c++ - 条件语句对 C++ 中的多态对象有什么作用？ (包含多态性)

Question

我遇到了一个有趣的错误，我很确定它与条件语句上下文中的包含多态性有关。

例子的亮点如下:

   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 变量。

我的整个函数看起来像这样:(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() 并强制输入错误时，它起作用了。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，我能想到的最佳答案是，当范围在第一个代码片段中解析时，程序以某种方式丢失了指针的内容。

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

注意:我没有包含整个程序(因为它有数百行)，但足以说明，当我将代码更改为第二个代码段时，行为是预期的。

Answer 1

具有自动存储的对象在它们周围的 { } 大括号中是本地的，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，则访问该指针是 UB。

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

这就是您将 refitem 设置为指向本地对象所做的事情。相反，使用 new 创建一个 Perishable* 或 NonPerishable*，并在 block 结束时将该指针分配给 refitem。多态性将按您预期的那样工作，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
}