[Flex-help] undefined reference to `yylex' when compiling as C++

[Filipp B. <b_...@zo...>]

Hello. I am just starting with flex and trying to compile a simple
program, no bison yet.

/* calc.l */

%{
#include <iostream>
#include <exception>
%}

%option debug
%option nodefault
%option noyywrap
%option c++

%%
\+          {
                std::cout << "PLUS" << std::endl;
            }

\*          {
                std::cout << "MULTIPLY" << std::endl;
            }

-           {
                std::cout << "MINUS" << std::endl;
            }

\/          {
                std::cout << "DIVIDE" << std::endl;
            }

.|\n        {
                std::terminate();
            }
%%

/* calc.l end */

The problem is the following:

$ flex calc.l && g++ lex.yy.cc -lfl
/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.3/../../../../lib64/libfl.a(libmain.o):
In function `main':
libmain.c:(.text.startup+0xb): undefined reference to `yylex'
collect2: error: ld returned 1 exit status


Writing pure C code using printf instead, with no %option c++, and
compiling it with gcc works fine, but this doesn't. Why?

-- 
Filipp B.