CODEWITHSUNDEEP
c++lambdacallbackmember-variables
Amiram Stark
Amiram Stark

Reputation: 2208

Storing C++ Lambda in a member variable to be used as a callback?

I'm trying to implement a c++ function that gets a Lambda callback as a parameter. The thing is, the callback is initiated asynchronously from another function in the same (called) class. I therefore need to store the Lambda in a member variable so that it can be accessed by the asynchronous function that needs to initiate the callback.

I tried all the ways I could think of to declare, set and call the Lambda using a member variable, but the code always crashes either in the assignment or in the call.

Here's a stripped-out version of what I'm trying to do.

Declaring the function:

void function(const std::function<void()>callback);

Calling the function from the main code:

myClass->function([](){cout << "Callback called";});

If I execute callback from within function it works fine, but I couldn't find a way to store it in a member variable (e.g. m_callback) and invoke it from another function of the same class.

Upvotes: 3

Views: 4596

Answers (2)

BЈовић
BЈовић

Reputation: 64223

Just create a std::function variable, and call it.

#include <iostream>
#include <functional>

struct A{
    std::function<void()> cb;

    void function(const std::function<void()>callback){
        cb=callback;
    }
};

int main() {
    A a;

    a.function([](){std::cout << "Callback called";});

    a.cb();

}

Upvotes: 1

Kerrek SB
Kerrek SB

Reputation: 477040

This should work:

#include <functional>
#include <utility>

struct MyThing
{
    std::function<void()> f_;

    void SetCallback(std::function<void()> f) { f_ = std::move(f); }

    void Action() { f_(); }
};

Usage:

#include <iostream>

MyThing thing;

thing.SetCallback([](){ std::cout << "Boo\n"; });
thing.Action();

Upvotes: 5

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