c++ - new with inheritance 的不当使用？

Question

我正在尝试找出崩溃的原因，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 Derived 实际上并没有被分配正确的大小。将 ppBases 更改为三重指针(像这样)使一切正常:

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;
        }
    }
}

虽然我不明白为什么。

谢谢

Answer 1

问题出在这一行:

 ppBases[x] = new Derived[yLen];

您正在将一组 Derived 对象分配给 Base * 指针。因此，当您编写 ppBases[x][y] 时，编译器会使用指针的类型，即 Base *，并进行适当的索引。所以 ppBases[x][1] 不引用分配数组中的第二个 Derived 对象。