution - capture by reference

We can capture the current class by reference, and this allows you to use add_number without any qualifiers:

#include <iostream>

class Hello
{
   public:
    double test_lambda(double in)
    {
        auto a_lambda = [&](double in) {
            // add_number can be called normally
            return add_number(in);
        };
        return a_lambda(in);
    }

   private:
    double add_number(double in) { return in + 2.0; }
};

int main()
{
    Hello h;
    std::cout << "Hello World: " << h.test_lambda(4.0); // Expect 6
    return 0;
}

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Capture class method in a lambda

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Capturing and using a pointer to member function

Simpler solution - capture by reference

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