成员指针指向成员对象和声明顺序 [英] Member pointer to member object and declaration order

问题描述

#include <iostream>

class FooParent
{
    public:
        FooParent(int* new_p_bar)
        {
            p_bar = new_p_bar;
        }
    public:
        int* p_bar;
};

class FooChild : public FooParent
{
    public:
        int bar;
    public:
        FooChild(int new_x)
        :FooParent(&bar)
        ,bar(new_x) \\ point of concern
            {} 
};

int main()
{ 
    FooChild foo(8);
    std::cout << foo.bar << std::endl;

}

上面的例子工作原理就像我想要的。将指针 p_bar 链接到 bar 。但是，我的问题是我指向的构造函数尚未被调用的成员。

The above example works as I want it to .i.e. link the pointer p_bar to bar. However, my concern is that I am pointing to a member whose constructor is not yet called.

这个代码是否有效，或者标准有什么要说的。

Is this code valid, or does the standard have something to say about it. If not what is the alternative.

注意：在我的应用程序 bar 对象 Bar （不是 int ），这有什么含义吗？

NOTE: In my application bar is an Object Bar (not int), does this have any implications?

推荐答案

看看这个：

class FooParent {
    public:
        FooParent(int* new_p_bar)
        {
            p_bar = new_p_bar;
            *p_bar = 99; // this has no sense
        }
        void set99() {
            *p_bar = 99; // this - has
        }
    public:
        int* p_bar;
};

class FooChild : public FooParent
{
    public:
        int bar;
    public:
        FooChild(int new_x)
        :FooParent(&bar)
        ,bar(new_x) // point of concern
            {} 
};

int main()
{ 
    FooChild foo( 42 );
    std::cout << foo.bar << std::endl;
    foo.set99();
    std::cout << foo.bar << std::endl;
}

LWS 。

我的意思是如果 FooParent 构造函数只向外部存储指针 int （或 Bar - 无关紧要）

I mean that if FooParent's constructor only stores a pointer to external int (or Bar - doesn't matter) then there will be no problem.

另一方面，如果您给予 bar code>到 FooParent - 像这样

In other hand, if you'll give a copy of bar to FooParent - like this

class FooParent
{
    public:
        FooParent(Bar new_p_bar)
        {
            p_bar = new_p_bar;
        }
        void set99() {
            p_bar = 99; // this - has
        }
    public:
        Bar p_bar;
};

class FooChild : public FooParent
{
    public:
        Bar bar;
    public:
        FooChild(Bar new_x)
        :FooParent(bar)
        ,bar(new_x) // point of concern
            {} 
};

int main()
{ 
    FooChild foo( 42 );
    std::cout << foo.bar << std::endl;
    foo.set99();
    std::cout << foo.bar << std::endl;
}

LWS 。

这不会工作。即使 Bar 将有一个副本操作符或/和赋值运算符

this will not work. Even if Bar will have a copy c-tor or/and assignment operator

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