Steven Watanabe schrieb:

AMDG

ope wrote:

...

Hi,

I've the following code:

---8<--- #include <boost/variant.hpp>

#include <iostream> #include <string>

struct available : public boost::static_visitor<bool> { template <typename T, typename U> bool operator()( const T&, const U& ) const { return false; // cannot compare different types }

template <typename T> bool operator()( const T& lhs, const T& rhs ) const { static const std::string NA( "NA" );

return lhs != NA; //??? } };

using namespace boost; using namespace std;

int main() { variant< double, std::string > R; variant< double, std::string > L;

R = 3.14; L = "NA";

cout << R << endl; cout << L << endl;

bool r = boost::apply_visitor(available(), R ); cout << ( r ? "true" : "false" ) << endl;

bool l = boost::apply_visitor(available(), L ); cout << ( l ? "true" : "false" ) << endl; }

--->8---

which does not compile. Anyway, from a data parser I get a double or a "NA" (not available) string. This test shall handle both cases. To process using the double I have to check if it available (than use

otherwise use default, this wasn't clear, sorry.

...

defaults). Unfortunately, the static_visitor<bool> needs two arguments. How can I get the syntax (or simpler) as used in the example

Let me make sure that I understand what you want. If the variant contains a double the result should be true. If the variant contains the string "NA" the result should be false.

That's correct. If it contains a double the string value is empty.

The variant cannot contain a string that is not "NA"

struct available : public boost::static_visitor<bool> { bool operator()( const double&) const { return true; }

bool operator()( const string& lhs) const { assert(lhs == "NA");

return false; //??? } };

May this work? There is always a string - empty or holding "NA". Thanks, Olaf

Hi Steven, the problem is related to the static_visitor, e.g. (from docs): boost::variant< int, std::string > u; ... class my_visitor : public boost::static_visitor<int> { public: int operator()(int i) const {return i;} int operator()(const std::string & str) const {return str.length(); } }; same for static_visitor<> etc. BUT static_visitor<bool>: class are_strict_equals : public boost::static_visitor<bool> { public: template <typename T, typename U> bool operator()( const T &, const U & ) const { return false; } template <typename T> bool operator()( const T & lhs, const T & rhs ) const { return lhs == rhs; } }; is binary! I've got compilation errors due to. Thanks, Olaf