Re: [Boost-users] bind return value

10 Sep 2003

      Darren Vincent Hart wrote:
...
Boost gurus,
I am using boost::bind in combination with boost::signals for a GUI
toolkit.  The signals return bool for convenience, but it is often
desireable to bind a void function and connect it to a signal.  Is
there a convenient way to assign a return value for a void function.
As an example:
-----------------------------------------------------------------
struct panel
{
    void print() { std::cout << "print" << std::endl; }
}
struct widget
{
    boost::signal on_event;
}
panel p;
widget w;
// is there some way to do something like this?
w.on_event.connect(boost::bind_return(panel::print, &p, true));
You can do this with Lambda:

using namespace boost::lambda;

    w.on_event.connect( (bind(panel::print, &p), true) );

but not with boost::bind, sorry.

Not out of the box, anyway.

template class override_return_t
{
private:

    F f_;
    V v_;

public:

    typedef V result_type;

    override_return_t(F const & f, V const & v): f_(f), v_(v)
    {
    }

    result_type operator()()
    {
        f_();
        return v_;
    }

    result_type operator()() const
    {
        f_();
        return v_;
    }
};

template override_return_t override_return(F const &
f, V const & v)
{
    return override_return_t(f, v);
}

#include 
#include 
#include <iostream>

void g()
{
    std::cout << "g()\n";
}

int main()
{
    boost::function f = override_return(boost::bind(g), true);
    std::cout << f() << std::endl;
}

You can even go a bit further and bring boost::bind one step closer to its
lambda counterpart:

template
override_return_t, V>
operator,(boost::_bi::bind_t const & f, V const & v)
{
    return override_return_t, V>(f, v);
}

int main()
{
    boost::function f = (boost::bind(g), true);
    std::cout << f() << std::endl;
}

Re: [Boost-users] bind return value

Peter Dimov