...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
The directives omit[]
and skip[]
consumes the attribute type of the embedded generator without generating
any output. The omit[]
directive will still execute the embedded
generator while discarding the generated output afterwards. The skip[]
directive will not execute the embedded generator, but will use it only
to extract the exposed attribute type.
// forwards to <boost/spirit/home/karma/directive/omit.hpp> #include <boost/spirit/include/karma_omit.hpp>
Also, see Include Structure.
Name |
---|
|
|
Notation
a
A generator object
A
Attribute type of generator a
Semantics of an expression is defined only where it differs from, or
is not defined in UnaryGenerator
.
Expression |
Semantics |
---|---|
|
The |
|
The |
See Compound Attribute Notation.
Expression |
Attribute |
---|---|
|
a: A --> omit[a]: A a: Unused --> omit[a]: Unused
|
|
a: A --> skip[a]: A a: Unused --> skip[a]: Unused
|
The overall complexity of the
omit[]
directive depends on the complexity of the embedded generator. The overall complexity of theskip[]
generator directive is O(1) as it does not generate any output.
Note | |
---|---|
The test harness for the example(s) below is presented in the Basics Examples section. |
Some includes:
#include <boost/spirit/include/karma.hpp> #include <boost/spirit/include/support_utree.hpp> #include <boost/phoenix/core.hpp> #include <boost/phoenix/operator.hpp> #include <boost/fusion/include/std_pair.hpp> #include <boost/proto/deep_copy.hpp> #include <iostream> #include <string>
Some using declarations:
using boost::spirit::karma::double_; using boost::spirit::karma::omit;
Basic usage of a omit
generator directive. It shows how it consumes the first element of the
provided attribute without generating anything, leaving the second element
of the attribute to the non-wrapped double_
generator.
std::pair<double, double> p (1.0, 2.0); test_generator_attr("2.0", omit[double_] << double_, p);
Generally, this directive is helpful in situations, where the attribute
type contains more information (elements) than need to be used to generate
the required output. Normally in such situations we would resolve to
use semantic actions to explicitly pass the correct parts of the overall
attribute to the generators. The omit
directive helps achieving the same without having to use semantic actions.
Consider the attribute type:
typedef fusion::vector<int, double, std::string> attribute_type;
where we need to generate output only from the first and last element:
typedef std::back_insert:iterator<std::string> iterator_type; karma::rule<iterator_type, attribute_type()> r; r = int_[_1 = phoenix::at_c<0>(_val)] << string[_1 = phoenix::at_c<2>(_val)]; std::string str; iterator_type sink(str); generate(sink, r, attribute_type(1, 2.0, "example")); // will generate: '1example'
This is error prone and not really readable. The same can be achieved
by using the omit
directive:
r = int_ << omit[double_] << string;
which is at the same time more readable and more efficient as we don't have to use semantic actions.
The semantics of using the skip[]
directive are identical to the omit[]
directive, except that it does not actually execute the embedded generator.
For this reason it is usually preferable to utilize the skip[]
directive instead of the omit[]
directive. On the other hand, the
omit[]
directive is very useful whenever the embedded generator produces side
effects (has semantic actions which need to be executed).