Passing class instance and method as argument to another function in C++

Question

I have two classes. One class has a method that takes a function as a parameter and that passed function is expected to have a string as an argument. For example:

class Dog{
public:
    void doDogThings(std::function <void(std::string)> theFunction);

}

class Cat{
public:
    void doCatThings(std::string stringInput);
}

And here they are in action

int main(){
   Cat aCat;
   Dog aDog;
   std::string theString = "Oh jeez";
   aDog.doDogThings(<< insert cat's method and argument here >>);
   .....

This is the part that is fouling me up here; I know I should use

bind(&Cat::doCatThings, ref(aCat) ....?.....);

But I am stuck on how to pass the argument for the Cat method as a parameter to this function pointer. Any help would be greatly be appreciated. Thanks.

aschepler · Accepted Answer · 2018-02-13 02:44:02Z

3

The correct bind syntax would be:

aDog.doDogThings(std::bind(
    &Cat::doCatThings, std::ref(aCat), std::placeholders::_1));

The "placeholder" _1 means "the first argument to the resulting functor will get used here to pass to the bound callable".

But I really don't recommend using std::bind in almost any circumstance. It has some annoying gotchas, is often tricky to get correct, tricky to read again after you've written a use of it, and results in the most terrible compiler errors any time you get something slightly wrong. Lambdas can do anything std::bind can do and quite a bit more, without most of those problems.

So instead, I'd suggest:

aDog.doDogThings(
    [&aCat](std::string stringInput)
    { aCat.doCatThings(std::move(stringInput)); }
);

answered Feb 13, 2018 at 2:44

aschepler

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4 Comments

elirg Over a year ago

So background motivation. I have a couple class managers and I am attempting to limit the amount of the coupling between the classes.I have a client manager that is responsible for managing incoming and outgoing messages and a window manager that displays stuff on screen using ncurses. Shared between the two class managers is a ringbuffer that is used to store ingoing and outgoing messages so that they can be displayed in a window. The window needs to be updated if a) a message is received b) a message is sent.

elirg Over a year ago

The goal as to minimize the amount of coupling between the two classes by passing class instances and a corresponding method to trigger updates. When I attempt to adapt your solution it throws an error message. I get the following Parameter type mismatch: Class '__bind<void (ClientManager::*)(string), reference_wrapper<ClientManager>, const placeholders::_1ph<1> &>' is not compatible with class "function<void(string)>' I don't really know if am passing and using the methods correctly at this point because I have never done this kind of thing before.

aschepler Over a year ago

Not sure what that could mean. Does the little program at coliru.stacked-crooked.com/a/00c817795b7ba652 compile on your system? If you can come up with the shortest possible program that gives you the same error message, that could make a good separate question here.

elirg Over a year ago

I got it to work. It was wanting some kind of import and once I did that when I adapted your example it worked exactly as intended.

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