Error is giving the following code while printing the content of vector of structure

[BABU Prasad G]

When I try to print the content of vector of structure of variant of vector the following big unbelievable error even it is very simple.

#include<iostream>
#include<boost/variant.hpp>
#include<vector>
#include<string>

using namespace std;


typedef std::string net_id;
typedef std::vector<net_id> netwk_id;

// Structure to hold the vector of netid's and some more information that I am not included here bcz it is //similar one.
struct network
{
boost::variant<netwk_id> _netwk_inc;

// operator overloading
friend std::ostream& operator<<(std::ostream& os, const network& nw)
{
os << " net_id " << boost::get<netwk_id>(nw._netwk_inc);  // Here also error no 
// match operator <<
}
};

// Here I am making vector containing the above structure  // this vector only available at other side //meaning I am sending this vector through socket programming to some other server so that there I //have to decode or extract this information.
  
typedef vector<network> network_list;


int main()
{
 
// here I am assigning my netid to vector 
netwk_id net1;
net1.push_back("202");

// here I am assigning vector to structure variable of type boost variant

network net2;
net2._netwk_inc = net1;

// finally I am pushing structure into vector 

network_list net3;
net3.push_back(net2);

// Now see here i am trying to print the content of vector what ever may be I assigned earlier . but here //only it is showing error
// I don't know exactly how to extract the content 
for(int i =0; i< net3.size(); ++i)
{
cout << net3[0]._netwk_inc ;
}

}

error : 
// pls tell me how to get out of this proble

In file included from /usr/include/boost/variant/variant.hpp:1892:0,
                 from /usr/include/boost/variant.hpp:17,
                 from bin_query.cpp:2:
/usr/include/boost/variant/detail/variant_io.hpp: In member function ‘void boost::detail::variant::printer<OStream>::operator()(const T&) const [with T = std::vector<std::basic_string<char> >, OStream = std::basic_ostream<char>]’:

[clanmjc]

I think I already hinted at this in your other thread, look at what the template parameter you are trying to use...

std::vector<std::basic_string<char> >

How is it going to print a vector? You need to loop through the vector, or choose an element and print that, but not the vector.

Forgetting about variants for a second. Here is an example of what I'm talking about.

struct MyStuff
{
   std::string Hobby;
}

std::ostream& operator<<(std::ostream& os, const MyStuff& stuff)
{
   os << stuff.Hobby;
   return os;
}

//later in the code...

std::vector<MyStuff> vecMyStuff;
//... Push back 100 my stuff's..

os << vecMyStuff; //Error won't work would have had to have been os << vecMyStuff[2]; or something like that 

Yeah ok that's not your problem, lets change MyStuff to look like yours...

struct MyStuff
{
   std::vector<std::string> Hobbies;
}

std::ostream& operator<<(std::ostream& os, const MyStuff& stuff)
{
   os << stuff.Hobbies; //Same problem as before, how does the stream know what to do with this, it doesn't.
   return os;
}

//You have to tell it how to handle it.  Here is an implementation, sparing optimizations...
std::ostream& operator<<(std::ostream& os, const MyStuff& stuff)
{
     for(MyStuff::const_iterator it = stuff.begin(); it != stuff.end(); ++it)
    {
       os << *it;  //If you just need one then you need to implement this the way you need it.
    }
    return os;
 }

Note: Also looking at your code a second time, double check the "Types" you think you are passing to the overloaded ostream operator, they don't look like they match. ^{hint: _netwk_inc != network}

[BABU Prasad G]

Hello Friend ,
I didn't understand what you mean about this line of code for(MyStuff::const_iterator it= stuff.begin(); it != stuff.end(); ++it) because how it possible to create iterator for structure Mystuff

[Pravesh Koirala]

@BABU Prasad G

You are right, that portion of @clanmjc is wrong. I'm sure he meant

std::ostream& operator<<(std::ostream& os, const MyStuff& stuff)
{
     for(std::vector<std::string>::const_iterator it = stuff.Hobbies.begin(); it != stuff.Hobbies.end(); ++it)
    {
       os << *it;  //If you just need one then you need to implement this the way you need it.
    }
    return os;
 }

[BABU Prasad G]

Dear Pravesh,

           Please will you tell me if you know instead of vector in the above Mystuff structure has boost::variant< > var_name, now how to print see please variant is containing with vector instance 

ie see 

typedef vector<std::string>  Hobbies

Struct Mystuff
{
boost::variant<Hobbies>var_name;
}


if it is like this means how to print is my real problem.

[JLBorges]

Something like this:

struct A
{
    std::vector< boost::variant< std::string, int > > data ;
};

std::ostream& operator<< ( std::ostream& stm, const A& a )
{
    stm << "[ " ;
    for( std::size_t i = 0 ; i < a.data.size() ; ++i )
    {
        if( a.data[i].type() == typeid(std::string) )
              stm << boost::get<std::string>( a.data[i] ) << ' ' ;
        else stm << boost::get<int>( a.data[i] ) << ' ' ;
    }
    return stm << ']' ;
}

typedef boost::variant< A, double > variant_type ;

void print( const std::vector<variant_type>& seq )
{
    for( std::size_t i = 0 ; i < seq.size() ; ++i )
    {
        if( seq[i].type() == typeid(A) ) std::cout << boost::get<A>( seq[i] ) << '\n' ;
        else std::cout << boost::get<double>( seq[i] ) << '\n' ;
    }
}

Alternatively, create a visitor and use boost::apply_visitor().

[BABU Prasad G]

Dear friend ,

                    Actually my case is boost::variant <vector> var_name, but you had written above is vector of variant ie vector<boost::variant<>>
 
is both are same ? that's my doubt and please will you tell me I have other doubt.

ie how to type cast the boost::variant<vector> into unsigned char* ? any idea?
 
in my program error is giving when I try to assign this content of variant to a variable of type unsigned char* is shown bellow.


error: cannot convert ‘boost::variant<odtone::mih::null, std::vector<std::basic_string<char> > >’ to ‘unsigned char*’ in assignment

 

[clanmjc]

@Pravesh Koirala - Yes exactly.