local objects

Here in this code the function "foo" adds local objects to a global list.
Then the function "bar" tries to access the list. In this simple example it worked.
(I was expecting some garbage values or other errors like invalid memory access).
So to ensure that "foo"'s stack values are destroyed, I made a large array of doubles
(which I later changed to smaller and smaller, and ultimately to 1)
in "bar". And now I get invalid values from the list and mem errors too.
I was expecting this to happen. Because the objects were created on stack so they were
gone!
Now is there a better way to do the same i.e. :
the "foo" function will create new objects
and the "bar" function will access them.

There is only way on my mind right now i.e. to create objects using "new".
Help Please!

Code:

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#include <iostream>
#include <string>
#include <list>
#include <cmath>

using namespace std;

typedef list<string> mylist;

mylist msg_list;

void foo()
{
        string h("Hello");
        string w("World");

        msg_list.push_back(h);
        msg_list.push_back(w);
}

void bar()
{
        double vals[1024];
        for (int i=0; i<sizeof(vals)/sizeof(double); ++i)
        {
                vals[i] = i * sin(3.1415/180.0 * i);
        }

        mylist::const_iterator it = msg_list.begin();
        while (it != msg_list.end())
        {
                cout << "Msg: " << *it << endl;
                ++it;
        }
}

int main()
{
        foo();
        bar();

        return 0;
}

---

sizeof() tells you the number of bytes, not the number of subscripts.

Salem, sorry for that stupid error!
Now the program works just fine :shock:
I am expecting it to show invalid data or make invalid memory errors.
Why is it working just fine? (Contrary to my belief!)

Your variables are static globals so all functions have access to them.

Quote:

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Originally Posted by siavoshkc

Your variables are static globals so all functions have access to them.

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I think it is a special case for string objects only.
What if the objects are of other types?
I am trying to do it with my own objects, and I will post them here.

> Here in this code the function "foo" adds local objects to a global list.
Except it doesn't, it makes a copy.

> msg_list.push_back(h);
This creates a copy of h, and stores that.

Try it using a small class of your own so you can trace the instances of constructor, destructor, copy and assignment.

Yeah Salem I am doing that!
But I think (I've seen it in MSVC while writing this example) that the function "push_back"
takes a reference. So no new copy should have been made.

The function call itself might take a reference, but that doesn't take into account what happens inside push_back().

Well I tried whatever I could think of but the program runs fine.
So why worry :)

Yes, standard containers make a copy of what you put in. Push-back takes a reference to be able to do it with only one copying of the object instead of two (one for the container, one when passing a copy to the function in the first place).

Quote:

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Originally Posted by msp

Yeah Salem I am doing that!
But I think (I've seen it in MSVC while writing this example) that the function "push_back"
takes a reference. So no new copy should have been made.

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That doesn't follow at all. In fact, a new copy MUST be made because a reference is not reassignable. You can't for instance, have "a vector of references." Ain't no such thing.