Statically initialize anonymous union in C++

问题:

I am trying to statically initialize the following structure in Visual Studio 2010:

struct Data
{
   int x;
   union
   {
      const Data* data;
      struct {int x; int y; };
   };
};

The following is fails with error C2440: 'initializing' : cannot convert from 'Data *' to 'char'.

static Data d1;
static Data d = {1, &d1};
static Data d2 = {1, {1, 2}};

I have found references to some ways this can be initialized properly but none of them work in VS2010. Any ideas?

回答1:

Can you do it by defining overloaded constructors? Untested code ahead:

struct Data 
{ 
    int x; 
    union 
    { 
        const Data* data; 
        struct {int a; int b; } z; 
    } y;

    Data()
    {
        x = 0;
        y.data = 0;
        y.z.a = 0;
        y.z.b = 0;
    }

    Data(int x_, Data* data_)
    {
        x = x_;
        y.data = data_;
    }

    Data(int x_, int a_, int b_)
    {
        x = x_;
        y.z.a = a_;
        y.z.b = b_;
    }
}; 

static Data d1; 
static Data d(1, &d1); 
static Data d2(1, 1, 2); 

回答2:

ISO C++03 8.5.1[dcl.init.aggr]/15:

When a union is initialized with a brace-enclosed initializer, the braces shall only contain an initializer for the first member of the union. [Example:

union u { int a; char* b; };
u a = { 1 };
u b = a;
u c = 1; // error
u d = { 0, "asdf" }; // error
u e = { "asdf" }; // error

—end example]

So, generally speaking, it can't be done.

回答3:

Change it to:

struct Data
{
   int x;
   union
   {
      const Data* data;
      char ch;
   };
};

static Data d1;
static Data d = {1, &d1};

回答4:

You can make this work if all your union types are pointers, by using a void pointer as the first element of the union. All pointers can be converted to a void pointer, so your union can be initialized with an arbitrary pointer type. For the given example, you get:

struct Data
{
   int x;
   union
   {
      const void* unused;
      const Data* data;
      struct {int x; int y; }*; //Not sure this works written like this
      const char* asChar;
      const int*  asInt;
   };
};

static Data d1;
static Data d2 = {2, &d1};
static Data d3 = {1, "Hello, world!"};

The other possibility is to do this in C instead. In C you can specify which part of the union is initialized. Using your original struct (and giving your structure the name asStruct):

static Data d1;
static Data d2 = {2, &d1};
static Data d3 = {3, {.asStruct = {0,0}}