...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
While parsing input or generating output it is often desirable to combine
some constant elements with variable parts. For instance, let us look at
the example of parsing or formatting a complex number, which is written
as (real, imag)
, where real
and imag
are the variables
representing the real and imaginary parts of our complex number. This can
be achieved by writing:
Library |
Sequence expression |
---|---|
Qi |
|
Karma |
|
Fortunately, literals (such as '('
and ", "
) do not
expose any attribute (well actually, they do expose the special type unused_type
, but in this context unused_type
is interpreted as if the
component does not expose any attribute at all). It is very important to
understand that the literals don't consume any of the elements of a fusion
sequence passed to this component sequence. As said, they just don't expose
any attribute and don't produce (consume) any data. The following example
shows this:
// the following parses "(1.0, 2.0)" into a pair of double std::string input("(1.0, 2.0)"); std::string::iterator strbegin = input.begin(); std::pair<double, double> p; qi::parse(strbegin, input.end(), '(' >> qi::double_ >> ", " >> qi::double_ >> ')', // parser grammar p); // attribute to fill while parsing
and here is the equivalent Spirit.Karma code snippet:
// the following generates: (1.0, 2.0) std::string str; std::back_insert_iterator<std::string> out(str); generate(out, '(' << karma::double_ << ", " << karma::double_ << ')', // generator grammar (format description) p); // data to use as the attribute
where the first element of the pair passed in as the data to generate is
still associated with the first double_
,
and the second element is associated with the second double_
generator.
This behavior should be familiar as it conforms to the way other input
and output formatting libraries such as scanf
,
printf
or boost::format
are handling their variable parts.
In this context you can think about Spirit.Qi's and
Spirit.Karma's primitive components (such as the
double_
above) as of being
type safe placeholders for the attribute values.
Tip | |
---|---|
Similarly to the tip provided above, this example could be rewritten using Spirit's multi-attribute API function: double d1 = 0.0, d2 = 0.0; qi::parse(begin, end, '(' >> qi::double_ >> ", " >> qi::double_ >> ')', d1, d2); karma::generate(out, '(' << karma::double_ << ", " << karma::double_ << ')', d1, d2);
which provides a clear and comfortable syntax, more similar to the placeholder
based syntax as exposed by |
Let's take a look at this from a more formal perspective. The sequence
attribute propagation rules define a special behavior if generators exposing
unused_type
as their attribute
are involved (see Generator
Compound Attribute Rules):
Library |
Sequence attribute propagation rule |
---|---|
Qi |
|
Karma |
|
which reads as:
Given
a
andb
are parsers (generators), andA
is the attribute type ofa
, andunused_type
is the attribute type ofb
, then the attribute type ofa >> b
(a << b
) will beA
as well. This rule applies regardless of the position the element exposing theunused_type
is at.
This rule is the key to the understanding of the attribute handling in
sequences as soon as literals are involved. It is as if elements with
unused_type
attributes
'disappeared' during attribute propagation. Notably, this is not only true
for sequences but for any compound components. For instance, for alternative
components the corresponding rule is:
a: A, b: Unused --> (a | b): A
again, allowing to simplify the overall attribute type of an expression.