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Read-Only Data Member
Submitted by |
Here is a modest but handy tip. Sometimes it would
be useful to have read-only public data member.
Let's take a very basic example:
class counter {
public:
int n;
counter() : n(0) {}
void increment() {
n++;
}
}; |
The problem with this code is that n can be freely
modified by anybody. We could prevent that by making
n private, but then we would have to write a method to
read its value.
The solution is to make n private and to add a public
constant reference to it:
class counter {
int n;
public:
const int &value;
counter() : n(0), value(n) {}
void increment() {
n++;
}
}; |
Thus, n can't be modified outside the counter class,
while it can be read through the value data member:
void f() {
counter c;
cout << c.value << endl; // allowed, will output 0
c.increment();
cout << c.value << endl; // allowed, will output 1
c.value++; // forbidden, will produce a compile error
} |
Cheers,
Francois.
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