Re: template problem: local variable as non-type argument

From:

Jeff Koftinoff <jeff.koftinoff@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 9 Feb 2009 05:12:19 CST

Message-ID:

<a99d51ac-2a44-43fc-9f01-6c936732efc4@z6g2000pre.googlegroups.com>

On Feb 5, 6:05 am, potapo...@gmail.com wrote:

Option 1, "switch" statement is much faster than "if" one:
Base& Factory::createInstance(int i) {
     switch(i){
         case 1: return C<1>();
         case 2: return C<2>();
         ...
     }

}

Option 2, linear recursive helper function:
const int max_n = 100;
template<int n>
struct helper
{
     static Base& func(int i)
     {
         return i == n? C<n>() : helper<n+1>::func(i);
     }

};

template<>
struct helper<max_n>
{
     static Base& func(int i)
     {
         return C<max_n>();
     }

};

Base& Factory::createInstance(int i) {
     return helper<0>(i);

}

<snip>

Base& func0()
{
     return C<0>();

}

Base& func1()
{
     return C<1>();

}

Base& func2()
{
     return C<2>();}

...

Base& Factory::createInstance(int i) {
     const Base& (*ptrs)()[]={func0,func1,func2,... /* can be filled by
Boost.Preprocessor too */};
     return ptrs[i]();

}

Unfortunately all of those examples fail as they are returning
references to locals that go out of scope.

An appropriate fix for all of them, including the helper version which
I quite like, is to make a local static object like this:

Base & func0()
{
   static const C<0> item;
   return item;
}

BTW this pattern is very useful in cases where you benchmarked your
code and absolutely need the optimization of template specializations
to factor out variable coefficients into constants.

--jeffk++
www.jdkoftinoff.com

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