C++ · 2 · Classes & RAII9 / 36 · 25%

Access Control, static & friend

Encapsulation knobs: who can touch what, and what belongs to the class itself.

Examples: public/private/protected, static members, friend

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Access Specifiers

struct defaults to public, class defaults to private.

Example

example

class Account {
public:
    void deposit(double x);      // interface
protected:
    void audit();                // derived classes only
private:
    double balance_ = 0;         // nobody outside
};

static Members

One shared instance for the whole class, not per object.

Example

example

class Counter {
    static inline int live_ = 0;      // C++17: define inline in the header
public:
    Counter()  { ++live_; }
    ~Counter() { --live_; }
    static int live() { return live_; }   // no this pointer
};

friend

Grants one specific function or class access to privates.

Example

example

class Vec2 {
    float x_, y_;
    friend std::ostream& operator<<(std::ostream&, const Vec2&);
};

std::ostream& operator<<(std::ostream& os, const Vec2& v) {
    return os << '(' << v.x_ << ", " << v.y_ << ')';
}

TIP

friend is not a hole in encapsulation when used for operators that are logically part of the class.

explicit & Member Init

Stop accidental implicit conversions; initialise in the member list.

Example

example

class Buffer {
    std::size_t size_;
    std::vector<char> data_;
public:
    explicit Buffer(std::size_t n) : size_(n), data_(n) {}
    // without explicit: Buffer b = 42; would compile
};

Members initialise in declaration order, not the order you write in the init list.