Question

I have the following problem. I create a class, and store the pointer to that class in an other class. Upon creating, everything is OK. However, a single step later it seems that the class has disappeared.

I’ve written a very simple test scenario here:

#include <iostream>

using namespace std;

class test
{
    public:
        test();
        bool ok;
};

test::test()
{
    ok = false;
}

class func
{
    public:
        func();
        void check();
        test *pTest;
};

func::func()
{
    test temptest = test();
    cout << temptest.ok << endl;
    pTest = &temptest;
    cout << pTest->ok << endl;
}

void func::check()
{
    cout << pTest->ok << endl;
};

int main( int argc, char *argv[] )
{
    func mFunc = func();
    // what happens here
    mFunc.check();
}

The above program outputs the following:

0
0
204

From 204 I’d guess that somehow the class I’ve created before has disappeared.

Can you tell me what is happening and why?

Answer 1

The problem is that you are creating a an object with a “limited” scope.

func::func()
{
    test temptest = test();             // temptest construction
    cout << temptest.ok << endl;
    pTest = &temptest;
    cout << pTest->ok << endl;
}                                       // temptest descrution 

After the construction of func, pTest now refer to an invalid object.

You have to use dynamic memory or shared pointers to manage pointers.

#include <iostream>

using namespace std;

class test
{
    public:
        test();
        bool ok;
};

test::test()
{
    ok = false;
}

class func
{
    public:
        func();
       ~func();
        void check();
        test *pTest;
};

func::func()
{
    pTest = new Test();
    cout << pTest->ok << endl;
    cout << pTest->ok << endl;
}
func::~func() { delete pTest; }

void func::check()
{
    cout << pTest->ok << endl;
};

int main( int argc, char *argv[] )
{
    func mFunc = func();
    // what happens here
    mFunc.check();
}

Now the constructor of test allocate a new object and store the address of that object, and the destructor can deallocate memory. Manage memory in this way is not a good prectice.

Use shared pointers instead like shared_ptr or unique_ptr, but this requires some other knowledge such move semantic.