A reentrant C++ flex scanner.

flex-scanner.l.c

%option reentrant
%option noyywrap
%option extra-type="Scanner *"
%option header-file="flex-scanner.h"
%option outfile="flex-scanner.c"

%{
#include <cstdio>
#include "scanner.hpp"
%}

%%

[ \t\n] {
}

[0-9]+\.[0-9]+ {
  printf("FLOAT: %s\n", yytext);
}

[0-9]+ {
  printf("INTEGER: %s\n", yytext);
}

[a-zA-Z_]+[a-zA-Z0-9_]* {
  printf("IDENTIFIER: %s\n", yytext);
}

. {
  fprintf(stderr, "Unexpected token: %s\n", yytext);
}

main.cc

class Scanner;

#include <cstdio>
#include "flex-scanner.h"
#include "scanner.hpp"

int main(int argc, const char *argv[])
{
  do {
    char input[80] = {0};
    printf(">>> ");
    if (fgets(input, 80, stdin)) {
      do {
        Scanner *scanner = Scanner::create(input);
        scanner->scan();
      } while (0);
    }
  } while (1);

  return 0;
}

Makefile

all:
	flex flex-scanner.l
	g++ -o d flex-scanner.c scanner.cc main.cc

test: all
	./d

clean:
	rm ./d
	rm ./flex-scanner.c
	rm ./flex-scanner.h

ReentrantFlexCpp.md

A reentrant C++ flex scanner.

scanner.cc

class Scanner;

#include "flex-scanner.h"
#include "scanner.hpp"

Scanner*
Scanner::create(const char* string)
{
  Scanner*        self = NULL;
  yyscan_t        flex;
  YY_BUFFER_STATE buffer;

  if (string) {
    yylex_init(&flex);
    buffer = yy_scan_string(string, flex);
    self   = new Scanner(flex, buffer);
    yyset_extra(self, flex);
  }

  return self;
}

Scanner::Scanner(yyscan_t flex, YY_BUFFER_STATE buffer)
  : _flex(flex),
    _buffer(buffer)
{
}

Scanner::~Scanner()
{
  yy_delete_buffer(_buffer, _flex);
  yylex_destroy(_flex);
}

void
Scanner::scan()
{
  yylex(_flex);
}

scanner.hpp

#pragma once

class Scanner {
public:
  static Scanner* create(const char* string);

  void scan();
  ~Scanner();

private:
  Scanner(yyscan_t flex, YY_BUFFER_STATE buffer);

  yyscan_t        _flex;
  YY_BUFFER_STATE _buffer;
};