Re: [Boost-users] boost::function/boost::bind compilation error

Subject: Re: [Boost-users] boost::function/boost::bind compilation error
From: Ahmed Badran (ahmed.badran_at_[hidden])
Date: 2009-09-05 20:52:06

Steven Watanabe wrote:
> AMDG
> 
> Ahmed Badran wrote:
>> I was trying to use boost function/bind to bind the first argument of 
>> a member function that takes two arguments, when I got into a 
>> compilation error that I can't quite decipher or understand, below is 
>> a sample representation of the problem, and then there's the 
>> compilation error, am I doing something incorrectly here?
>>
>> I have tried removing the boost::mem_fn instantiation with no difference.
>>
>>
>> #include <boost/function.hpp>
>> #include <boost/bind.hpp>
>> #include <boost/mem_fn.hpp>
>> #include <iostream>
>>
>> using namespace std;
>> using namespace boost;
>>
>> class my_class
>> {
>>     public:
>>     void test(bool b)
>>     {
>>         cout << "got " << b << endl;
>>     }
>>     private:
>> };
>>
>>
>>
>> int main()
>> {
>>     my_class c;
>>     boost::function1<void, bool> test_fn;
>>     boost::bind(&my_class::test, &c, _2);
>>
>>     /* causes a compilation error */
>>     test_fn = boost::bind<void>(boost::mem_fn(&my_class::test), &c, _2);
> 
> Use _1 instead of _2.  The placeholder number indicates
> the first argument to the function object created by bind.
> You're only passing a single bool argument in, so only _1
> is a valid placeholder.
> 

Thanks, for some reason I thought, the function originally takes two and since I'm binding the first, I want to let the second pass through, the documentation also has

"The placeholder _N selects the argument at position N from the argument list passed at "call time."" 

But since I wasn't calling it then and there, I somehow seemed to skip it.
So the _N is essentially the positional parameter of the output function from bind rather than the input function.

Thanks again.
-Ahmed