Assigning a non-static function to a variable in c++

Question

I was trying to assign a non-static function to a variable in C++. I have two functions, which are onMessageReceived and enableCommand. Here are definitions:

NetworkUtil.h:

class NetworkUtil{
public:
    std::function<void(char* message)> onMessageReceived;
    void enableCommand(char* cmd);
private:
// some private variables
}

NetworkUtil.cpp:

void NetworkUtil::enableCommand(char* cmd) {
    if (strcmp(cmd, "INIT_ACK") == 0){
        mCreateEnabled = true;
    }
    else if (strcmp(cmd, "START_ACK")){
        mStartEnabled = true;

    }
    else{
        std::cerr << "Unknown Command: " << cmd << std::endl;
    }
}

Here is a program that assigns a function to a variable:

void NetworkUtil::onMessage(WebSocket* ws, const WebSocket::Data &data)
{
// not working
    onMessageReceived = &(NetworkUtil::enableCommand);
// not working either
    onMessageReceived = std::bind(&NetworkUtil::enableCommand, data.bytes);
}

I tried both, but non of them are working. Can I get some advice on assigning a non-static function to a variable?

`if (strcmp(cmd, "START_ACK"))`?? Did you mean to use `if (strcmp(cmd, "START_ACK") == 0)`? — R Sahu, Jul 26 '15 at 04:43
Not up on my std::bind, but I doubt you need to take the address of the function. OK. Maybe you do. Quite surprised by this. — user4581301, Jul 26 '15 at 04:44
@cpplearner: `std::bind(&NetworkUtil::enableCommand, this);` does not work. — Peter Hwang, Jul 26 '15 at 04:45
@cpplearner: Error Message: Term does not evaluate a function taking 1 arguments. — Peter Hwang, Jul 26 '15 at 04:49
@ user4581301: It accepts two arguments, whose types are `Websocket*` and `data`. `data` is nothing but `struct` that contains `char*` type instance variable. — Peter Hwang, Jul 26 '15 at 04:51
I think @cpplearner almost had it. Give `std::bind(&NetworkUtil::enableCommand, this, data.bytes);` a spin, but I think this will get bind right and give a different error because you can't stuff bind into whatever onMessageReceived is. — user4581301, Jul 26 '15 at 04:53
By the way, `onMessageReceived` takes one input, which is `char*`. Sorry for misreading your question. — Peter Hwang, Jul 26 '15 at 04:56
No worries. I wasn't reading well. Your Question answered my question. The trick is NetworkUtil::enableCommand is a class member so it has `NetworkUtil * this` as a hidden extra param. — user4581301, Jul 26 '15 at 04:58
@user4581301 I wish to learn more about this `std::bind` and how it works and when should it be used. Any practical example or what is happening wrong in this case that normal assignment is not working properly. An answer with this explanation will be really helpful. — Vinay Shukla, Jul 26 '15 at 04:58
@VinayShukla I think you'll find your answer in the comment just above yours. — user4581301, Jul 26 '15 at 05:00
I think this is a "pointer to member" issue e.g.http://stackoverflow.com/questions/670734/c-pointer-to-class-data-member — mksteve, Jul 26 '15 at 08:56

score 1 · Answer 1 · answered Jul 26 '15 at 09:58

1

Try to do like this:

onMessageReceived = std::bind(&NetworkUtil::enableCommand, this, _1);

answered Jul 26 '15 at 09:58

JerryYoung

267
1
3

may need `using std::placeholders;` – M.M Jul 26 '15 at 10:46

UnknownGosu · Answer 2 · 2015-07-26T08:03:28.817

Since you're using std::function (hence C++11 or later), I assume you can give lambda functions a try:

// Create a local variable for capturing purposes.
// You must make sure the source data is never outlived by the callback
// to avoid the "use after free" scenario.
auto bytes = data.bytes;
onMessageReceived = [=](char * message) { this->enableCommand(bytes); };

= means to capture everything actually used by the closure by value (including the value of the this pointer), and due to use of type erasure you can assign it to a signature-compatible std::function.

Assigning a non-static function to a variable in c++

2 Answers2