Tagged pointer sample implementation

TaggedPointer.h

#pragma once

#include <cassert>

// alignedTo defaults to 8 (on 64 bit)
template <typename T, int alignedTo = sizeof(T *)>
class TaggedPointer {
private:
    static_assert(
        alignedTo != 0 && ((alignedTo & (alignedTo - 1)) == 0),
        "Alignment parameter must be power of two"
    );

    // tagMask = alignedTo - 1 = 7 = 0b111
    // i.e. the lowest three bits are set, which is where the tag is stored
    static const size_t tagMask = alignedTo - 1;

    // pointerMask is the exact contrary: 0b...11111000
    // i.e. all bits apart from the three lowest are set, which is where the pointer is stored
    static const size_t pointerMask = ~tagMask;

    // Save us some reinterpret_casts with a union
    union {
        T *asPointer;
        size_t asBits;
    };

public:
    inline TaggedPointer(T *pointer = 0, int tag = 0) {
        set(pointer, tag);
    }

    inline void set(T *pointer, int tag = 0) {
        // make sure that the pointer really is aligned
        assert((reinterpret_cast<size_t>(pointer) & tagMask) == 0);
        // make sure that the tag isn't too large
        assert((tag & pointerMask) == 0);

        asPointer = pointer;
        asBits |= tag;
    }

    inline T *getPointer() {
        return reinterpret_cast<T *>(asBits & pointerMask);
    }
    inline int getTag() {
        return asBits & tagMask;
    }
};