using a union-like class in an std::initializer_list

Question

In the code below I show union-like class S which contains two non-related structs B and C. I show how to instantiate the non-POD std::string and delete it again and then switch S to S::CC and set the num int.

#include <vector>
#include <string>
#include <iostream>
#include <memory>

struct B
{
  B() {}
  ~B() {}
  std::string str;
  void Func1() {}
};

struct C
{
  C() {}
  ~C() {}
  int num;
  void Func2() {}
};

struct S
{
  S() { tag = CC; }
  S( const S& s ) 
  {
    switch( s.tag )
    {
      case BB:
        new ( &b.str ) std::string;
        b.str = s.b.str;
        break;

      case CC:
        c.num = s.c.num; 

      default:
        break;
    }
  }

  ~S() 
  {
    switch( tag )
    {
      case BB:
        b.str.~basic_string< char >();
        break;

      case CC:
        c.num = 0;
        break;

      default:
        break;
    }
  }

  enum { BB, CC } tag;
  union
  {
    B b;
    C c;
  };
};

struct H
{
  H( std::initializer_list< S > initializerList ) : initListVect( initializerList ) {}
  std::vector< S > initListVect;
};

int main()
{
  S s;
  s.tag = S::BB;
  new ( &s.b.str ) std::string; // docs say use new placement to create memory
  s.b.str = "bbb";
  s.b.str.~basic_string< char >(); // string usage in B ok

  s.tag = S::CC;
  s.c.num = 333; // int usage in C ok

  H h {  }; // what should the init list be if I wanted 3 list elements S::BB, S::CC, S::BB?

  return 0;
}

My goal, however, is to use S in an std::initializer_list. I don’t know what the format should be for initializeing h. What should the arguments be if I wanted to initialize h with these S::BB, S::CC, S::BB?

My compiler is VS2015.

Edit: This post’s history: my posting comes from a need for a definitive answer to the question of storing compile-time-deduceable heterogeneous objects in an std::initializer_list. This question has been asked many times before and there have been many attempts at answers (see Heterogeneous containers in C++). The most simplistic answer is to use polymorphism, but this ignores the power of being able to define a type at compile time (templates). Besides, heterogeneous, non-related objects grouped together polymorphically means a lot of derived data members are useless, which sows usage and maintenance confusion downstream. Other advice given was to use boost::any or boost::variant, but this has the same weakness as polymorphism and reduces message declaration clarity. Another attempt at container object heterogeneity was the use of std::tuple, but although an initializer_list can certainly contain tuples, this approach too ignores compile-time type resolution. I even found a paper written in 1999 called Heterogeneous, Nested STL Containers in C++ which uses template template arguments to solve the heterogeneity problem. After all this, I settled on class-like unions which led to my posting here. Class-like unions for non-related/heterogeneous container objects has perfect message declaration clarity, no object size ambiguity, and is compile time template-able, and it leads to excellent downstream maintenance scenarios.

Edit2: (5 weeks later) Here is what has happened. 1) I implemented a full class-like union solution given the advice in this posting. The result was tedious and unwieldy with ‘tag’ being used to identify which sub-method to call for each new functionality. Low grade regarding code maintenance. 2) c++17 has accepted std::variant. Since that is currently not yet implemented in VS2015 Update 2, I set about using boost::variant. See What is the right c++ variant syntax for calling a member function set to a particular variant? which uses the Visitor pattern to allow access to initialized variant members and member functions. This eliminates the ‘tag’ switches and variant ‘get’ calls. Bottom line: I dropped my class-like union and adopted variant for creating maintainable code that uses initializer_list to store variant member functionality all being initializable at compile time (read: highly maintainable).

Answer 1

Alright, I'm feeling generous and I've made custom unions myself so he're some stuff that'll get you set up. I've rewritten your S structure to be more compliant and usable. (I've made changes marked by comments)

struct S
{
  S() : tag(CC) // initializer
  {
    new (&c) C; // make C object
  } 
  S(int num) : tag(CC) // added integer constructor
  {
    new (&c) C;
    c.num = num;
  }
  S(const std::string& str) : tag(BB) // added string constructor
  {
    new (&b) B; 
    b.str = str;
  }
  S( const S& s ) : tag(s.tag)
  {
    if (tag == CC)
    {
      new (&c) C; // construct c
      c.num = s.c.num;
    }
    else if (tag == BB)
    {
      new (&b) B; // construct b, not b.str
      b.str = s.b.str;
    }
  }
  S& operator= (const S& s) // added assignment operator
  {
    if (tag == s.tag) // just copy b or c
    {
      if (tag == CC)
        c = s.c;
      else
        b = s.b;
    }
    else // reconstruct b or c
    {
      if (tag == CC)
      {
        c.~C(); // destroy c
        new (&b) B; // construct b
        b.str = s.b.str;
      }
      else
      {
        b.~B(); // destroy b
        new (&c) C; // construct c
        c.num = s.c.num;
      }
      tag = s.tag;
    }

    return *this;
  }

  ~S() 
  {
    if (tag == CC)
    {
      c.~C(); // destroy c
    }
    else if (tag == BB)
    {
      b.~B(); // destroy b, not b.str
    }
  }

  enum { BB, CC } tag;
  union
  {
    B b;
    C c;
  };
};

One of the things that you were doing improperly was skipping the construction and destruction of B and C and going straight for the internal variables. You should always create and destroy types properly even when they may be trivial. While this may work out, not initializing these objects properly is only asking for trouble (It also makes it easier should you change B or C in the future).

To make using the class easier, I added in the proper constructors for std::string and int as well as an assignment operator. Because now that we can construct the objects how we want, your main() could look like this:

int main()
{
  S s;                    // default S
  s = std::string("bbb"); // set to string
  s = 333;                // set to number

  // use initialization list
  H h { std::string("bb"), 33, std::string("bb") }; 

  return 0;
}

I encourage you to modify B and C to use constructors to build their internals rather than relying on S.