Flex C++ VTable Error - parsing

Question

I am using Flex and Bison to create a compiler. As I am trying to create an AST (Abstract Syntax Tree) for my program, I need to port it to C++. So far I have been successful, until a encountered a rather obscure error from my compiler:
Kraken.o: In function Kraken::FlexScanner::FlexScanner()':
Kraken.cc:(.text._ZN6Kraken11FlexScannerC2Ev[_ZN6Kraken11FlexScannerC5Ev]+0x26): undefined reference to vtable for Kraken::FlexScanner'
Kraken.o: In function Kraken::FlexScanner::~FlexScanner()':
Kraken.cc:(.text._ZN6Kraken11FlexScannerD2Ev[_ZN6Kraken11FlexScannerD5Ev]+0xb): undefined reference to vtable for Kraken::FlexScanner'
Here is all the relevant code:
Kraken.cc:
#include "KrakenScanner.hh"
#include "KrakenParser.hh"
int main(int argc, char * argv[]) {
Kraken::Parser parser;
return parser.parse();
}
KrakenScanner.hh:
#ifndef KRAKENSCANNER_HH_
#define KRAKENSCANNER_HH_
#if ! defined(yyFlexLexerOnce)
#include <FlexLexer.h>
#endif
#undef YY_DECL
#define YY_DECL int Kraken::FlexScanner::yylex()
#include "parser.hh"
namespace Kraken {
class FlexScanner : public yyFlexLexer {
public:
int yylex(Kraken::BisonParser::semantic_type* lval);
private:
int yylex();
Kraken::BisonParser::semantic_type* yylval;
};
}
#endif /* KRAKENSCANNER_HH_ */
KrakenScanner.cc:
#include "KrakenScanner.hh"
int Kraken::FlexScanner::yylex(Kraken::BisonParser::semantic_type* lval) {
yylval = lval; return yylex();
}
Makefile:
OBJS := Kraken.o parser.o scanner.o KrakenScanner.o KrakenParser.o
%.cc: %.y
bison -o $(#:%.o=%.d) $<
%.cc: %.l
flex -o$(#:%.o=%.d) -i $<
all: $(OBJS)
g++ -okraken $(OBJS)
Kraken.o: Kraken.cc KrakenScanner.o KrakenParser.o
KrakenScanner.o: KrakenScanner.hh KrakenScanner.cc parser.o
parser.o: parser.hh parser.cc
parser.cc: parser.y
scanner.o: scanner.cc
scanner.cc: scanner.l
KrakenParser.o: KrakenParser.hh KrakenParser.cc KrakenScanner.o`
I don't know if this will help, but FlexLexer.h defines the classes FlexLexer and yyFlexLexer. FlexLexer declares just a virtual destructor, and yyFlexLexer defines both a constructor and destructor. Also, when I attempt to overload the constr. and destr. in Kraken.cc, I get an error saying that the two are "implicitly defined".

Answer 1

Try a clean rebuild (rm *.o) and recompile. The compiler is supposed to generate this stuff automatically. Some compilers have special non-portable magic to influence v-table linking, but I don't see anything like that in your code.
Also, I see in your makefile that you've written a rule for linking with g++, but you haven't written any rule for compilation. So make is using its built-in rules, which might by the C++ compiler provided by your OS, not g++.
Another thing is that makefile rules should put the primary source first. For example:
wrong:
KrakenScanner.o: KrakenScanner.hh KrakenScanner.cc parser.o
right:
KrakenScanner.o: KrakenScanner.cc KrakenScanner.hh
Finally, object files aren't used to build other object files, only during linking.

Answer 2

First off, that error message is from your linker, not your compiler. It looks like you aren't linking in KrakenScanner.o. It also helps to apply the name demangler for your compiler to your compiler/linker error output.
What compiler are you using?