C++ for Scientists - Technische Universität Dresden

102 CHAPTER 4. GENERIC PROGRAMMING
∼vector() { if (data) delete [] data ; }
private:
int my size;
unsigned char∗ data ;
};
One thing we realize is that the default constructor and the destructor are identic with the
non-specialized version (in the following also referred to as general version). Unfortunately, this
is not ‘inherited’ to the specialization. If we write a specialization we have to define everything
from scratch. We are free to omit member functions or variables from the general but for the
sake of consistency we do this only for good reasons, for very good reasons. For instance, we
might omit the operator+ because we have no addition for bool. The constant access operator
is implemented with shifting and bit masking:
template class vector
{
bool operator[](int i) const { return (data[i/8] ≫i%8) & 1; }
};
The mutable access is trickier because we cannot refer to single bits. The trick is to returns
some helper type — called ‘Proxy’ — that can perform the assignment and returning boolean
from a byte reference and the position within the byte.
template class vector
{
vector bool proxy operator[](int i)
{
return vector bool proxy(data[i/8], i%8);
}
};
Let us now implement our proxy:
class vector bool proxy
{
public:
vector bool proxy(unsigned char& byte, int p) : byte(byte), mask(1 ≪ p) {}
private:
unsigned char& byte;
unsigned char mask;
};
To simplify further operations we create a mask that has 1 on the position in question and 0
on all other positions.
The reading access is implemented by simply masking in the conversion operator:
class vector bool proxy
{
operator bool() const { return byte & mask; }
};
Setting a bit is realized by an assignment operator for bool: