An addition to what @huw-walters demonstrated above: you don't necessarily need to provide the 'store' with the constructor

CountValue(std::size_t* store):

But with slight changes, you can just use the po::variables_map::at() to get the verbosity value, just add a store_ member variable and you can remove the parameterized constructor:

class CountValue : public po::typed_value<std::size_t>
{
public:
    
    CountValue(/*std::size_t* store*/):
        po::typed_value<std::size_t>(&store_),
        store_(0)
    {
....

    virtual void xparse(boost::any& store, const std::vector<std::string>& /*tokens*/) const
    {
        // Replace the stored value with the access count.
        store_ = ++count_;
        store = boost::any(store_);
    }

private:
    mutable std::size_t count_{ 0 };
    mutable size_t store_;
};

And then you can get the 'verbose' value as:

size_t verbosity = varMap.at("verbose").as<size_t>();