Re: [Boost-users] [mpl] trouble with conditionals

On 10/6/05, David Abrahams wrote:

Geoffrey Romer writes:

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If so, doesn't that mean Cromwell's fix is incorrect?

I'm afraid I've not seen his fix.

I'm also afraid I don't have time to look at it right now.

Fair enough. It's sort of a side issue anyway.

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What I don't understand is when I should be saying "typename metafunc<args>::type" and when I should be using plain "metafunc<args>". This isn't a language-correctness issue, it's an MPL usage issue.

Oh. Maybe mapping into the runtime world will help. If you think of metafunc as a regular function, then "metafunc::type" is equivalent to a regular function call, and "metafunc" is equivalent to boost::bind(func, args...)

These are exact analogies, AFAICT.

OK, that's what I thought, but it seems not to apply to lambda-expressions. If I want to call foo<> on a placeholder, I seem to be required to write foo<_1>, and not typename foo<_1>::type, even though I want foo<> to actually be applied to the placeholder (after substitution). What am I missing?