Initializing a union with a non-trivial constructor

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Question 1: Default constructors do initialize POD members to 0 according to the C++ standard. See the quoted text below.

Question 2: If a constructor must be specified in a base class, then that class cannot be part of a union.

Finally, you can provide a constructor for your union:

union U 
{
   A a;
   B b;

   U() { memset( this, 0, sizeof( U ) ); }
};

For Q1:

From C++03, 12.1 Constructors, pg 190

The implicitly-defined default constructor performs the set of initializations of the
class that would be performed by a user-written default constructor for that class with an empty mem-initializer-list (12.6.2) and an empty function body.

From C++03, 8.5 Initializers, pg 145

To default-initialize an object of type T means:

  • if T is a non-POD class type
    (clause 9), the default constructor
    for T is called (and the
    initialization is ill-formed if T
    has no accessible default
    constructor);
  • if T is an array type, each element is default-initialized;
  • otherwise, the object is zero-initialized.

To zero-initialize an object of type T means:

  • if T is a scalar type (3.9), the object is set to the value of 0 (zero) converted to T;
  • if T is a non-union class type, each non static data member and each base-class subobject is zero-initialized;
  • if T is a union type, the object’s first named data member is zero-initialized;
  • if T is an array type, each element is zero-initialized;
  • if T is a reference type, no initialization is performed.

For Q2:

From C++03, 12.1 Constructors, pg 190

A constructor is trivial if it is an implicitly-declared default constructor and if:

  • its class has no virtual functions (10.3) and no virtual base classes (10.1), and
  • all the direct base classes of its class have trivial constructors, and
  • for all the nonstatic data members of its class that are of class type (or array
    thereof), each such class has a trivial constructor

From C++03, 9.5 Unions, pg 162

A union can have member functions (including constructors and destructors), but not virtual (10.3) functions. A union shall not have base classes. A union shall not be used as a base class.An object of a class with a non-trivial constructor (12.1), a non-trivial copy constructor (12.8), a non-trivial destructor (12.4), or a non-trivial copy assignment operator (13.5.3, 12.8) cannot be a member of a union, nor can an array of such objects

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