Boost C++ Libraries

Until now, we have been working around the fact that std::complex<> is not a native Boost.Fusion sequence. We have not been able to use it with the same simplicity and natural grace of a fusion::tuple<> or a similar Boost.Fusion data structure. Fortunately, starting with Boost V1.43 it is possible to adapt any data structure (not only, as before, structures with publicly accessible members) as a Boost.Fusion sequence. All we have to do is to employ one of the new BOOST_FUSION_ADAPT_ADT macros.

Adapting a Class As a Fusion Sequence

Let us start with the code again, following up with the explanations afterwards.

Wouldn't it be optimal if we could pass our instance of a std::complex<> directly to Karma's generate() function:

template <typename OutputIterator>
bool generate_complex(OutputIterator sink, std::complex<double> const& c)
{
    using boost::spirit::karma::double_;
    using boost::spirit::karma::bool_;
    using boost::spirit::karma::true_;
    using boost::spirit::karma::omit;
    using boost::spirit::karma::generate;

    return generate(sink,

        //  Begin grammar
        (
           &true_ << '(' << double_ << ", " << double_ << ')'
        |   omit[bool_]  << double_
        ),
        //  End grammar

        c     //  Data to output
    );
}

Indeed, this is possible! All we have to supply to make this work is a magic incantation (somewhere in the global namespace):

// We can leave off the setters as Karma does not need them.
BOOST_FUSION_ADAPT_ADT(
    std::complex<double>,
    (bool, bool, obj.imag() != 0, /**/)
    (double, double, obj.real(), /**/)
    (double, double, obj.imag(), /**/)
)

Most of the formatting grammar itself has not changed from the last section. We still utilize a very similar scheme. We have an alternative providing the formatting rules for our both use cases: one for the full complex format and one for complex numbers with a zero imaginary part. But instead of selecting the required alternative by comparing the imaginary part to zero in the grammar we assume to receive a boolean attribute carrying this information:

&true_ << "(" << double_ << ", " << double_ << ")"

This reads as: 'if the first (boolean) element of the supplied fusion sequence is true, proceed as specified, else select the next alternative'. The next alternative now accounts for the boolean element as well, but is otherwise (almost) unchanged from the last section's example.

Now it should be clear why our adapt construct above exposes a three element Boost.Fusion sequence: a boolean and two double values (the real and the imaginary part of the complex number). We want it to match the requirements of our formatting grammar, which expects those exact values. The BOOST_FUSION_ADAPT_ADT macro allows us to specify an arbitrary accessor construct, not necessarily limited to just calling a member function of the object instance (represented by obj in the context of this macro). This allows us to nicely encapsulate the decision logic into the class adaptation.

Here is the last new bit of information. If you look closely you realize the second alternative to be 'shorter' than the first one. It consumes only two elements of the supplied fusion sequence: it ignores the boolean and uses the real part of the complex number to generate its output. If there are more elements in our attribute than needed, we now can safely omit them from the grammar (which is a new 'feature' added to Spirit in V1.43 as well). Note, we could have written the alternative as

&false_ << double_

but this would have been a bit less efficient as we needed to compare the boolean value again, while the final solution provided will just ignore it.