Re: [Boost-users] [variant] ideas needed

From: Olaf (ope-devel_at_[hidden])
Date: 2008-02-21 01:10:08

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