How to mix boost::bind with C function pointers to implement callbacks

Question

I'm trying to shoehorn in some boost::bind to substitute member functions for straight up C function pointer style callbacks, but I'm running into problems doing the obvious thing. Can someone tell me why the following code snippet can't seem to match up the types in the function call?

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

using namespace std;

class Foo {
public:
  Foo(const string &prefix) : prefix_(prefix) {}
  void bar(const string &message)
  {
    cout << prefix_ << message << endl;
  }
private:
  const string &prefix_;
};

static void
runit(void (*torun)(const string &message), const string &message)
{
  torun(message);
}

int
main(int argc, const char *argv[])
{
  Foo foo("Hello ");
  runit(boost::bind<void>(&Foo::bar, boost::ref(foo), _1), "World!");
}

Why would you do this? What benefit are you trying to achieve? — AJG85
– AJG85, Commented Jul 7, 2011 at 20:58
@AJG85 - I have a C library that is used by multiple C programs (which precludes me from modifying the headers/implementation) and I want to develop a new C++ program using some of the provided functionality. In particular there are places where I want to call one of a possible set of library functions (with identical signatures save the function name) through a generic callback mechanism. — BD at Rivenhill
– BD at Rivenhill, Commented Aug 16, 2011 at 7:30
Ah well a typedef of a boost::function and a function that takes a const& to the typedef in a namespace ought to work if you must. Then you can use boost::bind when calling the namespace function with whatever you want. — AJG85
– AJG85, Commented Aug 16, 2011 at 15:38

ildjarn · Accepted Answer · 2011-07-07 20:57:12Z

4

The result type of bind is not a function pointer, it's a function object which does not happen to be implicitly convertible to a function pointer. Use a template:

template<typename ToRunT>
void runit(ToRunT const& torun, std::string const& message)
{
    torun(message);
}

Or use boost::function<>:

static void runit(boost::function<void(std::string const&)> const& torun,
                  std::string const& message)
{
    torun(message);
}

answered Jul 7, 2011 at 20:57

ildjarn

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2 Comments

sehe Over a year ago

The actual term for it is bind_expression (see the related is_http://msdn.microsoft.com/en-us/library/bb981965.aspx mpl predicate)

BD at Rivenhill Over a year ago

Tested both of these methods and they worked just fine. Thanks!

Jason · Accepted Answer · 2011-07-07 20:55:12Z

2

Rather than having a specific function pointer signature for your first argument to runit, use a template. So for instance:

template<typename function_ptr>
void runit(function_ptr torun, const string &message)
{
  torun(message);
}

answered Jul 7, 2011 at 20:55

Jason

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Comments

Olympian · Accepted Answer · 2011-07-07 20:57:03Z

1

You can use boost::function type for boost::bind objects

answered Jul 7, 2011 at 20:57

Olympian

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Comments

DaveR · Accepted Answer · 2011-07-07 20:57:21Z

0

Pasting the error you get could be useful; however at a guess it's probably due to the fact that "World!" is a string literal (i.e. char[]), not a std::string. Try:

runit(boost::bind<void>(&Foo::bar, boost::ref(foo)), std::string("World!"));

answered Jul 7, 2011 at 20:57

DaveR

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1 Comment

AJG85 Over a year ago

implicit conversion will take place via copy constructor overloads on std::string

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