Using a void function as a callback, convert from "std::function?

Question

I have a boost::Asio based serial port reader that uses a callback, as follows:

void received(const char *data, unsigned int len)
{

        string s(data, len);
        cout <<  s  << endl;    

}

int main()
{

    try {
        CallbackAsyncSerial serial("COM4", 115200);
        serial.setCallback(received);
        for(;;)
        {
            if(serial.errorStatus() || serial.isOpen()==false)
            {
                cerr<<"Error: serial port unexpectedly closed"<<endl;
                break;
            }
            char c;
            cin.get(c); //blocking wait for standard input
           serial.write(&c,1);
        }
        quit:
        serial.close();
    } catch (std::exception& e) {
        cerr<<"Exception: "<<e.what()<<endl;
    }

}

This, as above, compiles and runs. Now I need to merge it into an existing class, so i have:

void MyClass::received(const char *data, unsigned int len)
{

        string s(data, len);
        cout <<  s  << endl;    

}

int MyClass::main()
{

    try {
        CallbackAsyncSerial serial("COM4", 115200);
        serial.setCallback(received); //COMPILE ERROR HERE
        for(;;)
        {
            if(serial.errorStatus() || serial.isOpen()==false)
            {
                cerr<<"Error: serial port unexpectedly closed"<<endl;
                break;
            }
            char c;
            cin.get(c); //blocking wait for standard input
           serial.write(&c,1);
        }
        quit:
        serial.close();
    } catch (std::exception& e) {
        cerr<<"Exception: "<<e.what()<<endl;
    }

}

However, adding MyClass::received stops the compile, giving me:

Error (active)  E0415   no suitable constructor exists to convert from "void (const char *data, unsigned int len)" to "std::function<void (const char *, size_t)>"

I have tried adjusting it to:

std::function< void() >received(const char *data, unsigned int len);

But i see the same thing. What do i need to do here?

Like this? `serial.setCallback(std::bind(&MyClass::received, this, std::placeholders::_1, std::placeholders::_2));` — anti, Sep 04 '19 at 09:06
Have you tried making your `MyClass:received` function a `static` member? — Adrian Mole, Sep 04 '19 at 09:07
Sadly the most direct duplicate is [stuck in pre-C++11 land](https://stackoverflow.com/questions/400257/how-can-i-pass-a-class-member-function-as-a-callback)... Do we not have a canonical dupe target for this? — Max Langhof, Sep 04 '19 at 09:37

score 1 · Answer 1 · edited Sep 04 '19 at 09:17

1

Answered in comments by rafix07

What I needed was to adjust the setCallback line to:

serial.setCallback(
    std::bind(&MyClass::received, this, std::placeholders::_1, std::placeholders::_2));

edited Sep 04 '19 at 09:17

Holt

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answered Sep 04 '19 at 09:17

anti

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Do you not have lambdas available? `std::bind` is basically obsolete since C++11... – Max Langhof Sep 04 '19 at 09:21
How would this look using lambdas? – anti Sep 04 '19 at 09:22
2

Like that `serial.setCallback([this] (auto data, auto len) { received(data, len); })`. – Timo Sep 04 '19 at 09:23
Also works great. Thank you! Is this the more recommended approach? – anti Sep 04 '19 at 09:25
1

Yes it is. Watch [this video](https://www.youtube.com/watch?v=ZlHi8txU4aQ) for more details on the subject. – Timo Sep 04 '19 at 09:28

Using a void function as a callback, convert from "std::function?

1 Answers1