Tutorial :How do I use C++ in flex and bison?



Question:

I have a project for school where we need to use flex and bison. I want to use C++ so that I have access to STL and my own classes that I wrote. We were provided with the following Makefile:

CC = gcc   CFLAGS = -g     OBJs = parse.tab.o symtab.o attr.o lex.yy.o     default: parser    parser: ${OBJs}      ${CC} ${CFLAGS} ${OBJs} -o parser -lfl    lex.yy.c: scan.l parse.tab.h attr.h      flex -i scan.l    parse.tab.c: parse.y attr.h symtab.h      bison -dv parse.y    parse.tab.h: parse.tab.c    clean:      rm -f parser lex.yy.c *.o parse.tab.[ch] parse.output    depend:      makedepend -I. *.c  

scan.l and parse.y have some initial flex/bison stuff to generate the scanner and parser. I need to add my own stuff to those files. symtab.{h, c} is supposed to be a implementation of a symbol table. attr.{h, c} are for some attribute magic. I want to make symtab.c a .cc file so I can use STL. I also have other reasons for wanting to use C++.

I tried to use a parse.ypp file, so that a .cpp file would be generated. But the problem is that I'm not getting the right .h file. I changed the Makefile to look like this:

CC = g++          # Change gcc to g++  CFLAGS = -g     OBJs = lex.yy.o parse.tab.o symtab.o attr.o    default: lex.yy.c parser    # added lex.yy.c so I could just keep lex stuff in C since I don't really need C++ there    parser: ${OBJs}      ${CC} ${CFLAGS} ${OBJs} -o parser -lfl    lex.yy.o: scan.l parse.tab.h attr.h      # added this rule to use gcc instead of g++      gcc -c -o lex.yy.o lex.yy.c    lex.yy.c: scan.l parse.tab.h attr.h      flex -i scan.l    parse.tab.cpp: parse.ypp attr.h symtab.h      bison -dv parse.ypp    parse.tab.h: parse.tab.cpp      # I want a parse.tab.h but I get parse.tab.hpp    clean:      rm -f parser lex.yy.c *.o parse.tab.cpp parse.tab.h parse.output    depend:      makedepend -I. *.c  

Can someone tell me what I need to add or do to get C++ working? It should be noted I added some stuff in the .y (or .ypp) file to deal with moving from C to C++. In particular, I had to declare some stuff as extern. My main problem is that when I run make, scan.l has a bunch of syntax errors, and they seem to be because it cannot include parse.tab.h (because it is never generated).


Solution:1

You don't need to do anything with flex or bison to use C++, I have done it many times. You just have to make sure you use g++, not gcc.

Your problems are with the Makefile, not the code.


Solution:2

For using flex with C++:   1: read the flex docs:   2: use flex -+ -o file.cc parser.ll   3: In the .ll file:    %option c++  %option yyclass="Your_class_name"  %option batch     4: In your .hh file, derive Your_class_name from  public yyFlexLexer   5: you can then use your_class_instance.yylex()  


Solution:3

There are some differences that you can check out in detail here.


Solution:4

Use either a C Compiler or a C++ compiler but not both (till you know what you are upto). You are sure to shoot yourself many times on both your feet otherwise. Mixing gcc and g++ isn't good.

This line is suspect:

lex.yy.o: scan.l parse.tab.h attr.h      # added this ...  gcc -c -o lex.yy.o lex.yy.c  

Also, you don't seem to be using CC anywhere, using that'd have made life easier.

Assuming you don't change a single line of the C code, you will possibly hit some errors and quite a few warnings (like deprecated headers etc). You'll have to fix them as well.


Solution:5

If you are doing parsers in C++ I would recommend to look at Boost Spirit. It is so much nicer to handle than bison/yacc.

From here:

Spirit is an object-oriented recursive-descent parser generator framework implemented using template meta-programming techniques. Expression templates allow us to approximate the syntax of Extended Backus-Normal Form (EBNF) completely in C++.


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