Re: [boost] boost::function overhead?

From: Doug Gregor (dgregor_at_[hidden])
Date: 2006-05-02 15:15:12

On May 2, 2006, at 2:46 PM, Tobias Schwinger wrote:
> Curious question:
>
> Say we use a functor class template adapter with a function pointer  
> as non-type template parameter...
>
> In case we pass it to function templates that treat functors  
> uniformly (such as std algorithms), the compiler has (at least  
> theoretically) a fair chance to make an absolute call or even to  
> inline.
>
> Boost.Function, however, implements distinct handling for function  
> objects and function pointers. So, is there any chance for reducing  
> the work at the call site with this technique?

Sure. When you stick a function pointer into a boost::function, you  
actually get two indirect calls for each invocation: the first goes  
from the boost::function::operator() to a "manager" that encodes the  
type of the function pointer, and the second goes through the  
function pointer to the function. If you encode the function pointer  
in the type of the function object that you stick into  
boost::function, the compiler can eliminate the second call.

        Doug