Question

When I run the following C++ code from CodeBlocks on Windows using the mingw compiler, all is fine. But, when I run it on Mac OS X it doesn’t work:

void func (vector<int> &v1, vector<vector< int *> > &v2);

int main()
{
    vector<int> v1;
    v1.push_back(0);
    vector<vector <int *> > v2;
    vector<int *> vTemp;
    int x = 0;
    int * ptr = &x;
    vTemp.push_back(ptr);
    v2.push_back(vTemp);

    func(v1,v2);
    cout<<*(v2[0][1])<<endl;

    return 0;
}
void func (vector<int> &v1, vector<vector< int *> > &v2)
{
    v1.push_back(1);
    int *ptr = &(v1[1]);
    v2[0].push_back(ptr);
    cout<<*(v2[0][1])<<endl;
    v1.push_back(2);
    int *ptr2 = &(v1[2]);
    v2[0].push_back(ptr2);
    v1.push_back(3);
    int *ptr3 = &(v1[3]);
    v2[0].push_back(ptr3);
}

The output I expect (and get on Windows) is

1
1

But on the Mac, I get

1
0

Does anyone have any idea why this should be happening?

Answer 1

Your program is invalid, and you’re seeing undefined behavior. Your pointer value holds the address of a vector element which may be invalid after the vector resizes.

Running the program with GuardMalloc enabled catches this for you.