Re: [Boost-users] bind member function of a member of a class...

4 Mar 2009


      Alan M. Carroll <amc <at> network-geographics.com> writes:
...
That's not a good usage scenario for bind - you should just call the method
...
want to either (1) pass the functor to something else or (2) store the 
functor for future use.
Conceptually, as a rough guide, you should think of the result of bind as a
...
naturally, the result of bind of anything isn't going to be equal to 1, just
...
equal to one. Instead you want to compare the result of invoking / 
dereferencing the functor, which is done
with the function operator (). So your example would work if you added that, 
e.g.
return (boost::bind (&B::getA::test, b)() ==1);
But a more standard use case would look like (untested)
template < typename F > bool check_property(F const& func, B const& b) {
  return func(b);
}
bool testbind() {
  B b;
  return check_property(boost::bind(&A::test, boost::bind(&B::getA, _1)) == 
1, b);
}
The == operator of a bind result and a int creates a functor that compares
directly. Bind is useful when you
pointer, not a value. So,
like you won't have a pointer
the result of invoking the functor
...
to that int. This passes that to check_property which invokes it with the 
function operator.
P.S. If you want to store a Bind result for later use, look at 
Boost.Function. These two libraries are duals
of each other - Bind to *pass* function objects to somebody else, Function to 
*receive* function objects
from somebody else.
At 04:33 PM 3/4/2009, you wrote:
...
I cannot figure out how to use boost::bind in following scenario:
class A
{
 int i_;
public:
 int test() {return i;}
};
class B
{
  A _a;
public:
  A& getA() {return _a;}
}
bool testbind()
{
 B b;
// here I am trying to find out if b.getA().test() equals 1
 // I understand that B::getA::test is incorrect, and that's my question-
 // how to use bind here?
 return (boost::bind (&B::getA::test, b) ==1);
}
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Thank you for the quick reply and comments that I agree with. In the meantime I 
managed to make it work with double bind. When I looked at your suggestion I 
tried to implement it. Unfortunately I am getting compiled errors. Here is the 
complete test example (vc2008):

#include "stdafx.h"
#include <sstream>
#include <algorithm>
#include <vector>
#include <boost\bind.hpp>

class A
{
	int _i;
public:
	A(int val) : _i(val){}
	A(const A& another) : _i(another._i){}

	int value () {return _i;}
};

class B
{
	A _a;
public:
	B (int val) : _a(val){};
	B (const B& another) : _a(another._a) {}
	A& getA () {return _a;}
};

int _tmain(int argc, _TCHAR* argv[])
{
	std::vector<B> lstb;
	lstb.push_back(B(1));

	std::vector<B>::iterator it = std::find_if (lstb.begin(), lstb.end(),
		boost::bind (&B::getA::value, _1)() == 1);
	if (it == lstb.end())
		return 1;

	return 0;
}

Compiler errors are:
error C3083: 'getA': the symbol to the left of a '::' must be a type
error C2039: 'value' : is not a member of 'B'
see declaration of 'B'

Re: [Boost-users] bind member function of a member of a class...

Archie14