String comparison?

I am attempting to write a function (I shall call it findcode()) that
makes sure that a code read in from a file is an actual code, one found
within the library of books.

Here is what I have:
---
#include <iostream>
#include <string>
#include <vector>
#include <fstream>
#include <sstream>

class Book {
      private:
            std::string author;
            std::string title;
            std::string code;
            int ncopies;
            int onloan;
      public:
            Book( const std::string &auth, const std::string &tit,
const std::string &cd, int ncop, int nonloan );
            Book( const std::string &auth, const std::string &tit,
const std::string &cd, int ncop );
            const std::string &getAuthor( ) const;
            const std::string &getTitle( ) const;
            const std::string &getCode( ) const;
            int getNcopies( ) const;
            int getOnLoan( ) const;
            void Borrow( int qty );
            void nReturn( int qty );
};

Book::Book( const std::string &auth, const std::string &tit, const
std::string &cd, int ncop, int nonloan ) {
      author.assign( auth.begin( ), auth.end( ) );
      title.assign( tit.begin( ), tit.end( ) );
      code.assign( cd.begin( ), cd.end( ) );
      ncopies = ncop;
      onloan = nonloan;
      return;
}

Book::Book( const std::string &auth, const std::string &tit, const
std::string &cd, int ncop ) {
      author.assign( auth.begin( ), auth.end( ) );
      title.assign( tit.begin( ), tit.end( ) );
      code.assign( cd.begin( ), cd.end( ) );
      ncopies = ncop;
      onloan = 0;
      return;
}

const std::string &Book::getAuthor( ) const {
      return author;
}

const std::string &Book::getTitle( ) const {
      return title;
}

const std::string &Book::getCode( ) const {
      return code;
}

int Book::getNcopies( ) const {
      return ncopies;
}

int Book::getOnLoan( ) const {
      return onloan;
}

void Book :: Borrow( int qty ) {
      onloan += qty;
      return;
}

void Book :: nReturn( int qty ) {
      onloan -= qty;
      return;
}

typedef std::vector<Book> Library;

int findcode( Library &lib, std::string code );
void printFull( Library &lib );
void processTransactions( Library &lib );
void readLibrary( Library &lib );

int main( ) {
     Library lib;

     readLibrary( lib );
     printFull( lib );
     processTransactions( lib );
     printFull( lib );

     return 0;
}

int findcode( Library &lib, std::string tcode ) {
     for( Library::iterator itor = lib.begin( ); itor != lib.end( );
++itor ) {
          Book &b = *itor;
          if( ( tcode.compare( b.getCode ) ) == 0 )
               return 1;
     }

     return -1;
}

// function printFull() goes here
// (code I have is functional, so I
// did not include it here to save space)

// this function is yet to be completed
// if you can offer any help on how I can
// do that, it would be greatly appreciated!
void processTransactions( Library &lib ) {
     std::ifstream trans( "trans.txt" );

     std::string action, code;
     int copies;

     for( Library::iterator itor = lib.begin(); itor != lib.end( );
++itor ) {
          Book &b = *itor;
          trans >> action;
          trans >> code;
          trans >> copies;
     }

// findcode() would be called here somewhere...

// ...
}

// function readLibrary( ) goes here
// (again, I left it out because it's functional)

TRANS.TXT:
b H01 3
b S01 2
b H04 3
r G01 1
r M01 1
b P12 5
b J01 2
b K04 2
r H04 4
b H04 3
r M01 5
b G02 10
r G01 3

(The code P12, for example, is nonexistent in the library of books. The
function findcode() should recognize that and print an error message.)

However, my code does not work. I get the following error when I try to
compile the code in Dev-C++ 4.9.9.2:
C:\CIS\22\asn2\asn2.cpp no matching function for call to
`std::basic_string<char, std::char_traits<char>, std::allocator<char>


Obviously, I'm doing something wrong. Where is my mistake? =/

Thanks so much! =)