NAME¶

bisonc++ - Application programmer’s interface of bisonc++ generated classes

DESCRIPTION¶

Bisonc++ derives from bison++(1), originally derived from bison(1). Like these programs bisonc++ generates a parser for an LALR(1) grammar. Bisonc++ generates C++ code: an expandable C++ class.

Refer to bisonc++(1) for a general overview. This manual page covers the application programmer’s interface of classes generated by bisonc++. It contains the following sections:

o

DESCRIPTION: this section;

o

PUBLIC SYMBOLS: constructor, enums, members, and types that can be used by calling software;

o

PRIVATE ENUMS AND -TYPES: enumerations and types only available to the Parser class;

o

PRIVATE MEMBER FUNCTIONS: member functions that are only available to the Parser class;

o

PRIVATE DATA MEMBERS: data members that are only available to the Parser class;

o

TYPES AND VARIABLES IN THE ANONYMOUS NAMESPACE: an overview of the types and variables that are used to define and store the grammar-tables generated by bisonc++;

o

SEE ALSO: references to other programs and documentation;

o

AUTHOR: at the end of this man-page.

All identifiers ending in an underscore character are reserved for bisonc++. Member functions ending in an underscore character must not be redefined. Data members ending in an underscore character are available in the generated parser class, and may be modified by user-defined members of the parser class. Some members like (error, exceptionHandler, lex) are defined in the parser class and must remain present, but their implementations may be altered by the user. Members for which no default implementation is provided in the parser’s internal header file (e.g., Parser.ih) may not be redefined or masked by user-provided code.

UNDERSCORES¶

Starting with version 6.02.00 bisonc++ reserved identifiers no longer end in two underscore characters, but in one. This modification was necessary because according to the C++ standard identifiers having two or more consecutive underscore characters are reserved by the language. In practice this could require some minor modifications of existing source files using bisonc++’s facilities, most likely limited to changing Tokens__ into Tokens_ and changing Meta__ into Meta_.

The complete list of affected names is:

Enums:

Enums values:

Type / namespace designators:

Member functions:

Protected data members:

PUBLIC SYMBOLS¶

Parser classes generated by bisonc++ offer the following public constructor, enums, members and types (in the following overview parser class-name prefixes (e.g., Parser::) prefixes were omitted):

o

Constructors: the generated parser class merely defines the default constructor. Copy and move constructors are not available. The default constructor is a real default: it is declared as such in the parser’s header file. Additional constructors can easily be added to the parser class’s interface. Since the initialization of the parser’s base class is performed by the parser base class’s default constructor, constructors that are added to the generated parser class automatically call the base class constructor, so additional constructors do not have to explicitly initialize the parser’s base class.

o

DebugMode_:
The values of this enum are used to configure the type of debug information that will be displayed (assuming that the debug option/directive was specified when bisonc++ generated the parser’s code). It has three values:

OFF: no debug information is displayed when the generated parser’s parse function is called;

ON: extensive debug information about the parsing process is displayed when the generated parser’s parse function is called;

ACTIONCASES: just before executing the grammar’s action blocks the action block number is written to the standard output stream. These action block numbers refer to case labels of the switch that is defined in the parser’s executeAction function. It is commonly used to find the action block where a fatal semantic value mismatch was observed.

The bit_or operator can be used to combine ON and ACTIONCASES (see the member function setDebug(DebugMode_ mode) below).

o

LTYPE_:
The parser’s location type (user-definable). Available only when either %lsp-needed, %ltype or %locationstruct has been declared.

o

STYPE_:
The parser’s stack-type (user-definable), defaults to int.

o

Tokens_:
The enumeration type of all the symbolic tokens defined in the grammar file (i.e., bisonc++’s input file). The scanner should be prepared to return these symbolic tokens. Note that, since the symbolic tokens are defined in the parser’s class and not in the scanner’s class, the lexical scanner must prefix the parser’s class name to the symbolic token names when they are returned. E.g., return Parser::IDENT should be used rather than return IDENT.

o

int parse():
The parser’s parsing member function. It returns 0 when parsing was successfully completed; 1 if errors were encountered while parsing the input.

o

void setDebug(bool mode):
This member can be used to activate or deactivate the debug-code compiled into the parsing function. It is always defined but is only operational if the debug directive option was specified when bisonc++ generated the parse function. If so, it is not active by default; to activate the debug output call setDebug(true), to suppress the debug output call setDebug(false).

o

void setDebug(DebugMode_ mode):
This member can also be used to activate or deactivate the debug-code compiled into the parsing function. Like setDebug(bool) it is always defined but only operational if the debug directive option was specified when bisonc++ generated the parse function. If so, it is not active by default; to activate, call setDebug(Parser::ON), setDebug(Parser::ACTIONCASES), or setDebug(Parser::ON | Parser::ACTIONCASES). To suppress the debug code output call setDebug(Parser_::OFF) or simply setDebug(false).

When the %polymorphic directive is used:

o

Meta_:
Templates and classes that are required for implementing the polymorphic semantic values are all declared in the Meta_ namespace. The Meta_ namespace itself is nested under the namespace that may have been declared by the %namespace directive.

o

Tag_:
The (strongly typed) enum class Tag_ contains all the tag-identifiers specified by the %polymorphic directive. It is declared outside of the Parser’s class, but within the namespace that may have been declared by the %namespace directive.

PRIVATE ENUMS AND -TYPES¶

The following enumerations and types can be used by members of parser classes generated by bisonc++. They are actually protected members inherited from the parser’s base class.

o

Base::ErrorRecovery_:
This enumeration defines one values:

    UNEXPECTED_TOKEN_


        

When the parsing process throws UNEXPECTED_TOKEN_ the recovery procedure is started (i.e., it is started whenever a syntactic error is encountered or ERROR() is called).

The recovery procedure consists of (1) looking for the first state on the state-stack having an error-production, followed by (2) handling all state transitions that are possible without retrieving a terminal token. Then, in the state requiring a terminal token and starting with the initial unexpected token (3) all subsequent terminal tokens are ignored until a token is retrieved which is a continuation token in that state.

If the error recovery procedure fails (i.e., if no acceptable token is ever encountered) error recovery falls back to the default recovery mode: the parsing process terminates.

o

Base::Return_:
This enumeration defines two values:

    PARSE_ACCEPT = 0,


    PARSE_ABORT = 1


        

(which are also used as the parse function’s return values).

When the %polymorphic directive is used:

o

Meta_::sizeofTag_:
sizeofTag_ defines the number of tags that were defined for polymorphic semantic values.

PRIVATE MEMBER FUNCTIONS¶

The following members can be used by members of parser classes generated by bisonc++. When prefixed by Base:: they are actually protected members inherited from the parser’s base class. These members are shown below. Following the description of those members several more are listed: those members are used during the parsing process, andshould not be modified or masked by user-defined code.

o

void Base::ABORT() const throw(Return_):
This member can be called from any member function (called from any of the parser’s action blocks) to indicate a failure while parsing thus terminating the parsing function with an error value 1. Note that this offers a marked extension and improvement of the macro YYABORT defined by bison++ in that YYABORT could not be called from outside of the parsing member function.

o

void Base::ACCEPT() const throw(Return_):
This member can be called from any member function (called from any of the parser’s action blocks) to indicate successful parsing and thus terminating the parsing function. Note that this offers a marked extension and improvement of the macro YYACCEPT defined by bison++ in that YYACCEPT could not be called from outside of the parsing member function.

o

void Base::ERROR() const throw(ErrorRecovery_):
This member can be called from any member function (called from any of the parser’s action blocks) to generate an error, and results in the parser executing its error recovery code. Note that this offers a marked extension and improvement of the macro YYERROR defined by bison++ in that YYERROR could not be called from outside of the parsing member function.

o

void error():
By default implemented inline in the parser.ih internal header file, it writes a simple message to the standard error stream. It is called when a syntactic error is encountered, and its default implementation may safely be altered.

o

void exceptionHandler(std::exception const &exc):
This member’s default implementation is provided inline in the parser.ih internal header file. It consists of a mere throw statement, rethrowing a caught exception.

The parse member function’s body essentially consists of a while statement, in which the next token is obtained via the parser’s lex member. This token is then processed according to the current state of the parsing process. This may result in executing actions over which the parsing process has no control and which may result in exceptions being thrown.

Such exceptions do not necessarily have to terminate the parsing process: they could be thrown by code, linked to the parser, that simply checks for semantic errors (like divisions by zero) throwing exceptions if such errors are observed.

The member exceptionHandler receives and may handle such exceptions without necessarily ending the parsing process. It receives any std::exception thrown by the parser’s actions, as though the action block itself was surrounded by a try ... catch statement. It is of course still possible to use an explicit try ... catch statement within action blocks. However, exceptionHandler can be used to factor out code that is common to various action blocks.

The next example shows an explicit implementation of exceptionHandler: any std::exception thrown by the parser’s action blocks is caught, showing the exception’s message, and increasing the parser’s error count. After this parsing continues as if no exception had been thrown:

void Parser::exceptionHandler(std::exception const &exc)
{


    std::cout << exc.what() << ’\n’;


    ++d_nErrors_;
}


        

o

int lex():
By default implemented inline in the parser.ih internal header file, it can be pre-implemented by bisonc++ using the scanner option or directive (see above); alternatively it must be implemented by the programmer. It interfaces to the lexical scanner, and should return the next token produced by the lexical scanner, either as a plain character or as one of the symbolic tokens defined in the Parser::Tokens_ enumeration. Zero or negative token values are interpreted as `end of input’.

o

void print():
By default implemented inline in the parser.ih internal header file, this member calls print_ to display the last received token and corresponding matched text. The print_ member is only implemented if the --print-tokens option or %print-tokens directive was used when the parsing function was generated. Calling print_ from print is unconditional, but can easily be controlled by the using program, by defining, e.g., a command-line option.

o

size_t stackSize_() const:
Returns the current number of elements in the parser’s state-stack.

o

size_t state_() const:
Returns the current parsing-state.

o

bool Base::recovery_() const:
Returns true while recovering from a syntax error.

o

int Base::token_() const:
Returns the currently considered token.

The following members are required during the parsing process. They should not be modified or masked by user-defined code:

o

Base::ParserBase()

o

void Base::clearin_()()

o

void errorRecovery_()

o

void Base::errorVerbose_()

o

void executeAction_(int)

o

int lex_(int token)

o

int Base::lookup()

o

LTYPE_ const &lsp_(int) const
(only available when %lsp-needed, %ltype or %locationstruct was specified).

o

void nextCycle_()

o

void nextToken_()

o

void Base::pop_()

o

void Base::popToken_()

o

void print_()()

o

void Base::push_()

o

void Base::pushToken_()

o

void Base::shift_(int state)

o

void Base::redoToken_(int rule)

o

void Base::reduce_(int rule)

o

void Base::savedToken_()

o

void Base::symbol_()

o

void Base::startRecovert_()

o

void Base::top_()

o

int Base::token_() const

o

void Base::vs_(int idx)

PRIVATE DATA MEMBERS¶

The following data members can be used by members of parser classes generated by bisonc++. All data members are actually protected members inherited from the parser’s base class.

o

size_t d_acceptedTokens_:
Counts the number of accepted tokens since the start of the parse() function or since the last detected syntactic error. It is initialized to d_requiredTokens_ to allow an early error to be detected as well.

o

bool d_actionCases_:
When the debug option has been specified, this variable (false by default) determines whether the number of action block which is about to be executed by the parser’s member executeAction will be displayed to the standard output stream.

o

bool d_debug_:
When the debug option has been specified, this variable (true by default) determines whether debug information is actually displayed.

o

LTYPE_ d_loc_:
The location type value associated with a terminal token. It can be used by, e.g., lexical scanners to pass location information of a matched token to the parser in parallel with a returned token. It is available only when %lsp-needed, %ltype or %locationstruct has been defined.
Lexical scanners may be offered the facility to assign a value to this variable in parallel with a returned token. In order to allow a scanner access to d_loc_, d_loc_’s address should be passed to the scanner. This can be realized, for example, by defining a member void setLoc(STYPE_ *) in the lexical scanner, which is then called from the parser’s constructor as follows:

            d_scanner.setSLoc(&d_loc_);


       

Subsequently, the lexical scanner may assign a value to the parser’s d_loc_ variable through the pointer to d_loc_ stored inside the lexical scanner.

o

size_t d_nErrors_:
The number of errors counted by parse. It is initialized by the parser’s base class initializer, and is updated while parse executes. When parse has returned it contains the total number of errors counted by parse. Errors are not counted if suppressed (i.e., if d_acceptedTokens_ is less than d_requiredTokens_).

o

size_t d_requiredTokens_:
Defines the minimum number of accepted tokens that the parse function must have processed before a syntactic error can be generated.

o

STYPE_ d_val_:
The semantic value of a returned token or nonterminal symbol. With nonterminal tokens it is assigned a value through the action rule’s symbol $$. Lexical scanners may be offered the facility to assign a semantic value to this variable in parallel with a returned token. In order to allow a scanner access to d_val_, d_val_’s address should be passed to the scanner. This can be realized, for example, by passing d_val_’s address to the lexical scanner’s constructor.

Subsequently, the lexical scanner may assign a value to the parser’s d_val_ variable through the pointer to d_val_ stored in a data member of the lexical scanner.

Note that in some cases this approach must be used to make the correct semantic value available to the parser. In particular, when a grammar state defines multiple reductions, depending on the next token, the reduction’s action only takes place following the retrieval of the next token, thus losing the initially matched token text.

If STYPE is a polymorphic semantic value, specific requirements for assigning values to d_val_ apply. .

BUGS¶

With bisonc++ version 6.00.00 the following members were modified. Where necessary alternatives are mentioned:

o

bool Base::debug() const: use the d_debug_ data member;

o

void error(char const *): replaced by void error();

o

void exceptionHandler_(std::exception const &exc): omit the final underscores: void exceptionHandler(std::exception const &exc)

o

void executeAction(int): add one underscore to the declaration in the parser class interface: void executeAction_(int)

o

int lookup(bool): omit this member from the parser class interface.

o

void nextCycle_(int): add this member declaration to the parser class interface.

o

void nextToken(int): add one underscore to the declaration in the parser class interface: void nextToken_(int)

o

size_t d_nextToken_: removed from the interface.

o

int d_state_: use state_().

o

int d_token_: use token_().

o

LTYPE_ d_vsp_: removed from the interface. Use vsp_() instead.

TYPES AND VARIABLES IN THE ANONYMOUS NAMESPACE¶

In the file defining the parse function the following types and variables are defined in the anonymous namespace. These are mentioned here for the sake of completeness, and are not normally accessible to other parts of the parser.

o

char const author[]:
Defining the name and e-mail address of Bisonc++’s author.

o

Reserved_:
This enumeration defines some token values used internally by the parsing functions. They are:

    UNDETERMINED_ = -2,


    EOF_          = -1,


    errTok_        = 256,


       

These tokens are used by the parser to determine whether another token should be requested from the lexical scanner, and to handle error-conditions.

o

StateType:
This enumeration defines several additional token values used internally by the parsing functions. They are:

        NORMAL,


        ERR_ITEM,


        REQ_TOKEN,


        ERR_REQ,    // ERR_ITEM | REQ_TOKEN


        DEF_RED,    // state having default reduction


        ERR_DEF,    // ERR_ITEM | DEF_RED


        REQ_DEF,    // REQ_TOKEN | DEF_RED


        ERR_REQ_DEF // ERR_ITEM | REQ_TOKEN | DEF_RED


       

These tokens are used by the parser to define the types of the various states of the analyzed grammar.

o

StateTransition This enumeration only defines a single symbolic constant: ACCEPT_, which is used in the state transition tables to indicate that the accepting state has been reached.

o

PI_ (Production Info):
This struct provides information about production rules. It has two fields: d_nonTerm is the identification number of the production’s nonterminal, d_size represents the number of elements of the productin rule.

o

static PI_ s_productionInfo:
Used internally by the parsing function.

o

SR_ (Shift-Reduce Info):
This struct provides the shift/reduce information for the various grammatic states. SR_ values are collected in arrays, one array per grammatic state. These array, named s_<nr>, where tt<nr> is a state number are defined in the anonymous namespace as well. The SR_ elements consist of two unions, defining fields that are applicable to, respectively, the first, intermediate and the last array elements.
The first element of each array consists of (1st field) a StateType and (2nd field) the index of the last array element; intermediate elements consist of (1st field) a symbol value and (2nd field) (if negative) the production rule number reducing to the indicated symbol value or (if positive) the next state when the symbol given in the 1st field is the current token; the last element of each array consists of (1st field) a placeholder for the current token and (2nd field) the (negative) rule number to reduce to by default or the (positive) number of an error-state to go to when an erroneous token has been retrieved. If the 2nd field is zero, no error or default action has been defined for the state, and error-recovery is attepted.

o

STACK_EXPANSION_:
An enumeration value specifying the number of additional elements that are added to the state- and semantic value stacks when full.

o

static SR_ s_<nr>[]:
Here, <nr> is a numerical value representing a state number. Used internally by the parsing function.

o

static SR_ *s_state[]:
Used internally by the parsing function.

SEE ALSO¶

bison(1), bison++(1), bisonc++(1), bisonc++input(7), bison.info (using texinfo), flexc++(1), https://fbb-git.gitlab.io/bisoncpp/

Lakos, J. (2001) Large Scale C++ Software Design, Addison Wesley.
Aho, A.V., Sethi, R., Ullman, J.D. (1986) Compilers, Addison Wesley.

AUTHOR¶

Frank B. Brokken (f.b.brokken@rug.nl).