`static_cast` caution

static_cast caution

It is likely to lead unexpected behavior and maybe dangerous to invoke static_cast on wrong C++ object. Below example demostrates it.

On the second invocation of foo, foo(d2), the instance of class D2 is casted into instance of class D1 and the memory address for access to member variable b of D1 is calculated as d2's address + offset of b. The resulted address is actually out of the available memory of instance d2 because d2 is instance of class D2 which is smaller than class D1(D1 has a big member variable arr). That causeses unexpected behavior: if the memory which the address points is already allocated to the current process, the wrong data(the memory is occupied by another objects or values) are read; otherwise a "memory access violation" exception occurs.

#include <iostream>
#include <array>

using namespace std;

class Base
{
public:
    virtual void say()
    {
        cout << "hello, base;" << endl;
    }
};

class D1 : public Base
{
public:
    char a = '*';
    std::array<char, 10000> arr{};
    char b = '+';

public:
    void say() override
    {
        cout << "hello, D1;" << a << endl;
    }

    void jump()
    {
        cout << "jump " << b << endl;
    }
};

class D2 : public Base
{
public:
    long long v = 7777;

public:
    void say() override
    {
        cout << "hello, D2;" << v << endl;
    }
};

void foo(Base & bb);

int main()
{
    D1 d1;
    D2 d2;
    foo(d1);
    foo(d2); // cause memory access violation!!!
}

void foo(Base& bb)
{
    D1& d = static_cast<D1&>(bb);
    d.jump();
}

In my experiment, the "memory access violation" exception happens.