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18 Generating C++ Scanners

IMPORTANT: the present form of the scanning class is experimental and may change considerably between major releases.

flex provides two different ways to generate scanners for use with C++. The first way is to simply compile a scanner generated by flex using a C++ compiler instead of a C compiler. You should not encounter any compilation errors (see section Reporting Bugs). You can then use C++ code in your rule actions instead of C code. Note that the default input source for your scanner remains ‘yyin’, and default echoing is still done to ‘yyout’. Both of these remain FILE * variables and not C++ streams.

You can also use flex to generate a C++ scanner class, using the ‘-+’ option (or, equivalently, %option c++), which is automatically specified if the name of the flex executable ends in a ’+’, such as flex++. When using this option, flex defaults to generating the scanner to the file ‘’ instead of ‘lex.yy.c’. The generated scanner includes the header file ‘FlexLexer.h’, which defines the interface to two C++ classes.

The first class, FlexLexer, provides an abstract base class defining the general scanner class interface. It provides the following member functions:

const char* YYText()

returns the text of the most recently matched token, the equivalent of yytext.

int YYLeng()

returns the length of the most recently matched token, the equivalent of yyleng.

int lineno() const

returns the current input line number (see %option yylineno), or 1 if %option yylineno was not used.

void set_debug( int flag )

sets the debugging flag for the scanner, equivalent to assigning to yy_flex_debug (see section Scanner Options). Note that you must build the scanner using %option debug to include debugging information in it.

int debug() const

returns the current setting of the debugging flag.

Also provided are member functions equivalent to yy_switch_to_buffer(), yy_create_buffer() (though the first argument is an istream* object pointer and not a FILE*), yy_flush_buffer(), yy_delete_buffer(), and yyrestart() (again, the first argument is a istream* object pointer).

The second class defined in ‘FlexLexer.h’ is yyFlexLexer, which is derived from FlexLexer. It defines the following additional member functions:

yyFlexLexer( istream* arg_yyin = 0, ostream* arg_yyout = 0 )

constructs a yyFlexLexer object using the given streams for input and output. If not specified, the streams default to cin and cout, respectively.

virtual int yylex()

performs the same role is yylex() does for ordinary flex scanners: it scans the input stream, consuming tokens, until a rule’s action returns a value. If you derive a subclass S from yyFlexLexer and want to access the member functions and variables of S inside yylex(), then you need to use %option yyclass="S" to inform flex that you will be using that subclass instead of yyFlexLexer. In this case, rather than generating yyFlexLexer::yylex(), flex generates S::yylex() (and also generates a dummy yyFlexLexer::yylex() that calls yyFlexLexer::LexerError() if called).

virtual void switch_streams(istream* new_in = 0, ostream* new_out = 0)

reassigns yyin to new_in (if non-null) and yyout to new_out (if non-null), deleting the previous input buffer if yyin is reassigned.

int yylex( istream* new_in, ostream* new_out = 0 )

first switches the input streams via switch_streams( new_in, new_out ) and then returns the value of yylex().

In addition, yyFlexLexer defines the following protected virtual functions which you can redefine in derived classes to tailor the scanner:

virtual int LexerInput( char* buf, int max_size )

reads up to max_size characters into buf and returns the number of characters read. To indicate end-of-input, return 0 characters. Note that interactive scanners (see the ‘-B’ and ‘-I’ flags in Scanner Options) define the macro YY_INTERACTIVE. If you redefine LexerInput() and need to take different actions depending on whether or not the scanner might be scanning an interactive input source, you can test for the presence of this name via #ifdef statements.

virtual void LexerOutput( const char* buf, int size )

writes out size characters from the buffer buf, which, while NUL-terminated, may also contain internal NULs if the scanner’s rules can match text with NULs in them.

virtual void LexerError( const char* msg )

reports a fatal error message. The default version of this function writes the message to the stream cerr and exits.

Note that a yyFlexLexer object contains its entire scanning state. Thus you can use such objects to create reentrant scanners, but see also Reentrant C Scanners. You can instantiate multiple instances of the same yyFlexLexer class, and you can also combine multiple C++ scanner classes together in the same program using the ‘-P’ option discussed above.

Finally, note that the %array feature is not available to C++ scanner classes; you must use %pointer (the default).

Here is an example of a simple C++ scanner:

        // An example of using the flex C++ scanner class.

    int mylineno = 0;

    string  \"[^\n"]+\"

    ws      [ \t]+

    alpha   [A-Za-z]
    dig     [0-9]
    name    ({alpha}|{dig}|\$)({alpha}|{dig}|[_.\-/$])*
    num1    [-+]?{dig}+\.?([eE][-+]?{dig}+)?
    num2    [-+]?{dig}*\.{dig}+([eE][-+]?{dig}+)?
    number  {num1}|{num2}


    {ws}    /* skip blanks and tabs */

    "/*"    {
            int c;

            while((c = yyinput()) != 0)
                if(c == '\n')

                else if(c == @samp{*})
                    if((c = yyinput()) == '/')

    {number}  cout  "number "  YYText()  '\n';

    \n        mylineno++;

    {name}    cout  "name "  YYText()  '\n';

    {string}  cout  "string "  YYText()  '\n';


    int main( int /* argc */, char** /* argv */ )
        @code{flex}Lexer* lexer = new yyFlexLexer;
        while(lexer->yylex() != 0)
        return 0;

If you want to create multiple (different) lexer classes, you use the ‘-P’ flag (or the prefix= option) to rename each yyFlexLexer to some other ‘xxFlexLexer’. You then can include ‘<FlexLexer.h>’ in your other sources once per lexer class, first renaming yyFlexLexer as follows:

    #undef yyFlexLexer
    #define yyFlexLexer xxFlexLexer
    #include <FlexLexer.h>

    #undef yyFlexLexer
    #define yyFlexLexer zzFlexLexer
    #include <FlexLexer.h>

if, for example, you used %option prefix="xx" for one of your scanners and %option prefix="zz" for the other.

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