Accessing class members using reference_wrapper instead of standard pointers in C++11

Question

I am trying to build an unordered_map to vector variables which are members of my class. I can do this using standard pointers * but this requires the use of (*x) to access the vector. I wondered if std::reference_wrapper would be cleaner, but can't get it to work.

#include <unordered_map>
#include <iostream>
#include <vector>
#include <functional>

class model_class {

private:

  static constexpr auto MaxHerds = 1 ;
  static constexpr int MaxInitValues = 5 ;
  static constexpr int MaxAnimals = 2 ;

  std::vector< double > MamCellsF ;
  std::vector< std::vector< double > > InitCond ;

public:

  std::unordered_map< std::string , std::reference_wrapper< std::vector< double > > > vector;
  std::unordered_map< std::string , std::vector< std::vector< double > >* > array;

  model_class ( ) :
    // initialise vectors
    MamCellsF( MaxHerds , 0.07 ) ,
    InitCond( MaxAnimals , std::vector< double > ( MaxInitValues , 0.7 ) )
  {
     // point to variables from model
     vector.insert({"MamCellsF", std::ref(MamCellsF)});
     array["InitCond"] = &InitCond;

     // assign to vectors
     MamCellsF = { 0.001 , 0.002 }  ; // warning: automatic resizing
     InitCond = { { 1.0 , 550 , 3.5 , 1 , 4 } ,
     { 2.0 , 625 , 3.5 , 5 , 4 } } ;
  }

  void print_values(){

    // access the vectors
    double a = vector["MamCellsF"].get()[1]; // error: "no matching function call to `std::reference_wrapper<std::vector<double>>::reference_wrapper()"
    double b = (*array["InitCond"])[0][1];
    std::cout << a << std::endl;
    std::cout << b << std::endl;

  }

};

void test()
{

  model_class model;
  model.print_values();

}

Quimby · Accepted Answer · 2018-11-05 21:08:53Z

2

vector["MamCellsF"] returns a reference to the value in the map. So if there isn't one it must be constructed first. That uses default constructor but std::reference_wrapper is not default constructible hence the error.

I think that STL containers are safe with T that are not default constructible but their operations might be limited. So I don't recommend that.

edited Nov 5, 2018 at 21:08

answered Nov 5, 2018 at 20:58

Quimby

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4 Comments

Simon Woodward Over a year ago

I thought I have already inserted the reference into the map.

Quimby Over a year ago

@SimonWoodward That doesn't matter. This is compile error, because in that operator is code that does the adding and the default construction and it cannot be compiled.

Simon Woodward Over a year ago

Are you saying that std::reference_wrapper is not the right tool for what I am trying to do? Is it ok to use standard pointers then?

Quimby Over a year ago

@SimonWoodward It has the sematics that you want, but the cost to it is that it's not default-constructible and STL containers don't work nicely with that. So I would stick to pointers probably.

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