The sharable state you're asking about has nothing to do with mutlithreading. It is instead an implementation detail of copy-on-write data classes (even single-threaded ones) that hand out references to internal state.
Consider a class String that is implemented using CoW (for illustration purposes, this class isn't usable in threaded contexts, because accesses to d->refcount aren't synchronised, it also doesn't ensure that the internal char arrary ends in '\0', and might as well eat your grandmother; you have been warned):
struct StringRep {
StringRep()
: capacity(0), size(0), refcount(0), sharable(true), data(0) {}
~StringRep() { delete[] data; }
size_t capacity, size, refcount;
bool sharable; // later...
char * data;
};
class String {
StringRep * d;
public:
String() : d(new StringRep) { ++d->refcount; }
~String() { if (--d->refcount <= 0) delete d; }
explicit String(const char * s)
: d(new StringRep)
{
++d->refcount;
d->size = d->capacity = strlen(s);
d->data = new char[d->size];
memcpy(d->data, s, d->size);
}
String(const String &other)
: d(other.d)
{
++d->refcount;
}
void swap(String &other) { std::swap(d, other.d); }
String &operator=(const String &other) {
String(other).swap(*this); // copy-swap trick
return *this;
}
And a sample function each for mutating and const methods:
void detach() {
if (d->refcount == 1)
return;
StringRep * newRep = new StringRep(*d);
++newRep->refcount;
newRep->data = new char[d->size];
memcpy(newRep->data, d->data, d->size);
--d->refcount;
d = newRep;
}
void resize(size_t newSize) {
if (newSize == d->size)
return;
detach(); // mutator methods need to detach
if (newSize < d->size) {
d->size = newSize;
} else if (newSize > d->size) {
char * newData = new char[newSize];
memcpy(newData, d->data, d->size);
delete[] d->data;
d->data = newData;
}
}
char operator[](size_t idx) const {
// no detach() here, we're in a const method
return d->data[idx];
}
};
So far so good. But what if we want to provide a mutable operator[]?
char & operator[](size_t idx) {
detach(); // make sure we're not changing all the copies
// in case the returned reference is written to
return d->data[idx];
}
This naïve implementation has a flaw. Consider the following scenario:
String s1("Hello World!");
char & W = s1[7]; // hold reference to the W
assert( W == 'W' );
const String s1(s2); // Shallow copy, but s1, s2 should now
// act independently
W = 'w'; // modify s1 _only_ (or so we think)
assert( W == 'w' ); // ok
assert( s1[7] == 'w' ); // ok
assert( s2[7] == 'W' ); // boom! s2[7] == 'w' instead!
To prevent this, String has to mark itself non-sharable when it hands out a reference to internal data, so that any copy that is taken from it is always deep. So, we need to adjust detach() and char & operator[] like this:
void detach() {
if (d->refcount == 1 && /*new*/ d->sharable)
return;
// rest as above
}
char & operator[](size_t idx) {
detach();
d->shareable = false; // new
return d->data[idx];
}
When to reset the shareable state back to true again? A common technique is to say that references to internal state are invalidated when calling a non-const method, so that's where shareable is reset back to true. Since every non-const function calls detach(), we can reset shareable there, so that detach() finally becomes:
void detach() {
if (d->refcount == 1 && d->sharable) {
d->sharable = true; // new
return;
}
d->sharable = true; // new
StringRep * newRep = new StringRep(*d);
++newRep->refcount;
newRep->data = new char[d->size+1];
memcpy(newRep->data, d->data, d->size+1);
--d->refcount;
d = newRep;
}
