elementary question on vector and pop_back

hello,

I have a little experience with C, and I am trying to learn C++ (brave new world for me!). I have what I am sure is a most basic question. However, I am baffled...

Say I have the following code

Code:

---

    std::vector<int> intArray;

    for ( i = 0; i<=10; i++ )
    {
        intArray.push_back(2*i);
        printf("%d %d %d\n",i, intArray.size(), intArray[i]);
    }
   
    printf("***********\n");

    for ( i = 0; i<=5; i++ )
    {
        intArray.pop_back();
        printf("%d %d\n",i, intArray.size());
    }

---

this outputs what I expected after reading the vector tutorial

Code:

---

0 1 0
1 2 2
2 3 4
3 4 6
4 5 8
5 6 10
6 7 12
7 8 14
8 9 16
9 10 18
10 11 20
***********
0 10
1 9
2 8
3 7
4 6
5 5

---

Now, the tutorial I am reading (codeguru) says "...pop_back() removes the last element in the controlled sequence. The removed element becomes invalid, and size() is decremented". I am having a problem seeing the "becomes invalid" part.

To be explicit, can anyone tell me why the next code does not seg fault then when I add it immediately after the pop_back loop?

Code:

---

    for ( i = 0; i<=10; i++ )
    {
        printf("%d %d %d\n",i, intArray.size(), intArray[i]);
    }

---

I get

Code:

---

0 5 0
1 5 2
2 5 4
3 5 6
4 5 8
5 5 10
6 5 12
7 5 14
8 5 16
9 5 18
10 5 20

---

Thank you very much.

mc61

because it is undefined what happens when you access elements after they are removed. It can seem to work on some compilers but can crash on others. MSVC2008 throws an exception with your code

Thanks,

Indeed, after I submitted my post I found the answer a few threads earlier (and I did search for it before posting...).

Sorry for the bandwidth, and thanks for the help.

mc61

Yes -- it's called "undefined behavior," not "crashing behavior." What could possibly happen is, officially, the set of all things. A crash is something :)