Notes

const binds to whatever is immediately to its left; if there’s nothing
to its left, it binds to what’s on its right.

Code

const int* p;       // pointee is const, p itself can be reassigned
int* const p;       // p itself is const, pointee can be modified
const int* const p; // both are const

void draw(const Polygon2D& p);   // p can't be modified through this ref

class Node {
    void computeTransform() const;  // trailing const: can't modify
};                                   // members; only kind callable
                                      // through a const Node&

const objects — can only call const member functions

class Node {
public:
    void setColor(glm::vec3 c) { color = c; }        // NOT const
    glm::vec3 getColor() const { return color; }      // const
private:
    glm::vec3 color;
};

const Node n;
n.getColor();          // fine - const object calling a const function
n.setColor({1,0,0});   // error - can't call a non-const function on a const object

The compiler doesn’t check what setColor actually does — it just sees the function isn’t marked const and refuses, even if setColor happened to do nothing at all. This is exactly why marking every function that doesn’t modify state as const matters: it’s the only thing that decides what a const object is allowed to call.

mutable — the deliberate escape hatch

class Node {
public:
    int getAccessCount() const {
        accessCount++;   // would normally be illegal in a const function...
        return accessCount;
    }
private:
    mutable int accessCount = 0;   // ...but mutable makes this one member exempt
};

mutable marks one member as modifiable even through a const object or const member function. Reserve it for bookkeeping that doesn’t change the object’s logical state — a cache, a lazily-computed value, an access counter — never for something that actually changes what the object represents.

So basically, people will use const when they don’t want something to be modified. For safety or something else.

Gotchas

• std::move can’t strip const - you can’t move out of a const
  parameter.

Related

unique_ptr