Type un-erasure from a boost::program_options value

Question

I'm trying to retrofit my program with boost::program_options. Among other things, I've written a function which I just can't get to compile. Here's a sort-of-minimal .cpp file which fails compilation:

#include <boost/program_options.hpp>

template <typename C, typename E>
inline bool contains(const C& container, const E& element) {
    return container.find(element) != container.end();
}

template <typename K, typename V>
V& updateFromConfig(V& updatee, const K& key, const po::variables_map& vm) {
    if (contains(vm, key)) {
        // option 1
        updatee = vm[key];
        // option 2
        //updatee = vm[key].variable_value();
        // option 3
        // updatee = vm[key].as<V>();
        // option 4
        // updatee = vm[key].as();
    }
    return updatee;
}

template size_t& updateFromConfig<char*,size_t>(size_t& updatee, char* const& key, const po::variables_map& vm);

With any of the four options I get a different compiler error:

• With option 1:

  cannot convert ‘const boost::program_options::variable_value’ to ‘long unsigned int’ in assignment
     updatee = vm[key];
  

• With option 2:

  invalid use of ‘boost::program_options::variable_value::variable_value’
  

• With option 3:

  18:25: error: expected primary-expression before ‘>’ token
  18:27: error: expected primary-expression before ‘)’ token
  

• With option 4:

  no matching function for call to ‘boost::program_options::variable_value::as() const
  

What am I doing wrong?

Answer 1

C++ syntax for the win. Do:

updatee = vm[key].template as<V>();