c++ - unresolved external symbol for yyparse() -

my main.cpp looks this:

#include <assert.h> #include <fstream> #include <iostream> #include "astgen.h" #include "astexec.h"  extern int yyparse(); extern file *yyin;  int main() {     //yydebug = 0;     struct astelement *a = 0;     file *fp ;     fopen_s(&fp,"example.txt","r");     //std::cout<<fp;     if (fp==null)     {         std::cout<<"error";     }     yyin=fp;     yyparse();      assert(a);     struct execenviron* e = createenv();     execast(e, a);     freeenv(e);     /* todo: destroy ast */ } 

when try compile this, error: main.obj : error lnk2019: unresolved external symbol "int __cdecl yyparse(void)" (?yyparse@@yahxz) referenced in function _main

i newbie in flex/bison. appreciated. if helps here parser.y file:

%error-verbose /* instruct bison generate verbose error messages*/ %{ #include "astgen.h" #define yydebug 1  /* since parser must return ast, must parameter  * ast can stored. type of parameter void*. */ #define yyparse_param astdest extern int yylex(); %}  %union {     int val;     char op;     char* name;     struct astelement* ast; /* new member store ast elements */ }  %token token_begin token_end token_while token_do %token<name> token_id %token<val> token_number %token<op> token_operator %type<ast> program block statements statement assignment expression whilestmt call %start program  %{ /* forward declarations */ void yyerror(const char* const message);   %}  %%  program: statement';' { (*(struct astelement**)astdest) = $1; };  block: token_begin statements token_end{ $$ = $2; };  statements: {$$=0;}     | statements statement ';' {$$=makestatement($1, $2);}     | statements block';' {$$=makestatement($1, $2);};  statement:        assignment {$$=$1;}     | whilestmt {$$=$1;}     | block {$$=$1;}     | call {$$=$1;}  assignment: token_id '=' expression {$$=makeassignment($1, $3);}  expression: token_id {$$=makeexpbyname($1);}     | token_number {$$=makeexpbynum($1);}     | expression token_operator expression {$$=makeexp($1, $3, $2);}  whilestmt: token_while expression token_do statement{$$=makewhile($2, $4);};  call: token_id '(' expression ')' {$$=makecall($1, $3);};  %%  #include "astexec.h" #include <stdlib.h>  void yyerror(const char* const message) {     fprintf(stderr, "parse error:%s\n", message);     exit(1); } 

edit 1: using winflexbison in vs 2012 environment generate parser , lexer. process build rather simple. required done add <appropriate>_custom_build.targets build customizations project in vs , build solution. reference documentation can found here @ winflexbison visual studio.

i point out followed same method build sample flex , bison file , later integrated in project. worked fine me.

what doing wrong here?

your bison input includes well-commented obsolete definition [note 1]:

/* since parser must return ast, must parameter  * ast can stored. type of parameter void*. */ #define yyparse_param astdest 

as comment indicates, cause prototype yyparse be:

int yyparse(void* astdest); 

however, in main function, you've declared

extern int yyparse(); 

since compiling c++, yyparse declared in main.cpp , invoked main not same function yyparse declared in parser.tab.h , defined in parser.tab.cpp. consequently, linker error.

had put #include "parser.tab.h" main.cpp file instead of manually declaring yyparse, have seen more easily-understood error message, although can't vouch error messages produced vs 2012.

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notes:

1. yyparse_param has been deprecated since bison version 1.875, more decade ago, , works yacc-compatible parsers. should use %parse-param declaration, works bison-produced parsers, , allows specify actual parameter type rather having abandon type safety. see bison manual details.