Re: [Boost-users] What's wrong with my use of member-function bind?

Subject: Re: [Boost-users] What's wrong with my use of member-function bind?
From: Nathan Ridge (zeratul976_at_[hidden])
Date: 2011-07-28 04:50:31

> Thanks. Can someone tell me what's wrong? I thought I followed the 
> example from the manual. 
> 
> 
> void foo (int); 
> 
> 
> class S { 
> const char* const name; 
> public: 
> S(const char* x) : name(x) {} 
> void bar (int); 
> }; 
> 
> 
> 
> S s1 ("s1"); // some instance 
> 
> void subscribe (State_t st, const NotifyFunc& func); 
> 
> subscribe (STATE1, &foo); // this works, so the form of the function 
> parameters is right. 
> 
> subscribe (STATE1, bind(&S::bar, &s1)); // this one is wrong. Why? 
> 
> 
> Does 
> 
> subscribe (STATE1, bind(&S::bar, s1)); 
> 
> work? 

No, you need to do bind(&S::bar, s1, _1).
 
The _1 in this case means "pass the FIRST argument given to the 
result of the bind (FIRST because it's _1), as the SECOND argument 
to &S::bar (SECOND because the _1 appeared as the second argument 
to bind(), after the function itself). Note that as a member 
function, bind considers the first argument of &S::bar to be the 
object on which the member function is called.
 
In other words, if
 
f = bind(&S::bar, s1, _1)
 
then f(x) is equivalent to bar(s1, x) (which is really s1.bar(x)).
 
If you just do
 
f = bind(&S::bar, s1)
 
then f(x) will just be equivalent to bar(s1) (i.e. s1.bar())
(the x will just be ignored), but that is not a valid way to call
&S::bar, so it's an error.
 
Regards,
Nate.