Re: [Boost-users] Template type questions

Subject: Re: [Boost-users] Template type questions
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-12-18 09:56:29

AMDG

Glenn Macgregor wrote:
> Hi All,
>
> I am using the boost::variant with the binary visitor algorithm, apply_visitor.
>
> struct UnknownValue {};
> typedef boost::variant<UnknownValue, long, unsigned long, double, string, char> ValueType;
>
> I am using apply_visitor for binary visitation as below:
>
> class AddVisitor : public boost::static_visitor<> {
> public:
>                 template <typename T>
>                 void operator()(const T &lhs, const T &rhs) {
>                                 T val = lhs + rhs;
>                 }
>
>                 template <typename T, typename R>
>                 void operator()(const T &lhs, const R &rhs) {
>                                 // do nothing...different types
>                 }
>
>                 template <typename T>
>                 void operator()(const T &lhs, const UnknownValue &rhs) {
>                                 // do nothing...we have an unknown value
> }
>
> template <typename T>
>                 void operator()(const UnknownValue &lhs, const T &rhs) {
>                                 // do nothing...we have an unknown value
> }
>
> void operator()(const UnknownValue &lhs, const UnknownValue &rhs) {
>                                 // do nothing...we have an unknown value
> }
> };
>
> The above is not complete because I can do addition if the types are long and unsigned long or long and double or long and char...etc.
>
> What is the preferred method for doing this? Do I need to create an overridden operator() for every argument combination that is valid or can I get the argument types in the function?
>   

You can try using enable_if/disable_if with is_arithmetic.

In Christ,
Steven Watanabe