Template argument deduction / variable template: how can I make it work?

Question

I have a problem in wrapping a template class in a non-template one;

I'm trying to make this code work:

my_class.hpp

#ifndef MY_CLASS_HPP
#define MY_CLASS_HPP

#include <vector>
#include <iostream>

class VectorPrinter {

private:

  template<class value_type>
  static std::vector<value_type> vec;

public:

  template<class value_type>
  VectorPrinter(const std::vector<value_type>& vec_)
  {
    vec = vec_
    for (std::size_t i = 0; i<vec.size(); ++i)
      std::cout << vec[i] << std::endl;
  }

};

#endif /* MY_CLASS_HPP */

main.cpp

#include "my_class.hpp"
#include <string>

int main() {

std::vector<int> v1{1, 2, 3, 4, 5};
std::vector<std::string> v2{"Hello", "world", "!"};
std::vector<double> v3{2.5, 7.3, -5.1};

VectorPrinter vp1(v1);
VectorPrinter vp2(v2);
VectorPrinter vp3(v3);

return 0;

}

I get the following error message while trying to compile (I tried -std=c++11, -std=c++14, -std=c++17: no difference; no difference neither switching from g++ to clang):

./my_class.hpp:19:5: error: cannot refer to variable template 'vec' without a template argument list

Now: my goal is to circumvent defining VectorPrinter as a template class or, alternatively, avoid specifying the template argument in the case VectorPrinter cannot be non-template; so my problem reside within the scope of either variable template or template argument deduction.

This is just a test for my thesis project; what I need to achieve, at the end, is being able to define a template library for RNG and to encapsulate these classes in more complex ones which perform Direct Simulation Monte Carlo. So, at the end I would like something like:

template <class engineType>
class RngObject {
   // ...
};

class Ensemble {
   private:
      template<class engineType> RngObject<engineType> rng;
   // ...
};

It would be quite boring having to define each and every class that encapsulate a RNG as template; moreover, I am asked to avoid using dynamic time polymorphism, at leat at this stage.

I hope someone can provide me some useful suggestion. Thanks

@MooingDuck it's just a test; i actually, in my project I would use reference to RNG objects, instead — Michele Pellegrino

Max Langhof Max Langhof · Accepted Answer · 2019-01-23T17:50:21

The error message says quite clearly what you have to do: Specify the template argument for vec.

  template<class value_type>
  VectorPrinter(const std::vector<value_type>& vec_)
  {
    // Using a reference here so we don't have to type all this again and again.
    auto& v = vec<value_type>;
    v = vec_;
    for (std::size_t i = 0; i < v.size(); ++i)
      std::cout << v[i] << std::endl;
  }

You'll also have to provide the definition for your variable template:

template<class value_type>
std::vector<value_type> VectorPrinter::vec;

Together this should give you what you're asking for: https://godbolt.org/z/gVFvFw
(Whether this is a good design for your use case is a different question).

Template argument deduction / variable template: how can I make it work?

2 Answers