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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。

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，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：您能否在条件的上下文中实现包含多态性（如图所示），如果不能，这是否是导致我的程序崩溃的原因？

注意：我没有包含整个程序（因为它有数百行），但我只想说，当我将代码更改为第二个片段时，行为是应该预期的。

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1 回答 1

5

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

这就是您设置refitem指向本地对象所做的事情。相反，使用创建一个Perishable*或，当块结束时，将该指针分配给。多态性将按您的预期工作，错误只是对象的范围。NonPerishable*newrefitem

if (!isPerishable)
{
    NonPerishable* myitem = new NonPerishable(); //dynamic memory
    std::cin >> *myitem;
    refitem = myitem; //refitem is still valid after this scope ends
}
于 2015-12-05T04:37:14.070 回答