Question

我正在试图弄清楚崩溃，并将其归结为以下示例：

class Base
{
public:
    virtual unsigned int    GetNum  () = 0;
};

class Derived : public Base
{
    int data;   // <--- commmenting this out removes the crash

public:
    Derived() {}
    virtual unsigned int    GetNum() { return 1; }
};

void func() {

    Base** ppBases;

    unsigned int xLen = 3;
    unsigned int yLen = 4;

    ppBases = new Base*[xLen];
    for(unsigned int x = 0; x < xLen; ++x) {
        ppBases[x] = new Derived[yLen];
        for(unsigned int y = 0; y < yLen; ++y) {
            Base* curBase = dynamic_cast<Base*>(&(ppBases[x][y]));
            Derived* curDerived = dynamic_cast<Derived*>(&(ppBases[x][y])); // <--- crashes with a null dereference
            unsigned int result = curBase->GetNum();
            result = result;
        }
    }
}

我猜测Derived实际上并没有分配适当的大小。将ppBases更改为tripple指针（如此）会使一切正常工作：

void func() {

    Base*** ppBases;

    unsigned int xLen = 3;
    unsigned int yLen = 4;

    ppBases = new Base**[xLen];
    for(unsigned int x = 0; x < xLen; ++x) {
        ppBases[x] = new Base*[yLen];
        for(unsigned int y = 0; y < yLen; ++y) {
            ppBases[x][y] = new Derived();
            Base* curBase = dynamic_cast<Base*>(ppBases[x][y]);
            Derived* curDerived = dynamic_cast<Derived*>(ppBases[x][y]);
            unsigned int result = curBase->GetNum();
            result = result;
        }
    }
}

我不明白为什么。

由于

Answer 1

问题在于这一行：

 ppBases[x] = new Derived[yLen];

您将Derived个对象数组分配给Base *指针。因此，当您编写ppBases[x][y]时，编译器将使用指针的类型（Base *）并进行适当的索引。因此ppBases[x][1]不会引用已分配数组中的第二个Derived对象。

使用继承不正确的新用法？

1 个答案: