在 constness 和 volatility 上過載

如果可能，將指標引數傳遞給 T*引數比將其傳遞給 const T*引數更好。

struct Base {};
struct Derived : Base {};
void f(Base* pb);
void f(const Base* pb);
void f(const Derived* pd);
void f(bool b);

Base b;
f(&b); // f(Base*) is better than f(const Base*)
Derived d;
f(&d); // f(const Derived*) is better than f(Base*) though;
       // constness is only a "tie-breaker" rule

同樣，如果可能的話，將引數傳遞給 T& 引數比將其傳遞給 const T& 引數更好，即使兩者都具有完全匹配等級。

void f(int& r);
void f(const int& r);
int x;
f(x); // both overloads match exactly, but f(int&) is still better
const int y = 42;
f(y); // only f(const int&) is viable

此規則也適用於 const 限定的成員函式，其中允許對非 const 物件的可變訪問和對 const 物件的不可變訪問非常重要。

class IntVector {
  public:
    // ...
    int* data() { return m_data; }
    const int* data() const { return m_data; }
  private:
    // ...
    int* m_data;
};
IntVector v1;
int* data1 = v1.data();       // Vector::data() is better than Vector::data() const;
                              // data1 can be used to modify the vector's data
const IntVector v2;
const int* data2 = v2.data(); // only Vector::data() const is viable;
                              // data2 can't be used to modify the vector's data

以相同的方式，易失性過載將不如非易失性過載優選。

class AtomicInt {
  public:
    // ...
    int load();
    int load() volatile;
  private:
    // ...
};
AtomicInt a1;
a1.load(); // non-volatile overload preferred; no side effect
volatile AtomicInt a2;
a2.load(); // only volatile overload is viable; side effect
static_cast<volatile AtomicInt&>(a1).load(); // force volatile semantics for a1