How to assign value to union in VC++ - visual-c++

Question

There is an union in C and embedded into C++ as below:
typedef union MyUnion MyUnion_;
union MyUnion{
ULONG mLong;
char mChar;
...
};
When I trying to init it like:
MyUnion_ test;
test = (MyUnion_)NULL;
this is can compile by Mingw32, but gives
error: C2440: 'type cast': cannot convert from 'void *' to 'MyUnion_'
in VC++ (VS2015). So how to do cast & initialize of union in VC++ compiler?
Now I am doing like this:
MyUnion_ test;
test.mLong = NULL;
but this makes the program look bad when passing union as a parameter.
void test(MyUnion_ u)
ULONG i = 0;
// mingw32
test((MyUnion_)i);
// vc++
MyUnion_ temp;
temp.mLong = i;
test(temp);

Answer 1

Using a compiler that supports the C++11 uniform initialization syntax you can use a braced initializer with a single value which will be used to initialize the first non-static field of the union …
MyUnion test{ 0 };
You could use NULL instead of zero in the code above but it seems confusing to initialise mLong (which is a ULONG) with NULL.
You can also use braced initialization in an assignment statement if you have to set the variable after it was declared …
MyUnion test{ 0 };
// ...
test = { 3 };
Note that the braced initializer syntax may also be available in older compilers that offer experimental support for what used to be called C++0x
Visual Studio 2015 C++ supports braced initializers unless you are compiling a file with a .c extension or are using the /TC switch to compile as C code (rather than C++ code).
Older C++ (and C) compilers
When using compilers that don't support braced initialization the older assignment initialization syntax can be used in declarations ...
MyUnion_ test = { 0 };
… but not in assignment statements.
Casting to union type
According to this IBM Knowledge Center article casting to a union type is an extension to C99 "... implemented to facilitate porting programs developed with GNU C" - which suggests it's not standard C.
This Microsoft documentation indicates there are no legal casts in C for a union, struct or array.
In C++ a cast to a union type is possible if a suitable constructor exists...
union MyUnion {
unsigned long mLong;
char mChar;
MyUnion(unsigned long val) { mLong = val; };
};
// valid cast
test = (MyUnion)3ul;
// invalid cast - no suitable constructor exists
void * ptr = 0;
test = (MyUnion)ptr;

Answer 2

Default constructor?
typedef union MyUnion MyUnion_;
union MyUnion {
ULONG mLong;
char mChar;
MyUnion(): mLong(0) {}
};
int main()
{
MyUnion_ temp;
return 0;
}