creating hash (or list) of member functions

[mzimmers]

@Chris-Kawa yeah, I'd actually gotten something working, but I like yours better, because it accepts the callback as an argument, rather than hardcoding it in the timer.

I think this topic is closed, but I wonder if you could give me an explanation for the use of the std::bind. The list is just a list of QObjects; how does this "attach" the callback function?

Thanks!

[Chris Kawa]

@mzimmers In my example it's not a list of QObjects. It's a list of std::function objects.
Clock::update() is a class member function, so to call it you need an instance of that class i.e. instance->update(). The way class member functions work is that they really are just regular functions that have a hidden implicit this parameter, so in effect it's like Clock::update(instance).
std::bind, as the name suggests, creates a callable object that binds a functor with a parameter, so you can call it as if there was no parameter.
The way to think about it is that std::bind(cs, client) creates a struct with the operator(), something like this:

struct Something
{
   Clock* client;
   void operator()() { client->update(); }
}

so it turns a class member function with hidden this parameter into something that can be called without parameters. Then I just store it in a std::function object that can hold any type of callables (functions, functors, lambdas etc.).
In other words std::bind creates something that holds information about both object and a function pointer, so you don't need anything extra to call it.

[JonB]

@mzimmers said in creating hash (or list) of member functions:

but I wonder if you could give me an explanation for the use of the std::bind. The list is just a list of QObjects; how does this "attach" the callback function?

All of this in place of the typedef void (*clientSlot)(), with C life used to be so simple :) We can't use that to call a C++ class member function on an instance. So...

std::function<void()>

I can be used to call a C++ class member method.

registerClient(T* client, void(T::*cs)())

Here's my client object (of a certain type), and here is the class member function.

clients.push_back(std::bind(cs, client));

Creates and pushes an object which, when invoked, will call cs(client). Which turns out to be the same as client->cs(). Which I am just about to question @Chris-Kawa on...!

[JonB]

@Chris-Kawa said in creating hash (or list) of member functions:

The way class member functions work is that they really are just regular functions that have a hidden implicit this parameter, so in effect it's like Clock::update(instance).

OMG! But where does C++ tell you this and that you can write code to use it? I had no idea this was "documented" or "supported". I assumed implementation was opaque/abstract.

[Chris Kawa]

@JonB said:

But where does C++ tell you this and that you can write code to use it?

Well no, you can't currently write it like that. I meant it conceptually. That's just what the compiler does anyway (you can see it e.g. in the mangled function signatures when inspecting C++ library exports).

Although the so called Uniform Call Syntax has been proposed multiple times over the years, including by Mr. C++ himself: N4474, so you might see it in some future standard version.

[JonB]

@Chris-Kawa said in creating hash (or list) of member functions:

Well no, you can't currently write it like that. I meant it conceptually.

Oh, right! For a while there I thought you were saying literally.

I suppose I ought go look at what magic std::bind() actually does, then it would be clear. But I just know it's going to look complicated.... :(

[Chris Kawa]

@JonB said:

But I just know it's going to look complicated.... :(

It does look a bit complicated, but it has to deal with variable number of perfectly forwarded arguments and a lot of weird corner cases users come up with. Also it's the standard library, so it's mangled with all those underscore names and defensive programming style, but if you squint a little you'll see it basically returns a class with operator() like I mentioned.

[JonB]

@Chris-Kawa
Thanks. You gotta love hardcore C++, it's so... simple and clean.

[mzimmers]

@JonB said in creating hash (or list) of member functions:

@Chris-Kawa
Thanks. You gotta love hardcore C++, it's so... simple and clean.

Now, now...no sarcasm.

But yeah...wouldn't you love to have today's compute resources available for solving the problems of 30 years ago?

[J.Hilk]

@JonB said in creating hash (or list) of member functions:

All of this in place of the typedef void (*clientSlot)(), with C life used to be so simple :) We can't use that to call a C++ class member function on an instance

who says you can't ?

#include <array>

class SomeClass : public QObject
{
    Q_OBJECT
    typedef void (SomeClass::*SomeClassFunction)();

    std::array<SomeClassFunction,3> arrayOfSignalsPointers{&SomeClass::signal1,&SomeClass::signal2, &SomeClass::signal3};
    std::array<SomeClassFunction, 3> arrayOfSlotsPointers{&SomeClass::slot1, &SomeClass::slot2, &SomeClass::slot3};

public:
    explicit SomeClass(QObject *parent = nullptr) : QObject(parent)
    {
        QObject::connect(this, &SomeClass::signal1, this, &SomeClass::slot1);
        QObject::connect(this, &SomeClass::signal2, this, &SomeClass::slot2);
        QObject::connect(this, &SomeClass::signal3, this, &SomeClass::slot3);

        qDebug() << "Emit all signals";
        for(auto entry : arrayOfSignalsPointers){
            (this->*entry)();
        }

        qDebug() << "Call all slots directly";
        for(auto entry : arrayOfSlotsPointers){
            (this->*entry)();
        }
    }

signals:
    void signal1();
    void signal2();
    void signal3();

public slots:
    void slot1(){qDebug() <<  Q_FUNC_INFO;}
    void slot2(){qDebug() <<  Q_FUNC_INFO;}
    void slot3(){qDebug() <<  Q_FUNC_INFO;}
};

[JonB]

@J-Hilk said in creating hash (or list) of member functions:

typedef void (SomeClass::*SomeClassFunction)();

I said that you cannot use typedef void (*clientSlot)();, as the OP wrote and one would in C, to call a C++ member function. And you can't: as you show you need ClassName::*function not just plain *function.

Having said that, I was nonetheless not aware that you can use that to get a member function's address and then call (instance->*memberFunctionPointer)(). Thank you for clarifying.

So.... all this std::function<> and particularly std::bind() looks like the usual C++ "why would you want to write something simple when you can wrap it up to be complicated"? ;-)

[Chris Kawa]

@JonB said:

So.... all this std::function<> and particularly std::bind() looks like the usual C++ "why would you want to write something simple when you can wrap it up to be complicated"? ;-)

No, it's a way to be generic. To write typedef void (SomeClass::*SomeClassFunction)() you have to hardcode SomeClass i.e. know it up front. Notice that what @J-Hilk posted will work with one particular class only. I know he just shows how to call a member function from a pointer and that's fine, but it doesn't do much for the original problem.
std::function doesn't care. It just takes any functor you give it and std::bind creates a functor from anything callable you give it.

[JonB]

@Chris-Kawa
Ah yes, I get it.

Modern C++ programming is hugely about templates. But, correct me if I am wrong, C++ did not start out with templates, did it?

[J.Hilk]

@JonB proposed in 1988, realised in 1990 so a decade after the "initial commit" :P

[JonB]

@J-Hilk Ah, thanks, that is much earlier than I realized, I thought more like 10 years later.

[Chris Kawa]

@J-Hilk Where "initial commit" at that time would probably be Stroustrup saving it to a big floppy and physically carrying it to the cubicle of his coworkers. Good old times :D

[JonB]

@Chris-Kawa

We were beyond floppies by then. The first computer I used had 10.5 inch (I think, unless it was only 8 inch) floppies, https://www.computinghistory.org.uk/det/10247/Nord-ND305-355-Floppy-Disk/ :)