How to dynamically create a union instance in c++?

I need to have several instances of 开发者_开发问答a union as class variables, so how can I create a union instance in the heap? thank you

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The same as creating any other object:

union MyUnion
{
   unsigned char charValue[5];
   unsigned int  intValue;
};


MyUnion *myUnion = new MyUnion;

Your union is now on the heap. Note that a union is the size of it's largest data member.

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My C++ is a bit rusty, but:

   my_union_type *my_union = new my_union_type;
   ...
   delete my_union;

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Use the new operator.

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I'm not sure where you want to head with this. A union is a user-defined data or class type that, at any given time, contains only one object from its list of members. So starting from this, if you have a union defined like this:

union DataType
{
    char ch;
    integer i;
    float f;
    double d;
};

You can then use DataType as a type to define members in a class or as a type to define variables on the stack, just like regular types, struct or classes you define.

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Use the new operator:

#include <iostream>

union u {
  int a;
  char b;
  float c;
};

class c {
public:
  c() { u1 = new u; u2 = new u; u3 = new u; }
  ~c() { delete u1; delete u2; delete u3; }
  u *u1;
  u *u2;
  u *u3;
};

int main()
{
  c *p = new c;

  p->u1->a = 1;
  p->u2->b = '0' + 2;
  p->u3->c = 3.3;

  std::cout << p->u1->a << '\n'
            << p->u2->b << '\n'
            << p->u3->c << std::endl;

  delete c;

  return 0;
}

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Same as a struct :) You can use malloc() and do it the C way, or new for the C++ way. The secret is that structs, unions and classes are related; a struct is just a class (usually) without methods. There's more clarification in the following comments, should you care.