istream& operator>>( istream& i, line& s )
{
	s.clear();
	getline( i, s );
	return i;
}

struct funct : unary_function< string, line >
...
string operator()( line l )

#include <string>
#include <iostream>
#include <fstream>
#include <iterator>
#include <algorithm>
#include <functional>
#include <sstream>
#include <set>
using namespace std;
const char* ar_tokens[] = { "#if", "#ifdef", "#ifndef", "#endif", "#error", "#elif", "#else", "#include" };
string to_string( int i )
{
	stringstream ss;
	ss << i;
	return ss.str();
}
class line : public string {};
istream& operator>>( istream& i, line& s )
{
	s.clear();
	getline( i, s );
	return i;
}
struct funct : unary_function< const line&, string >
{
	set< string > tokens_;
	int line_number_;
	string token_;
	bool is_preprocessor_token( const string& l )
	{
		string::size_type tok_begin = l.find_first_not_of( " \t" );
		if ( string::npos != tok_begin && l[tok_begin] == '#' )
		{
			string::size_type tok_end = l.find_first_of( " \t" );
			token_ = ( string::npos == tok_end ) ? 
				l.substr( tok_begin ) : l.substr( tok_begin, tok_end - tok_begin );
			if ( tokens_.find( token_ ) != tokens_.end() )
				return true;
		}
		return false;
	}
public:
	funct() : line_number_(0)
	{
		for( int i = 0; i < sizeof( ar_tokens ) / sizeof( char* ); ++i )
			tokens_.insert( ar_tokens[ i ] );
	}
	string operator()( const line& l )
	{
		++line_number_;
		return is_preprocessor_token( l ) ? to_string( line_number_ ) : l;
	}
};
int main()
{
	transform( istream_iterator<line>( ifstream("source.txt") ),
		istream_iterator<line>(),
		ostream_iterator<string>( ofstream("dest.txt"), "\n" ),
		funct() );
}