c++ - 覆盖虚函数和隐藏非虚函数有什么区别？

Question

给定以下代码片段，函数调用有什么区别？什么是函数隐藏？什么是函数覆盖？它们与函数重载有什么关系？两者有什么区别？我在一个地方找不到对这些的详细描述，所以我在这里询问，以便我可以整合信息。

class Parent {
  public:
    void doA() { cout << "doA in Parent" << endl; }
    virtual void doB() { cout << "doB in Parent" << endl; }
};

class Child : public Parent {
  public:
    void doA() { cout << "doA in Child" << endl; }
    void doB() { cout << "doB in Child" << endl; }
};

Parent* p1 = new Parent();
Parent* p2 = new Child();
Child* cp = new Child();

void testStuff() {
  p1->doA();
  p2->doA();
  cp->doA();

  p1->doB();
  p2->doB();
  cp->doB();
}

Answer 1

什么是函数隐藏？

... 是名称隐藏的一种形式。一个简单的例子:

void foo(int);
namespace X
{
    void foo();
    
    void bar()
    {
        foo(42); // will not find `::foo`
        // because `X::foo` hides it
    }
}

这也适用于基类中的名称查找:

class Base
{
public:
    void foo(int);
};

class Derived : public Base
{
public:
    void foo();
    void bar()
    {
        foo(42); // will not find `Base::foo`
        // because `Derived::foo` hides it
    }
};

---

什么是函数覆盖？

这与虚函数的概念有关。 [class.virtual]/2

If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name, parameter-type-list, cv-qualification, and ref-qualifier (or absence of same) as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf.

class Base
{
private:
    virtual void vf(int) const &&;
    virtual void vf2(int);
    virtual Base* vf3(int);
};

class Derived : public Base
{
public: // accessibility doesn't matter!
    void vf(int) const &&; // overrides `Base::vf(int) const &&`
    void vf2(/*int*/);     // does NOT override `Base::vf2`
    Derived* vf3(int);     // DOES override `Base::vf3` (covariant return type)
};

调用虚函数时，最终的覆盖器变得相关:[class.virtual]/2

A virtual member function C::vf of a class object S is a final overrider unless the most derived class of which S is a base class subobject (if any) declares or inherits another member function that overrides vf.

即如果您有一个 S 类型的对象，则最终覆盖器是您在遍历 S 的类层次结构返回其基类时看到的第一个覆盖器。重要的一点是函数调用表达式的动态类型用于确定最终的覆盖器:

Base* p = new Derived;
p -> vf(42);    // dynamic type of `*p` is `Derived`

Base& b = *p;
b  . vf(42);    // dynamic type of `b` is `Derived`

---

覆盖和隐藏有什么区别？

本质上，基类中的函数总是被派生类中的同名函数隐藏；无论派生类中的函数是否覆盖基类的虚函数:

class Base
{
private:
    virtual void vf(int);
    virtual void vf2(int);
};

class Derived : public Base
{
public:
    void vf();     // doesn't override, but hides `Base::vf(int)`
    void vf2(int); // overrides and hides `Base::vf2(int)`
};

要查找函数名，使用表达式的静态类型:

Derived d;
d.vf(42);   // `vf` is found as `Derived::vf()`, this call is ill-formed
            // (too many arguments)

---

它们与函数重载有何关系？

由于“函数隐藏”是名称隐藏的一种形式，如果函数名称被隐藏，所有重载都会受到影响:

class Base
{
private:
    virtual void vf(int);
    virtual void vf(double);
};

class Derived : public Base
{
public:
    void vf();     // hides `Base::vf(int)` and `Base::vf(double)`
};

对于函数重写，只会重写基类中具有相同参数的函数；你当然可以重载一个虚函数:

class Base
{
private:
    virtual void vf(int);
    virtual void vf(double);
    void vf(char);  // will be hidden by overrides in a derived class
};

class Derived : public Base
{
public:
    void vf(int);    // overrides `Base::vf(int)`
    void vf(double); // overrides `Base::vf(double)`
};