using std::function, std::bind, perfect forwarding

From:
Andrew Tomazos <andrew@tomazos.com>
Newsgroups:
comp.lang.c++
Date:
Fri, 23 Dec 2011 01:51:57 -0800 (PST)
Message-ID:
<cca9e497-8e57-4e18-bd81-483d0dd3f660@i6g2000vbe.googlegroups.com>
I want to write a class that will wrap an arbitrary function and bind
a set of arguments to it so that I can execute it later (perhaps in
another thread, perhaps repeatedly):

    void f(const A& a) { ... }

    int main()
    {
        A a = ...;
        PFunction pf = MakeFunction(f,a);
        ...
        pf->call(); // executes f(a);
    }

I've implemented something using std::function and std::bind, and it
seems to work - but im having trouble getting the arguments to forward
perfectly.

The output of the below test program (which tracks the argument A) is:

    constructed at 0x7fff7f008d7f
    copied from 0x7fff7f008d7f to 0x7fff7f008a98 <----- HERE
    moved from 0x7fff7f008a98 to 0x20ce068
    desctructed at 0x7fff7f008a98
    desctructed at 0x7fff7f008d7f
    used at 0x20ce068
    desctructed at 0x20ce068

Is there a way to modify MakeFunction and FunctionT (perhaps using
std::forward or std::decay or something) such that the temporary A is
moved into the Function object, and not copied?

Thanks,
Andrew.

#include <iostream>
#include <memory>
#include <functional>

using namespace std;

class Function;

typedef shared_ptr<Function> PFunction;

class Function
{
public:
    virtual void call() = 0;
};

template<class F, class... Args>
class FunctionT : public Function
{
public:
    FunctionT(F f, const Args&... args)
        : m_f(bind(f,args...))
    {}

    void call() { m_f(); }

private:
    function<void()> m_f;
};

template<class F, class... Args>
shared_ptr<Function> MakeFunction(F&& f, Args&&... args)
{
    auto f2 = make_shared<FunctionT<F, Args...>>(f, args...);
    return dynamic_pointer_cast<Function> (f2);
}

class A
{
public:
    A()
    {
        cout << "constructed at " << this << endl;
    }

    A(const A& that)
    {
        cout << "copied from " << &that << " to " << this << endl;
    }

    A(A&& that)
    {
        cout << "moved from " << &that << " to " << this << endl;
    }

    ~A()
    {
        cout << "desctructed at " << this << endl;
    }
};

void f(const A& a)
{
    cout << "used at " << &a << endl;
}

PFunction pf;

void f1()
{
    pf = MakeFunction(f, A());
}

void f2()
{
    pf->call();
}

int main()
{
    f1();
    f2();
    return 0;
}

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