Chris Andrews Chris Andrews - 9 days ago 5
C++ Question

Complete example using Boost::Signals for C++ Eventing

I’m aware of the tutorial at boost.org addressing this:
Boost.org Signals Tutorial, but the examples are not complete and somewhat over simplified. The examples there don’t show the include files and some sections of the code are a little vague.

Here is what I need:

ClassA raises multiple events/signals

ClassB subscribes to those events (Multiple classes may subscribe)

In my project I have a lower-level message handler class that raises events to a business class that does some processing of those messages and notifies the UI (wxFrames). I need to know how these all might get wired up (what order, who calls who, etc).

Answer

The code below is a minimal working example of what you requested. ClassA emits two signals; SigA sends (and accepts) no parameters, SigB sends an int. ClassB has two functions which will output to cout when each function is called. In the example there is one instance of ClassA (a) and two of ClassB (b and b2). main is used to connect and fire the signals. It's worth noting that ClassA and ClassB know nothing of each other (ie they're not compile-time bound).

#include <boost/signal.hpp>
#include <boost/bind.hpp>
#include <iostream>

using namespace boost;
using namespace std;

struct ClassA
{
    signal<void ()>    SigA;
    signal<void (int)> SigB;
};

struct ClassB
{
    void PrintFoo()      { cout << "Foo" << endl; }
    void PrintInt(int i) { cout << "Bar: " << i << endl; }
};

int main()
{
    ClassA a;
    ClassB b, b2;

    a.SigA.connect(bind(&ClassB::PrintFoo, &b));
    a.SigB.connect(bind(&ClassB::PrintInt, &b,  _1));
    a.SigB.connect(bind(&ClassB::PrintInt, &b2, _1));

    a.SigA();
    a.SigB(4);
}

The output:

Foo
Bar: 4
Bar: 4

For brevity I've taken some shortcuts that you wouldn't normally use in production code (in particular access control is lax and you'd normally 'hide' your signal registration behind a function like in KeithB's example).

It seems that most of the difficulty in boost::signal is in getting used to using boost::bind. It is a bit mind-bending at first! For a trickier example you could also use bind to hook up ClassA::SigA with ClassB::PrintInt even though SigA does not emit an int:

a.SigA.connect(bind(&ClassB::PrintInt, &b, 10));

Hope that helps!

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