Re: [Boost-users] [function] How do you bind two boost::function objects together?

From: Jason House (jasonhouse_at_[hidden])
Date: 2006-10-01 13:27:15

Douglas Gregor wrote:
> On Sep 30, 2006, at 1:01 PM, Jason House wrote:
> 
> 
>>I'm trying to convert the input argument to a function object using a
>>small helper function object.  The code below looks relatively simple,
>>but it doesn't seem to compile.  Any ideas what could be wrong?
>>
>>template<typename inputType, typename outputType>
>>boost::function<outputType (int)>
>>convert(boost::function<outputType (inputType)> &object_to_convert){
>>   boost::function<intputType (int)> input_converter(...);
>>
>>   // The following fails to compile
>>   boost::function<outputType (int)> converted_function
>>     (boost::lambda::bind(object_to_convert, input_converter));
> 
> 
> The bind expression is going to result in a function object that  
> calls object_to_convert(input_converter). What you really want is a  
> function object that takes one parameter ("arg1") and call  
> object_to_object with the result of input_converter(arg1). To do so,  
> you will need a nested bind expression that will look something like  
> this:
> 
>    boost::function<outputType (int)> converted_function
>     (boost::lambda::bind(object_to_convert, boost::lambda::bind 
> (input_converter, arg1)));
> 
> 	Cheers,
> 	Doug

I want the result of the bound expression to have one input parameter 
that would be decided later.  For instance:

bool some_function(char x);
boost::function<bool (char) some_function_object(some_function);
boost::function<bool (int)> f = convert(some_function_object)
std::cout << f(32);

Unless I'm missing something, I don't think the proposed change allows 
that.  I tried using arg1 and got a compile error.  boost::lambda::arg1 
didn't work either.  Using boost::lambda::_1 instead produced an 
identical error to my original code.