How can I specialize a template member function for std::vector

Question

I need to define a get method in two different ways. One for simple types T. And once for std::vector.

template
const T& Parameters::ge         


        

Answer 1

Don't specialize function template.

• Why Not Specialize Function Templates?
• Template Specialization and Overloading

Instead, use overload.

Write a function template get_impl to handle the general case, and overload (not specialize) this to handle the specific case, then call get_impl from get as:

template<typename T>
const T& Parameters::get(const std::string& key)
{
     //read the explanation at the bottom for the second argument!
     return get_impl(key, static_cast<T*>(0) );
}

And here goes the actual implementations.

//general case
template<typename T>
const T& Parameters::get_impl(const std::string& key, T*)
{
    Map::iterator i = params_.find(key);
    return boost::lexical_cast<T>(boost::get<std::string>(i->second));
}

//this is overload - not specialization
template<typename T>
const std::vector<T>& Parameters::get_impl(const std::string& key, std::vector<T> *)
{
      //vector specific code
}

The static_cast<T*>(0) in get is just a tricky way to disambiguate the call. The type of static_cast<T*>(0) is T*, and passing it as second argument to get_impl will help compiler to choose the correct version of get_impl. If T is not std::vector, the first version will be chosen, otherwise the second version will be chosen.

Answer 2

Erm. call it something else? e.g.

template<typename T>
const T& Parameters::getVector(const std::string& key)
{
  Map::iterator i = params_.find(key);
  std::vector<std::string> temp = boost::get<std::vector<std::string> >(i->second)
  // T is already a vector
  T ret; ret.reserve(temp.size());
  for(int i=0; i<temp.size(); i++){
     ret.push_back(boost::lexical_cast<typename T::value_type>(temp[i]));
  }
  return ret;  
}

You'll have to call this as:

foo.getVector<std::vector<int> > ("some_key");

Nothing in your question precludes this.

Now, if you really do need to use get(), then you have to rely on partially specializing a structure, as function partial specialization is not supported by the language.

This is a lot more complicated, for example:

template <typename T>
struct getter
{
  const T& operator()(std::string const& key)
  {
    // default operations
  }
};

// Should double check this syntax 
template <typename T>
struct getter<std::vector<T, std::allocator<T> > >
{
  typedef std::vector<T, std::allocator<T> > VecT;
  const VecT& operator()(std::string const& key)
  {
    // operations for vector
  }
};

Then in you method becomes:

template<typename T>
const T& Parameters::get(const std::string& key)
{
  return getter<T>()(key); // pass the structures getter needs?
}