[boost] Re: parameter_traits aka constructor_parameter

From: Howard Hinnant (hinnant_at_[hidden])
Date: 1999-11-27 17:54:25

Dave Abrahams wrote on 11/27/99 5:37 PM
>And has anyone done any actual _measurement_ to find out if the supposed
>optimization is *really* an optimization? For example, contrary to
>assumptions I've seen posted to this list, I would bet that long long is
>really best passed by value most of the time. If you have a const T&
>parameter and there's a non-inlined function call anywhere in the function's
>body, the parameter still has to be reloaded from memory after the call,
>where it might otherwise have lived in a register or two.

Hi Dave!

I haven't done any measurement, but I don't feel too bad about that.  
Whether long long should be passed by value or by const& is a detail that 
may even vary from machine to machine.  But I can say with some certainty 
that there will be types small enough that they should be passed by 
value, and others large enough that they should be passed by const&.  
long long can always be specialized for a particular machine if it does 
not fit the general rule (this will probably frequently be true of 
double).  And if the "IsSmallAndFast" pattern is used, even the general 
rule can be easily customized.

You're a few steps ahead of us.  We're fairly convinced that the 
call_traits idea has merit.  We're still trying to figure out generally 
how to best implement it.  But we haven't gotten to the stage of being 
able to say with any certainty exactly what types should do what ('cept 
for maybe array types which Beman and Dietmar are working on for the 
make_pair patch).

-Howard