C++：从相同 class 的成员函数调用纯虚函数

Question

考虑以下 2 个程序。

#include <iostream>
using std::cout;
class Base {
    public:
        virtual void f()=0;
        void g() {
            f();
        }
        virtual ~Base() { }
};
class Derived : public Base
{
    public:
    void f() {
        cout<<"Derived::f() is called\n";
    }
     ~Derived() {}
};
class Derived1 : public Base
{
    public:
    void f() {
        cout<<"Derived1::f() is called\n";
    }
    ~Derived1() { }
};
int main() {
    Derived1 d;
    Base& b=d;
    b.g();
    b.f();
}

编译和运行良好并给出预期结果..

#include <iostream>
using std::cout;
class Base {
    public:
        virtual void f()=0;
        Base() {
            f();    // oops,error can't call from ctor & dtor
        }
};
class Derived : public Base
{
    public:
        void f() {
            std::cout<<"Derived::f() is called\n";
        }
};
int main() { Derived d; Base& b=d; b.f(); }

上述程序编译失败。为什么允许从声明纯虚函数的同一个 class 的成员函数调用纯虚函数？这样做可以吗，还是因为 derived class 仍然没有提供纯虚函数的实现而导致未定义的行为？为什么不能从同一个 class 的构造函数和析构函数中调用纯虚函数？我知道 Derived class 构造函数可以调用基础 class 的纯虚函数。 C++ 标准对此有何规定？

Answer 1

"Why pure virtual function can't be called from constructor ... ?"

因为此时最终的 class 尚未完全构建，并且 vtable 尚未完全设置，因此无法正确调度函数调用。

您也可以使用 static 基础关系和派生关系 class，就像 CRTP:

所建议的那样

template<class DerivedType>
class Base {
    public:
        void g() {
            static_cast<DerivedType*>(this)->f();
        }
        virtual ~Base() { }
};

class Derived : public Base<Derived>
{
    public:
    void f() {
        cout<<"Derived::f() is called\n";
    }
     ~Derived() {}
};

class Derived1 : public Base<Derived1>
{
    public:
    void f() {
        cout<<"Derived1::f() is called\n";
    }
    ~Derived1() { }
};

Answer 2

Why it is allowed to call pure virtual function from the member function of same class in which pure virtual function is declared?

因为这在技术上是可行的并且在实践中使用：请参阅模板方法模式。

Why pure virtual function can't be called from constructor & destructor of the same class?

这在技术上不能直接用 C++ 实现（还没有 vtable）。但更根本的是，您无论如何都不需要它，因为在调用构造函数时您总是知道对象的确切 class。

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