[boost] Boost.Exception and constexpr

Subject: [boost] Boost.Exception and constexpr
From: Marshall Clow (mclow.lists_at_[hidden])
Date: 2013-01-09 21:23:39

I was playing around with Boost.Array tonight, seeing if I could make the signatures more closely match std::array.

Specifically, I wanted to add 'constexpr' to "operator [] (size_t) const" and "at (size_t) const"as proposed in 
        http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3470.html

But I ran into a problem.
Here are the implementations:

        const_reference operator[](size_type i) const 
        {     
            BOOST_ASSERT_MSG( i < N, "out of range" );
            return elems[i]; 
        }

        const_reference at(size_type i) const { rangecheck(i); return elems[i]; }

        static void rangecheck (size_type i) {
            if (i >= size()) {
                std::out_of_range e("array<>: index out of range");
                boost::throw_exception(e);
            }
        }

BOOST_ASSERT does not fit into the constexpr world at all, and neither does rangecheck.

I could rewrite rangecheck so it could be constexpr, but only (I believe) by throwing the exception directly, rather than using boost::throw_exception.

        static void rangecheck (size_type i) {
            if (i >= size())
                throw std::out_of_range e("array<>: index out of range");
            }

or even just:
        BOOST_CONSTEXPR const_reference at(size_type i) const {
                return i >= size () ? throw std::out_of_range e("array<>: index out of range") : elems[i];
                }

Has anyone put any effort into making Boost.Exception (and BOOST_ASSERT) work with constexpr?

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists_at_[hidden]>

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