The callback-to-derived-member function problem

[ryan.mitchley]

Hi all

I have a class (cPort) that is designed to receive objects and,
depending on the type, call a handler (callback) in any descendant of a
cProcessBlock class. Callback functions take a shared_ptr<cBas eas a
parameter, and return void.

The code was working fine, although I have encountered problems (under
a Microsoft compiler, of course - VC 8.0) when I attempt to add
callbacks to a class with multiple inheritance. I hate multiple
inheritance in general, but this is necessary to interface with another
library. I think there is some issue regarding the size of member
function pointers in VC.

Anyway - I believe that this code can be converted to use
boost::function and/or boost::bind to iron out these issues. The
problem is that I am going more than slightly squint when I read the
boost docs.

I have attached a reduced set of my code below. I would be extremely
grateful if someone could point out how this code could be converted to
use the boost representations .

Thanks for any and all replies!

Ryan

typedef void (cProcessBlock: :*PortCallbackF nPtr)(shared_pt r<cBase>);

// structure that associates a callback function pointer with the
required caller
class cPortCallback : public cBase
{
public:
cPortCallback() :
m_pPortCallback Fn(NULL), m_pCaller(m_pNU LL) { }
cPortCallback(P ortCallbackFnPt r pPortCallbackFn ,
shared_ptr<cBas epCaller) :
m_pPortCallback Fn(pPortCallbac kFn), m_pCaller(pCall er) { }
PortCallbackFnP tr m_pPortCallback Fn;
shared_ptr<cBas em_pCaller;
};

class cPort : public cProcessBlock
{
public:
/// Add a new callback
void AddCallback(Por tCallbackFnPtr NewCallbackFnPt r, std::string
strClassName,
shared_ptr<cBas epCaller);

protected:
/// Callback registry (map of callback handlers)
map<string, cPortCallbackm_ CallbackRegistr y;
};

void cPort::AddCallb ack(PortCallbac kFnPtr NewCallbackFnPt r, std::string
strClassName,
shared_ptr<cBas epCaller)
{
m_CallbackRegis try[strClassName] = cPortCallback(N ewCallbackFnPtr ,
pCaller);
}

bool cPort::Timeslic e()
// (called repeatedly in thread) - continuously receive new objects and
process using callbacks
{
shared_ptr<cBas epNewObject;
pNewObject = Receive();

// Call appropriate callback depending on the received object's
class name
if (pNewObject) {
PortCallbackFnP tr CallbackFn =
m_CallbackRegis try[pNewObject->ClassName()].m_pPortCallbac kFn;
if (CallbackFn) // if appropriate callback handler is found

((*(shared_poly morphic_cast<cP rocessBlock>(m_ CallbackRegistr y[pNewObject->ClassName()].m_pCaller))).* (CallbackFn))(p NewObject);
}
return true;
}
An example of adding a callback:
AddCallback(sta tic_cast<PortCa llbackFnPtr>(&c Graph::OnNewGra phData),
"cRealMatri x", this);

[mlimber]

ry***********@g mail.com wrote:

Hi all

I have a class (cPort) that is designed to receive objects and,
depending on the type, call a handler (callback) in any descendant of a
cProcessBlock class. Callback functions take a shared_ptr<cBas eas a
parameter, and return void.

The code was working fine, although I have encountered problems (under
a Microsoft compiler, of course - VC 8.0) when I attempt to add
callbacks to a class with multiple inheritance. I hate multiple
inheritance in general, but this is necessary to interface with another
library. I think there is some issue regarding the size of member
function pointers in VC.

Anyway - I believe that this code can be converted to use
boost::function and/or boost::bind to iron out these issues. The
problem is that I am going more than slightly squint when I read the
boost docs.

I have attached a reduced set of my code below. I would be extremely
grateful if someone could point out how this code could be converted to
use the boost representations .

Thanks for any and all replies!

Ryan

typedef void (cProcessBlock: :*PortCallbackF nPtr)(shared_pt r<cBase>);

// structure that associates a callback function pointer with the
required caller
class cPortCallback : public cBase
{
public:
cPortCallback() :
m_pPortCallback Fn(NULL), m_pCaller(m_pNU LL) { }
cPortCallback(P ortCallbackFnPt r pPortCallbackFn ,
shared_ptr<cBas epCaller) :
m_pPortCallback Fn(pPortCallbac kFn), m_pCaller(pCall er) { }
PortCallbackFnP tr m_pPortCallback Fn;
shared_ptr<cBas em_pCaller;
};

class cPort : public cProcessBlock
{
public:
/// Add a new callback
void AddCallback(Por tCallbackFnPtr NewCallbackFnPt r, std::string
strClassName,
shared_ptr<cBas epCaller);

protected:
/// Callback registry (map of callback handlers)
map<string, cPortCallbackm_ CallbackRegistr y;
};

void cPort::AddCallb ack(PortCallbac kFnPtr NewCallbackFnPt r, std::string
strClassName,
shared_ptr<cBas epCaller)
{
m_CallbackRegis try[strClassName] = cPortCallback(N ewCallbackFnPtr ,
pCaller);
}

bool cPort::Timeslic e()
// (called repeatedly in thread) - continuously receive new objects and
process using callbacks
{
shared_ptr<cBas epNewObject;
pNewObject = Receive();

// Call appropriate callback depending on the received object's
class name
if (pNewObject) {
PortCallbackFnP tr CallbackFn =
m_CallbackRegis try[pNewObject->ClassName()].m_pPortCallbac kFn;
if (CallbackFn) // if appropriate callback handler is found

((*(shared_poly morphic_cast<cP rocessBlock>(m_ CallbackRegistr y[pNewObject->ClassName()].m_pCaller))).* (CallbackFn))(p NewObject);
}
return true;
}
An example of adding a callback:
AddCallback(sta tic_cast<PortCa llbackFnPtr>(&c Graph::OnNewGra phData),
"cRealMatri x", this);

You might want to ask on the Boost user's list, but since the
components you're talking about are mostly in TR1 (not counting
shared_polymorp hic_cast), if you post a more complete example that
demonstrates the problem and that we can cut and paste into our editors
unchanged (see the guidelines for posting code:
http://parashift.com/c++-faq-lite/ho....html#faq-5.8), we can
probably help you here.

Cheers! --M

[k04jg02]

Using boost::bind and boost::function for member function call backs is
actually surprisingly easy. You just make sure the first parameter you
bind is the 'this' object you want to call the member function on.

boost::function <void (void)f;
f = boost::bind(&My Class::MyMethod , this);

If you have additional parameters left that should be called on f:

boost::function <void (int, int)f;
f = boost::bind(&My Class::MyMethod , this, _1, _2);

This says, "MyMethod is a member function that takes two ints and
returns void. For the first parameter always give it 'this.'" which
effectively makes f a function that now takes 2 variables instead of 3
(before it took the 'this' pointer and two additional parameters). If
you did:

f = boost::bind(&My Class::MyMethod , this, _1, _1);

Now f will take only 1 parameter, and it will pass it as both the first
and second parameter to MyMethod.

I usually just make the particular boost::function <template needed a
typedef.

mlimber wrote:

ry***********@g mail.com wrote:

Hi all

I have a class (cPort) that is designed to receive objects and,
depending on the type, call a handler (callback) in any descendant of a
cProcessBlock class. Callback functions take a shared_ptr<cBas eas a
parameter, and return void.

The code was working fine, although I have encountered problems (under
a Microsoft compiler, of course - VC 8.0) when I attempt to add
callbacks to a class with multiple inheritance. I hate multiple
inheritance in general, but this is necessary to interface with another
library. I think there is some issue regarding the size of member
function pointers in VC.

Anyway - I believe that this code can be converted to use
boost::function and/or boost::bind to iron out these issues. The
problem is that I am going more than slightly squint when I read the
boost docs.

I have attached a reduced set of my code below. I would be extremely
grateful if someone could point out how this code could be converted to
use the boost representations .

Thanks for any and all replies!

Ryan

typedef void (cProcessBlock: :*PortCallbackF nPtr)(shared_pt r<cBase>);

// structure that associates a callback function pointer with the
required caller
class cPortCallback : public cBase
{
public:
cPortCallback() :
m_pPortCallback Fn(NULL), m_pCaller(m_pNU LL) { }
cPortCallback(P ortCallbackFnPt r pPortCallbackFn ,
shared_ptr<cBas epCaller) :
m_pPortCallback Fn(pPortCallbac kFn), m_pCaller(pCall er) { }
PortCallbackFnP tr m_pPortCallback Fn;
shared_ptr<cBas em_pCaller;
};

class cPort : public cProcessBlock
{
public:
/// Add a new callback
void AddCallback(Por tCallbackFnPtr NewCallbackFnPt r, std::string
strClassName,
shared_ptr<cBas epCaller);

protected:
/// Callback registry (map of callback handlers)
map<string, cPortCallbackm_ CallbackRegistr y;
};

void cPort::AddCallb ack(PortCallbac kFnPtr NewCallbackFnPt r, std::string
strClassName,
shared_ptr<cBas epCaller)
{
m_CallbackRegis try[strClassName] = cPortCallback(N ewCallbackFnPtr ,
pCaller);
}

bool cPort::Timeslic e()
// (called repeatedly in thread) - continuously receive new objects and
process using callbacks
{
shared_ptr<cBas epNewObject;
pNewObject = Receive();

// Call appropriate callback depending on the received object's
class name
if (pNewObject) {
PortCallbackFnP tr CallbackFn =
m_CallbackRegis try[pNewObject->ClassName()].m_pPortCallbac kFn;
if (CallbackFn) // if appropriate callback handler is found

((*(shared_poly morphic_cast<cP rocessBlock>(m_ CallbackRegistr y[pNewObject->ClassName()].m_pCaller))).* (CallbackFn))(p NewObject);
}
return true;
}
An example of adding a callback:
AddCallback(sta tic_cast<PortCa llbackFnPtr>(&c Graph::OnNewGra phData),
"cRealMatri x", this);

You might want to ask on the Boost user's list, but since the
components you're talking about are mostly in TR1 (not counting
shared_polymorp hic_cast), if you post a more complete example that
demonstrates the problem and that we can cut and paste into our editors
unchanged (see the guidelines for posting code:
http://parashift.com/c++-faq-lite/ho....html#faq-5.8), we can
probably help you here.

Cheers! --M

[ryan.mitchley]

k0*****@gmail.c om wrote:

Using boost::bind and boost::function for member function call backs is
actually surprisingly easy. You just make sure the first parameter you
bind is the 'this' object you want to call the member function on.

Thanks! The conversion did indeed turn out to be surprisingly more easy
than I was expecting . . .

I hope the performance penalty of the "bind" stuff is not too high, but
I think the clarity of the code is probably worth it.

Ryan