Why don't anonymous joins contain members with non-trivial constructors / destructors?

Question

I could be wrong, but the main explanation I discovered was that the union cannot initialize because it does not know which member constructor needs to be called. The compiler cannot automatically generate a constructor for concatenation.

Why is the user not allowed to define a union constructor? This will remove the indicated problem and allow the presence of union members that have a non-trivial constructor / destructor.

Also, why doesn't a union member have any custom constructors? The previous explanation does not match custom constructors.

Update 1:

Example:

struct SQuaternion
{
    union
    {
        S3DVector Axis;
        struct
        {
            float X;
            float Y;
            float Z;
        };
    };
    float W;
};

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Answer 1

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Answer 2

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Answer 3

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union X {
    int i;
    std::string s;
};

X x;
x.s = "Hello";
x.i = 23;

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Answer 4

:

typedef union uAA {
    double dVal;
    int iVal[2];

    uAA() : dVal(3.22) {}
} UAA;

main() {
    UAA rdata;

    printf("Array output: %d %d \nDouble output: %lf \n",
        rdata.iVal[0], rdata.iVal[1], rdata.dVal);
}

Answer 5

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Re- : 9.5

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