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Semantic actions may be attached to any point in the grammar specification. They allow to call a function or function object in order to provide the value to be output by the parser attached to the semantic action. Semantic actions are associated with a parser using the syntax p[], where p is an arbitrary parser expression.

// forwards to <boost/spirit/home/qi/action.hpp>
#include <boost/spirit/include/qi_action.hpp>

Also, see Include Structure.

Model of



a, p

Instances of a parser, P


Attribute type exposed by a parser, a


A (semantic action) function with signature void(Attrib&, Context, bool&). The third parameter is a boolean flag that can be set to false to force the parser to fail. Both Context and the boolean flag are optional. For more information see below.


The attribute obtained from the parse.


The type of the parser execution context. For more information see below.

Expression Semantics

Semantics of an expression is defined only where it differs from, or is not defined in UnaryParser.




If p is successful, call semantic action, fa. The function or function object fa is provided the attribute value parsed by the parser p, plus some more context information and a mutable bool flag which can be used to fail parsing.

The possible signatures for functions to be used as semantic actions are:

template <typename Attrib>
void fa(Attrib& attr);

template <typename Attrib, typename Context>
void fa(Attrib& attr, Context& context);

template <typename Attrib, typename Context>
void fa(Attrib& attr, Context& context, bool& pass);

The function or function object is expected to return the value to generate output from by assigning it to the first parameter, attr. Here Attrib is the attribute type of the parser attached to the semantic action.

The type Context is the type of the parser execution context. This type is unspecified and depends on the context the parser is invoked in. The value, context used by semantic actions written using Phoenix to access various context dependent attributes and values. For more information about Phoenix placeholder expressions usable in semantic actions see Nonterminal.

The third parameter, pass, can be used by the semantic action to force the associated parser to fail. If pass is set to false the action parser will immediately return false as well, while not invoking p and not generating any output.





a: A --> a[fa]: A


The complexity of the action parser is defined by the complexity of the parser the semantic action is attached to and the complexity of the function or function object used as the semantic action.

[Note] Note

The test harness for the example(s) below is presented in the Basics Examples section.


More examples for semantic actions can be found here: Examples of Semantic Actions.