How do I do bit operations on a struct?

Question

I have a bitfield struct on which I want to perform bitwise operations using masks. I want to know the simplest and most efficient way to do this. I have tried using my conversion operator (which seems an inefficient way of doing the & operation on two structs) but I get error C2440: 'type cast' : cannot convert from 'const test::dtType' to 'char'. No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

class test
{
   public:
    test() : startTime(0), endTime(5,23) {}
    ~test();

    struct dtType {
        // inline constructors with initialisation lists
        dtType() {dtType(0);}
        dtType(byte z) {dtType(z,z);}
        dtType(byte n,byte h) : mins(n), hrs(h){}

        // inline overloaded operator functions
        operator char() {return mins + hrs<<3;}; // allow casting the struct as a char

        // All I want to do is return (this & hrsMask == date & hrsMask)
        // I know that in this trivial case I can do return (hrs == date.hrs) but I have simplified it for this example.
        bool operator== (dtType date){return (char(this) & char(hrsMask)) == (char(date) & char(hrsMask));};

        // data members
        unsigned mins: 3; // 10's of mins
        unsigned hrs: 5; // 8 bits
    };
    const static dtType hrsMask; // initialised outside the declaraion, since a static is not associated with an individual object.
    dtType startTime; // initialised by test() in its initialisation list
    dtType endTime; // initialised by test() in its initialisation list
};

// Typically in a source file, but can be in the header.
const test::dtType hrsMask(0,31);

I have tried using void pointers to do the bitwise operation. It compiles, but I haven't tested it.

bool test::dtType::operator== (dtType date){
    const void * longThisDT = this;
    const void * longThatDT = & date;
    const void * longMaskDT = & hrsMask;
    return (*((long *)longThisDT) &  *((long *)longMaskDT) == *((long *)longThatDT) &  *((long *)longMaskDT));
};

Is this about as efficient as we can get? It involves three extra pointers, when all I really need is a cast to long.

Answer 1

It works when this is indirected and consts are removed:

bool operator== (dtType date) {
    return (char(*this) & char(hrsMask)) == (char(date) & char(hrsMask));};

/* ... */

static dtType hrsMask;

/* ... */

 test::dtType test::hrsMask(0,31);