Re: [Boost-users] C++ guru required!

Subject: Re: [Boost-users] C++ guru required!
From: Nathan Ridge (zeratul976_at_[hidden])
Date: 2012-02-17 16:04:12

> I've got a really dumb question. My question is illustrated by the
> following
> code snippit.
>
> template<class T, class U>
> T inline operator%(const T & lhs, const U & rhs) {
> if(0 == rhs)
> throw std::domain_error("Divide by zero");
> if(boost::is_same<T, boost::uintmax_t>::value
> && boost::numeric::is_signed<U>::value
> ){
> if(1 == rhs || -1 == rhs) // is this dropped?
> overflow("unsigned type can hold this result");
> }
> return lhs % rhs;
> }
>
> I would like to think that the second if is always
> dropped from the compile for a particular pair
> of types since this can be evaluated at compile time.

I don't think the standard requires that it be dropped, 
but any decent compiler will do it if optimizations are 
enabled.

> If I'm correct, I can replace some tedious
> template metaprogramming for some straightforward
> and transparent code inserted in a convenient code
> inserted in the most convenient place.
>
> Andre Alex... gave a talk at "Going Native" proposing
> a "static if" for this case. But I don't see the necessity for
> for this since I would assume that the compiler
> just optimises away the "dead" code. I've compiled
> the above and it seems to do what I want but
> still I wonder.
>
> Basically I see lots of applications of variations
> on this idea to get the benefits of tmp without
> the attendent pain.
>
> Am I missing anything here?

For a runtime "if",  the compiler will still type-check the body 
of the if statement even if it ends up being optimized away.

"static if" is useful in cases where you want to write code
in the body that is valid for the types for which the "static if"
passes, but not necessarily valid for other types.

Regards,
Nate