Segfault if change the pointer value from a derived class

Question

I, unfortunately, can't describe my problem properly, but:

When I am trying to change a pointer value bar defined in base class 'A' from a child class 'B' with method foo() it gives me a segfault, because the pointer bar is null:

#include <iostream>
class A {
    protected:
        virtual void foo() {};
        int* bar;
    public:
        void eval(){
            foo();
            std::cout << *bar;
        }

};

class B : public A {
    void foo() override {
        *bar = 10; //segfault
    }
};

int main(){
    B* foobar = new B();
    foobar->eval();
}

But, if the bar is not a pointer, but a normal variable it works fine.

`bar` is not initialized to point to a valid (properly allocated) block of memory (of at least `sizeof(int)` bytes). — goodvibration, Oct 25 '19 at 13:18
The question says "because the pointer bar is null". In fact, the pointer is not null but indeterminate because it is not initialized. — MikeCAT, Oct 25 '19 at 13:24
@MikeCAT The pointer is null because the class B is value initialized. — Vlad from Moscow, Oct 25 '19 at 13:25
@VladfromMoscow [Here is an example](https://wandbox.org/permlink/nepdzdZsCZMxjA2Q) in which `bar` happened not to be null. — MikeCAT, Oct 25 '19 at 13:28

Frodyne · Accepted Answer · 2019-10-25T13:43:51.270

5

Your bar is not pointing to anything, so as it does not point at an int you cannot assign a value to its int (*bar = 10 = segfault). Add a constructor to A that fixes that:

class A {
protected:
    A() {                       // <- Constructor
        bar = new int(0);
    }
    A(const A& other)           // <- Copy sonstructor
    {
        bar = new int(*other.bar);
    }
    A& operator=(A other)       // Assignment operator
    {
        std::swap(bar, other.bar);
        return *this;
    }
    virtual ~A() {              // <- Destructor
        delete bar;
    }

    virtual void foo() {}
    int* bar = nullptr;
public:
    void eval() {
        foo();
        std::cout << *bar;
    }
};

Then the rest of your code works, and eval() prints 10 as intended. Note, adding a destructor to clean up bar later is needed to prevent memory leaks.

Edit:

As very correctly pointed out by MikeCAT, I was much too hasty and violated the Rule of three in my answer. If you do dynamic allocation you will want both a destructor, a copy constructor, and an assignment operator. I have added those to the code above.

edited Oct 25 '19 at 13:43

answered Oct 25 '19 at 13:25

Frodyne

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1

This code violates [The Rule of Three](https://stackoverflow.com/questions/4172722/what-is-the-rule-of-three), so it has some risk to cause runtime error when the object is copied. – MikeCAT Oct 25 '19 at 13:27
@MikeCAT True, thanks for catching my hasty mistake - it has apparently been too long since I new'ed anything. :) – Frodyne Oct 25 '19 at 13:45
`A other` is created when the assignment operator is called. Better use `const A& other`. – goodvibration Oct 25 '19 at 14:21
@goodvibration Sorry for the late reply, but here the copy is very intentional to use the copy-and-swap ideom: https://stackoverflow.com/questions/3279543/what-is-the-copy-and-swap-idiom – Frodyne Oct 26 '19 at 15:11

Segfault if change the pointer value from a derived class

1 Answers1