cpp core · high

Const-Correctness

Const-correctness is a compile-time contract. A const member function promises not to modify the object, and a const object (or const reference) may call only const methods — calling a non-const method on it is ill-formed because it discards the qualifier. Members marked mutable are the exception: they may change inside a const method to support logical const (caches, counters, mutexes). With pointers, read the type right-to-left: 'const int*' is a pointer to const data, 'int* const' is a const pointer to mutable data. Default to const everywhere it applies and let the compiler enforce intent.

The code

class Widget {
    int v_;
    mutable int reads_ = 0; // mutable: writable even in a const method
public:
    int get() const {
        ++reads_;
        return v_;
    } // const: won't modify *this
    void set(int x) {
        v_ = x;
    } // non-const: may modify
};

const Widget w; // const object
Widget m;       // non-const object

w.get();  // OK    - const object may call const methods
w.set(3); // ERROR - discards the 'const' qualifier
m.get();
m.set(3); // OK    - non-const object may call anything

const int* p = &n;       // pointer to const int : *p read-only, p can repoint
int* const q = &n;       // const pointer to int : *q writable, q is fixed
const int* const r = &n; // both fixed

What this lesson walks through

1. 01const is a promise
2. 02const object -> const method: OK
3. 03const object -> non-const method: error
4. 04non-const object: full access
5. 05mutable: logical const
6. 06const with pointers: read right-to-left
7. 07Be const by default

Marking a member function const is a promise to the compiler: this call will not modify the object. The flip side: a const object (or const reference) may call only const member functions.