条件语句对C++中的多态对象有什么作用?(内含物多态性)

Question

我遇到了一个有趣的错误，我确信这与条件语句上下文中的包含多态性有关。

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???

应该清楚的是，条件语句在最后一行使*parentPointer变为变量。

我的整个功能看起来是这样的：（注意它崩溃的地方）

    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（）并在输入中强制出现错误时，它就工作了。代码现在看起来是这样的：

    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，我能想到的最好的答案是，当范围在第一个代码片段中解析时，程序丢失了指针的内容。

这个问题有两个方面：你能在条件条件的上下文中实现包含多态性吗（如图所示）？如果不能，这是导致我的程序崩溃的原因吗？

注意：我没有包括整个程序（因为它有数百行），但可以说，当我将代码更改为第二个代码段时，行为是应该预期的。

Answer 1

具有自动存储的对象是它们周围的{ }大括号的本地对象，包括if语句。如果您有一个指向本地的指针，而对象超出了范围，那么访问该指针就是UB。

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

这就是将refitem设置为指向本地对象时要执行的操作。相反，使用new创建一个Perishable*或NonPerishable*，当块结束时，将该指针分配给refitem。多态性将如您所期望的那样工作，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
}