The language already mostly lets you do this with a simple class; with the way const
cascades to access to members (combined with mutable
for the counter
member, which you've indicated should always be mutable), you can provide both read-only and read-write access to a buffer quite easily:
class C
{
public:
C(int* buffer) : buffer(buffer) {}
const int* get() const { return buffer; }
int* get() { return buffer; }
void increment() const { counter++; }
private:
int* buffer;
mutable int counter;
};
void function(int* n)
{
// These are both okay
C wn(n);
const C wc(n);
// Reading the value is always allowed
printf("%d %d", wn.get()[0], wc.get()[0]);
// Can increment either object's counter
wn.increment();
wc.increment();
// Can set non-const pointer
wn.get()[0] = 5;
// Generates a compiler error
wc.get()[0] = 5;
}
What you can't do with this is neatly arrange for the class to be instantiated with either a int*
or a const int*
; the two lead to totally different semantics for your class, so you should split it into two if you really need that.
Fortunately, templates make this easy:
template <typename T>
class C
{
public:
C(T* buffer) : buffer(buffer) {}
const T* get() const { return buffer; }
T* get() { return buffer; }
void increment() const { counter++; }
private:
T* buffer;
mutable int counter;
};
Now a C<int>
is as above, but a C<const int>
only provides read-only access to the buffer, even when the C<const int>
object itself is not marked as const
:
void function(int* n1, const int* n2)
{
C<int> a(n1);
C<const int> b(n2);
const C<int> c(n1);
const C<const int> d(n2);
// Reading the value is always allowed
printf("%d %d %d %d",
a.get()[0], b.get()[0],
c.get()[0], d.get()[0]
);
// Incrementing the counter is always allowed
a.increment();
b.increment();
c.increment();
d.increment();
// Can set non-const pointer
a.get()[0] = 5;
// Cannot set const pointer, or const/non-const pointer behind const object
//b.get()[0] = 5;
//c.get()[0] = 5;
//d.get()[0] = 5;
}