Need help about 'new' operator in C++

Question

I wanna understand about the new operator in C++ and what's going on my code. I have 2 files A and B,both can be compiled.But I'm facing the error "Segmentation fault" when running binary of A.Binary of B is working fine.

What does the "new" operator do in file B ?

File A

#include <iostream>
using namespace std;

class Animal
{
    public:
    virtual void eat() { std::cout << "I'm eating generic food."<< endl; }
};

class Cat : public Animal
{
    public:
    void eat() { std::cout << "I'm eating a rat." << endl;}
};

void func(Animal *xyz){ 
    xyz->eat();
}

int main()
{
    Animal *animal ;
    Cat *cat ;
    func(animal);
    func(cat);
    return 0;
}

File B

#include <iostream>
using namespace std;

class Animal
{
    public:
    virtual void eat() { std::cout << "I'm eating generic food."<< endl; }
};

class Cat : public Animal
{
    public:
    void eat() { std::cout << "I'm eating a rat." << endl;}
};

void func(Animal *xyz){ 
    xyz->eat();
}

int main()
{
    Animal *animal = new Animal;
    Cat *cat = new Cat;
    func(animal);
    func(cat);
    return 0;
}

The differences between file A and B are

Animal *animal = new Animal;
Cat *cat = new Cat;

When you define local variables in C++, like e.g. `animal` or `cat` in your examples, they are not automatically initialized but will have an *indeterminate* value. Using them in any way without initialization leads to *undefined behavior*. In the case of a pointer, it means it will point to a seemingly random location. — Some programmer dude, Jun 13 '17 at 05:42
You are missing the basic very concept of what pointers are and do. — eddie, Jun 13 '17 at 06:07
I do not want to explain. But because so many experts are here. I just started learning C ++ a few days ago. I will Google more before asking next time — Nam Pham, Jun 13 '17 at 06:12
As you continue studying C++ many other questions and doubts will arise ;) — eddie, Jun 13 '17 at 06:15

Mureinik · Answer 1 · 2017-06-13T05:45:12.467

1

The new operator allocates memory for the object and then invokes the constructor to initialize it. In file A you do not initialize your objects, thus passing uninitialized pointers to func, leading to the segmentation fault.

edited Jun 13 '17 at 05:45

answered Jun 13 '17 at 05:41

Mureinik

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5

He don't pass nullpointer. The pointer are not initialized, so the value is random. He should use `Animal animal` and pass the address `&animal`, to use the `func()` without the new operator. – Andre Kampling Jun 13 '17 at 05:42
1

No, they're not null pointer. And the behavior is undefined. – songyuanyao Jun 13 '17 at 05:43

Need help about 'new' operator in C++

1 Answers1