Minimize exposure of members

Reason

Encapsulation. Information hiding. Minimize the chance of unintended access. This simplifies maintenance.

Example

template<typename T, typename U>
struct pair {
    T a;
    U b;
    // ...
};

Whatever we do in the //-part, an arbitrary user of a pair can arbitrarily and independently change its a and b. In a large code base, we cannot easily find which code does what to the members of pair. This might be exactly what we want, but if we want to enforce a relation among members, we need to make them private and enforce that relation (invariant) through constructors and member functions. For example:

class Distance {
public:
    // ...
    double meters() const { return magnitude*unit; }
    void set_unit(double u)
    {
            // ... check that u is a factor of 10 ...
            // ... change magnitude appropriately ...
            unit = u;
    }
    // ...
private:
    double magnitude;
    double unit;    // 1 is meters, 1000 is kilometers, 0.001 is millimeters, etc.
};

Note

If the set of direct users of a set of variables cannot be easily determined, the type or usage of that set cannot be (easily) changed/improved. For public and protected data, that's usually the case.

Example

A class can provide two interfaces to its users. One for derived classes (protected) and one for general users (public). For example, a derived class might be allowed to skip a run-time check because it has already guaranteed correctness:

class Foo {
public:
    int bar(int x) { check(x); return do_bar(x); }
    // ...
protected:
    int do_bar(int x); // do some operation on the data
    // ...
private:
    // ... data ...
};

class Dir : public Foo {
    //...
    int mem(int x, int y)
    {
        /* ... do something ... */
        return do_bar(x + y); // OK: derived class can bypass check
    }
};

void user(Foo& x)
{
    int r1 = x.bar(1);      // OK, will check
    int r2 = x.do_bar(2);   // error: would bypass check
    // ...
}

Note

protected data is a bad idea.

Note

Prefer the order public members before protected members before private members; see NL.16.

Enforcement

Flag protected data.
Flag mixtures of public and private data

C.concrete: Concrete types

Concrete type rule summary:

View on isocpp.github.io →

class Distance { public: // ... double meters() const { return magnitude*unit; } void set_unit(double u) { // ... check that u is a factor of 10 ... // ... change magnitude appropriately ... unit = u; } // ... private: double magnitude; double unit; // 1 is meters, 1000 is kilometers, 0.001 is millimeters, etc. };