Eliminate second invocation of class (example_visitor) object

Newsgroup: comp.lang.c++

Subject: Eliminate second invocation of class (example_visitor) object

From: Mark <ma740988@...>

Date: Fri, 28 Jun 2013 20:43:50 -0700 (PDT)



Code referenced below compiles and runs. The issue I'm faced with is the constructor for example_visitor is invoked twice. The second invocation of the example_visitor constructor is predicated on invocation of dispatch in particular the line: typename Type::value_type visitor ; within dispatch



What modifications needs to occur such that I could use the 'this' pointer from the first instance of example_visitor in dispatch? I tried modifying the code to have example_visitor derive off dispatch but to no avail.



Sample appreciated. Thanks in advance.





# include <map>

# include <boost/variant.hpp>

# include <boost/mpl/vector.hpp>

# include <boost/mpl/contains.hpp>

# include <boost/utility/enable_if.hpp>

# include <boost/lexical_cast.hpp>

# include <boost/variant.hpp>

# include <boost/any.hpp>

# include <boost/shared_ptr.hpp>



# include <vector>

# include <string>

# include <iostream>



// Generic visitor

template <typename Visitor, typename TypeList>

struct generic_visitor :

public boost::static_visitor<void>,

public Visitor

{

template <typename T>

inline void operator ()

( T& v, typename boost::enable_if<

typename boost::mpl::contains< TypeList, T >::type >::type *dummy = NULL )

{

Visitor::operator () (v);

}



template <typename T>

inline void operator ()

( T& v,

typename boost::disable_if <

typename boost::mpl::contains< TypeList, T >::type >::type *dummy = NULL )

{}

};



class nil {

protected :

nil(const nil& );

nil& operator = (const nil& ) ;

unsigned int abc ;

public :

unsigned int const get() { return abc ; }

nil ()

: abc ( 5 )

{}

};



template < typename Type >

class dispatch {



typedef boost::variant <

nil&, char&, int&, double&

> sql_field;

typename Type::value_type visitor ;



private :

int test_int ;

double test_double ;



public :

dispatch ()

: test_int ( 1 )

, test_double ( 1. )

{}



template < typename T >

void doit ( T& in ) {

std::cout << "(typeid=" << typeid( in ).name() << std::endl;

typename dispatch< Type >::sql_field obj ( in );

boost::apply_visitor ( this->visitor, obj );

}



};



struct example_visitor

{

typedef generic_visitor

< example_visitor,

boost::mpl::vector<nil, char, int > > value_type;

typedef dispatch < example_visitor > dispatch_evisit ;





dispatch_evisit* ptr ;

int x ;

nil n ;



example_visitor()

: ptr ( 0 )

, x ( 4 )

{ std::cout << "." ; }



void operator () (char& v) {

std::cout << "character detected" << std::endl;

}



void operator () (int& v) {

std::cout << "(integer detected=" << v << std::endl;

}



void operator () (nil& v) {

std::cout << "nil detected=" << v.get() << std::endl;

}

void initialize () {

ptr = new dispatch_evisit ;

}

void execute () {

if ( ptr ) {

ptr->doit ( x ) ;

ptr->doit ( n ) ;

x += 2 ;

}

}

};





int main() {



example_visitor test;

test.initialize();

for ( unsigned int odx ( 0 ); odx < 10; ++odx ) {

test.execute() ;

}

std::cin.get();

}

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