[boost] mpl/loki

From: Drazen DOTLIC (Drazen.Dotlic_at_[hidden])
Date: 2003-07-11 09:19:16

Hello,

I've recently discovered that mpl provides all the functionality I was previously using from loki, so I decided to switch. There is one small thing driving me crazy, and I was wondering if I missed something...
I was using loki's TypeAtNonStrict "algorithm" to give me type from type list at a specified position, or NullType (loki's internal "null" class) if not found. Now, I need the same for mpl:vector, and I tried the following 'naïve' approach:

[TypeVector is boost::mpl::vector<bool>]
enum { numParams = boost::mpl::size<TypeVector>::type::value };
typedef typename boost::mpl::if_c<(numParams > 2), typename boost::mpl::at_c<TypeVector, 2>::type, boost::mpl::void_>::type Param1;

I was expecting to get Param1 to be boost::mpl::void_, but to my great surprise, my compiler (VC7.1) decided to fully evaluate "then" branch of if_c as well, even though the expression numParams > 2 was false, and failed.

What would be the "right" way to express my intention? Btw, I would like to congratulate authors of mpl on the job well done, I am most impressed not only with the features that mpl provides but also with the errors I get when something goes wrong - they are far more readable than most of the STL errors I am used to seeing.

Thanks in advance,

Drazen