条件对C++中的多态对象有什么作用？(包含多态性)

我遇到了一个有趣的错误,我很确定它与条件语句的上下文中的包含多态性有关.

该示例的亮点如下:

   ClassParent *parentPointer; //Declare pointer to parent

   if(condition){
       ClassChild1   = mychild; //Declare child1 object
       parentPointer = *mychild;//Parent pointer points to child
   }

   if(!condition){
       ClassChild2   = mychild; //Declare child2
       parentPointer = *mychild;//Parent pointer points to child2  
   }

   cout << *parentPointer; //What will this point to???

应该清楚,条件语句在最后一行中生成*parentPointer变量.

我的整个函数看起来像这样:(注意崩溃的地方)

    void PosApp::addItem(bool isPerishable) {
        Item *refitem;

        if (isPerishable) {
            Perishable myitem;
            std::cout   << "Enter the following: "  << std::endl
                        << "Sku: "                  << std::endl
                        << "Name:"                  << std::endl
                        << "Price: "                << std::endl
                        << "Taxed: "                << std::endl
                        << "Quantity: "             << std::endl
                        << "Expiry date: "          << std::endl;

            std::cin    >> myitem;
            refitem = &myitem; //Item now implements inclusion polymorphism,  be aware of dynamic/static types (dynamic is item, static Perishable)
        }

        if (!isPerishable) {
            NonPerishable myitem;
            std::cout   << "Enter the following: "  << std::endl
                        << "Sku: "                  << std::endl
                        << "Name:"                  << std::endl
                        << "Price: "                << std::endl
                        << "Taxed: "                << std::endl
                        << "Quantity: "             << std::endl;

            std::cin    >> myitem;
            refitem = &myitem; //Item now implements inclusion polymorphism,  be aware of dynamic/static types (dynamic is item, static NonPerishable)


        }

        if (cin.fail()) {//The inclusion polymorphism allows me to call this block only once regardless of persh/non-perishable
            cin.clear();
            cin.ignore(2000, '\n');

            //CRASH POINT***********
            cout << "Error: " << *refitem << endl;//Be aware of early/late binding, the write/dowrite must be child calls, not parent.
        }


    }

现在非常有趣的是,当删除cin.fail上的if()并强制输入错误时,它可以工作.代码现在看起来像这样:

    void PosApp::addItem(bool isPerishable) {
        Item *refitem;


        if (!isPerishable) {
            NonPerishable myitem;
            std::cout   << "Enter the following: "  << std::endl
                        << "Sku: "                  << std::endl
                        << "Name:"                  << std::endl
                        << "Price: "                << std::endl
                        << "Taxed: "                << std::endl
                        << "Quantity: "             << std::endl;

            std::cin    >> myitem;
            refitem = &myitem; //Item now implements inclusion polymorphism,  be aware of dynamic/static types (dynamic is item, static NonPerishable)


            cin.clear();
            cin.ignore(2000, '\n');

            //THIS DOES NOT CRASH NOW
            cout << "Error: " << *refitem << endl;//Be aware of early/late binding, the write/dowrite must be child calls, not parent.



    }

就崩溃而言,我能想出的最佳答案是,在第一个代码片段中解析范围时,程序丢失了指针的内容.

这个问题有两个问题:你可以在条件语境中实现包含多态性(如图所示),如果不是,这是导致程序崩溃的原因吗？

注意:我没有包含整个程序(因为它是数百行),但足以说,当我将代码更改为第二个代码段时,行为应该是预期的.

具有自动存储的对象是{ }它们周围的括号的本地对象,包括if语句.如果您有一个指向本地的指针,并且该对象超出范围,则访问该指针的是UB.

Object* ptr;
if (condition)
{
    Object obj;
    ptr = &obj;
} //obj is out of scope
*ptr; //undefined behaviour

这是您在设置refitem指向本地对象时所做的事情.相反,创建一个Perishable*或NonPerishable*通过使用new,当块结束时,将指针指定给refitem.多态性将按预期工作,错误只是对象的范围.

if (!isPerishable)
{
    NonPerishable* myitem = new NonPerishable(); //dynamic memory
    std::cin >> *myitem;
    refitem = myitem; //refitem is still valid after this scope ends
}