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Expectation Parser (a > b)

Like the Sequence, the expectation operator, a > b, parses two or more operands (a, b, ... etc.), in sequence:

a > b > ...

However, while the plain Sequence simply returns a no-match (returns false) when one of the elements fail, the expectation: > operator throws an expectation_failure<Iter> when the second or succeeding operands (all operands except the first) fail to match.

// forwards to <boost/spirit/home/qi/operator/expect.hpp>
#include <boost/spirit/include/qi_expect.hpp>

Also, see Include Structure.

Model of



a, b

A Parser


A ForwardIterator type

Expectation Failure

When any operand, except the first, fail to match an expectation_failure<Iter> is thrown:

template <typename Iter>
struct expectation_failure : std::runtime_error
    Iter first;           // [first, last) iterator pointing
    Iter last;            // to the error position in the input.
    info what_;       // Information about the nature of the error.
Expression Semantics

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



a > b

Match a followed by b. If a fails, no-match. If b fails, throw an expectation_failure<Iter>


See Compound Attribute Notation.



a > b

a: A, b: B --> (a > b): tuple<A, B>
a: A, b: Unused --> (a > b): A
a: Unused, b: B --> (a > b): B
a: Unused, b: Unused --> (a > b): Unused

a: A, b: A --> (a > b): vector<A>
a: vector<A>, b: A --> (a > b): vector<A>
a: A, b: vector<A> --> (a > b): vector<A>
a: vector<A>, b: vector<A> --> (a > b): vector<A>


The overall complexity of the expectation parser is defined by the sum of the complexities of its elements. The complexity of the expectation operator itself is O(N), where N is the number of elements in the sequence.

[Note] Note

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

Some using declarations:

using boost::spirit::ascii::char_;
using boost::spirit::qi::expectation_failure;

The code below uses an expectation operator to throw an expectation_failure with a deliberate parsing error when "o" is expected and "i" is what is found in the input. The catch block prints the information related to the error. Note: This is low level code that demonstrates the bare-metal. Typically, you use an Error Handler to deal with the error.

    test_parser("xi", char_('x') > char_('o')); // should throw an exception
catch (expectation_failure<char const*> const& x)
    std::cout << "expected: "; print_info(x.what_);
    std::cout << "got: \"" << std::string(x.first, x.last) << '"' << std::endl;

The code above will print:

expected: tag: literal-char, value: o
got: "i"