Re: [Boost-users] [variant] how to get 1st bounded type

From: John Femiani (JOHN.FEMIANI_at_[hidden])
Date: 2008-05-07 05:53:08

> Why don't you like visitor to dispatch the type stored in the variant?
 
I like it very much, and I use variant because of the power of the
static visiting mechanism.
The reason I wanted to test if the variant contains its 1st type is the
following: my variant is made of a list of shared-pointers to various
types; there's a routine that initializes it with one of the types
depending on some conditions, then it applies some visitor on the
resulting variant and stores it then in a container - however, there's
no "default" behavior, so if neither condition was met, the variant
remains "uninitialized" - i.e. it contains 1st type "zero" ptr. I want
neither apply the visitor on "empty" smartpointer nor store it in the
container, so I have to know whether it was "really" created or not. Two
simple ways I can think about are to test the resulting variant or to
introduce some flag - of course, both ways are far from ideal...
 
 
struct my_visitor {
    typedef bool return_type;
    vector<variant_type> & vec_;
 
    my_visitor (vector<variant_type> & vec) : vec_(vec) {} 
    my_visitor (my_visitor  & other) : vec_(other.vec_) {}
 
    bool opeator()(...) {return false;}
    bool operator()(type1  value) { vec_.push_back(variant_type(value));
} 
    bool operator()(type2  value) { vec_.push_back(variant_type(value));
}
} 
 
 
:boost::apply_visitor(my_visitor(my_vec), my_variant)