c++ - 前向声明是行不通的

Question

下面是两个代码片段(准备编译)。在第一个片段中，我仅使用结构的前向声明，同时未调用从 Guest 类的基类 dtor 中删除指向该结构的指针。
在第二个片段中，我没有使用前向声明，而是使用了这个 Guest 类的完整定义，使用了 Base 中的 delete 按预期工作。
为什么？为什么会有所作为？前向声明难道不应该只是编译器的注释，说明此类/结构的定义在其他地方吗？
我很惊讶它不能直观地工作。

//First just forward dclr  
#include "stdafx.h"
#include <iostream>
using std::cout;

struct Guest;

struct Base
{
    Guest* ptr_;
    Base(Guest* ptr):ptr_(ptr)
    {
        cout << "Base\n";
    }
    ~Base()
    {
        cout << "~Base\n";
        delete ptr_;
    }
};

struct Guest
{
    Guest()
    {
        cout << "Guest\n";
        throw std::exception();
    }
    Guest(int)
    {
        cout << "Guest(int)\n";
    }
    ~Guest()
    {
        cout << "~Guest\n";
    }
};

struct MyClass : Base
{
    Guest g;
    MyClass(Guest* g):Base(g)
    {
        cout << "MyClass\n";

    }
    ~MyClass()
    {
        cout << "~MyClass\n";
    }
};
int _tmain(int argc, _TCHAR* argv[])
{
    try
    {
        Guest* g = new Guest(1);
    MyClass mc(g);
    }
    catch(const std::exception& e)
    {
        std::cerr << e.what();
    }
    return 0;
}

//第二 - 全定义

#include "stdafx.h"
#include <iostream>
using std::cout;

struct Guest
{
    Guest()
    {
        cout << "Guest\n";
        throw std::exception();
    }
    Guest(int)
    {
        cout << "Guest(int)\n";
    }
    ~Guest()
    {
        cout << "~Guest\n";
    }
};

struct Base
{
    Guest* ptr_;
    Base(Guest* ptr):ptr_(ptr)
    {
        cout << "Base\n";
    }
    ~Base()
    {
        cout << "~Base\n";
        delete ptr_;
    }
};



struct MyClass : Base
{
    Guest g;
    MyClass(Guest* g):Base(g)
    {
        cout << "MyClass\n";

    }
    ~MyClass()
    {
        cout << "~MyClass\n";
    }
};
int _tmain(int argc, _TCHAR* argv[])
{
    try
    {
        Guest* g = new Guest(1);
    MyClass mc(g);
    }
    catch(const std::exception& e)
    {
        std::cerr << e.what();
    }
    return 0;
}

Answer 1

来自 C++ 标准 (5.3.5/5):

If the object being deleted has incomplete class type at the point of deletion and the complete class has a non-trivial destructor or a deallocation function, the behavior is undefined.

因此您不能对不完整的类型使用 delete。它会调用析构函数，而编译器还不知道。