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boost::variant and boost::recursive wrapper issue

unbekant (4)
Hi everyone,
I've recently started using boost libraries and im facing a problem.
I have the following boost::variant typedef:

typedef boost::variant<std::string,float,int,bool> Value;

I need Value to hold a vector of Values itself, can this be done?
A person I know told me it could be done using recursive_wrapper but it seems i'm not getting it right nor if it can actually be done. I tried to compile the following code and receive several invalid template arguments error because Value has not yet been declared:

typedef boost::variant<std::string,float,int,bool, std::vector< boost::recursive_wrapper<Value> > > Value;

Could anyone tell me if this can be done and if possible what am I doing wrong?
Thanks in advance! :)
jsmith (5804)
Have you tried

 
 
typedef boost::make_recursive_variant< std::string, float, int, bool, std::vector< boost::recursive_variant_ > >::type Value;

unbekant (4)
Thank you jsmith it actually works like that!
It brought me a lot of new troubles because of the use I gave to the old variant but that's got nothing to do with this.
Thanks again!
Cya
jenia (14)
I tried to initialize an instance of the Value, but I always get a compilation error.

Also I have the following code with boost pointer and would like also to initialize,

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struct nil { }; 
 
typedef boost::make_recursive_variant<nil,  
    std::pair<int, boost::scoped_ptr<boost::recursive_variant_> > >::type      Value;





Could anyone help me?
(I would like to have a simple example of initialization)
Thanks in advance!
Last edited on
jenia (14)
I did myself the first with no pointers.
If it might help someone, I put the code:

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typedef boost::make_recursive_variant< std::string, float, int, bool, 
std::vector< boost::recursive_variant_ > >::type Value;

	std::vector< Value > subresult;
	subresult.push_back(("hello", 0, 3, true, ("2nd", 2)));
	subresult.push_back(5);
	

	std::vector< Value> result;
	result.push_back(1);
	result.push_back(subresult);
	result.push_back(7);

	Value var(result);


It compiles.
To be clear, it represents the resultant content of var as
( 1 (hello 0 3 true (2nd 2) 5 ) 7 ).


To print everything:

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struct print_visitor : public boost::static_visitor<void>
{
// print int
void operator()(int i) const
{
std::cout << i << ' ';
}
// print vector of Value

void operator()(std::vector<Value> const & v) const
{
std::cout << " ( ";
for(std::size_t i=0; i<v.size(); ++i )
boost::apply_visitor(print_visitor(), v[i]);

std::cout << " ) ";
}

void operator()(float i) const
{
std::cout << i << ' ';
}

void operator()(bool i) const
{
std::cout << i << ' ';
}

void operator()(std::string i) const
{
std::cout << i << ' ';
}

};
boost::apply_visitor(print_visitor(), var); // output:  ( 1  ( 2 5  ) 7  )





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#include <boost/regex.hpp>
#include <iostream>
#include <string>
#include <boost\variant\recursive_wrapper.hpp>
#include <boost\make_shared.hpp>
#include <boost\variant\recursive_variant.hpp>
#include <boost\variant\variant.hpp>
#include <utility>
#include <boost\visit_each.hpp>
#include <boost/variant/apply_visitor.hpp>


With scoped_ptr pointer it is trivial.

Last edited on
jenia (14)
To finish with:

Note: Portability is unfortunately, due to standard conformance issues in several compilers, make_recursive_variant is not universally supported.

On these compilers the library indicates its lack of support via the definition of the preprocessor symbol BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT. Thus, unless working with highly-conformant compilers, maximum portability will be achieved by instead using recursive_wrapper.
jsmith (5804)
Yes. Boost has become a good stress test for compilers :)

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