CODEWITHSUNDEEP
c++pointerscrash
Jackie
Jackie

Reputation: 3

Using Pointers to Return a Value - Program Crashes

I am new to C++ and experimenting with using pointers to return a values. I have written up a simple program, but I cannot understand why it crashes.

main.cpp

int main() {
    bool *pMyBool;

    bar myBar;
    myBar.foo(pMyBool);

    cout << *pMyBool << endl;

    return 0;
}

bar.cpp

int bar::foo(bool *pMyBool) {
    bool myBool = true;
    *pMyBool = myBool;
    return 0;
}

output

1
//then it crashes

Upvotes: 0

Views: 311

Answers (4)

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

In C++, you don't want to use pointers in your case, but to use references:

int bar::foo(bool &MyBool) {
   MyBool = true;
   return 0;
}

Of course, you'll just call from your main 

   bool MyMainBool = false;
   bar.foo(MyMainBool);

Upvotes: 3

Lie Ryan
Lie Ryan

Reputation: 64875

it's because myBool is allocated in the "stack" (local variables); when the function foo exits, the stack associated with foo is released therefore the pointer pMyBool is pointing to a memory address that is no longer valid.

Upvotes: -1

Vinayak Garg
Vinayak Garg

Reputation: 6616

pMyBool is not pointing to any valid location. SO you cannot dereference it.
You should first allocate memory, and make the pointer, point to it. bool *pMyBool = new bool;
and later deallocate memory. delete pMyBool;

Upvotes: 2

Jonathan Leffler
Jonathan Leffler

Reputation: 754280

You don't initialize pMyBool in main(), so it points at random memory, and then the program crashes because the random memory isn't valid memory.

One possible fix:

int main() {
    bool  value;
    bool *pMyBool = &value;

    bar myBar;
    myBar.foo(pMyBool);

    cout << *pMyBool << endl;

    return 0;
}

With pointers, you always need to ensure you initialize them to point somewhere before you try using them. Always!

Upvotes: 4

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