Question

This seems like a fairly strange bug to me. My C++ program is segfaulting, and I found something strange using GDB. I have the following constructor and copy constructor:

Bank::Bank(char mode, int floor_dimensions_, int num_floors_) : floor_dimensions(floor_dimensions_), num_floors(num_floors_) {
    for (int i = 0; i < num_floors; i++) {
        floors[i] = new Floor(floor_dimensions);
    }

    if (mode == 'M') {
        read_map(floors);
    } else if (mode == 'C') {
        read_coords(floors);
    }
}

Bank::Bank(const Bank& b) {
    floor_dimensions = b.floor_dimensions;
    num_floors = b.num_floors;

    cout << floor_dimensions << endl;
    cout << num_floors << endl;

    for (int i = 0; i < num_floors; i++) {
        floors[i] = new Floor(*b.floors[i]);
        floors[i]->print_map();
}

The definition of this class is something like

class Bank {
    /** The number of floors the bank has */
    int num_floors;

    /** The dimension of each of the floors */
    int floor_dimensions;

    /** The floors in the bank */
    Floor* floors[];

private:
    Bank(char mode, int floor_dimensions_, int num_floors_);
    Bank(const Bank& bank);
    ~Bank();
    void read_map(Floor** floor);
void read_coords(Floor** floor);
}

Now I set a breakpoint at the first line of the constructor using GDB. After executing the next two lines, the value of b.floors[1] has changed, which leads to the segfault when I call b.floors[i]->foo().

    Breakpoint 1, Bank::Bank (this=0x7fffffffe050, b=...) at bank.cpp:29
29      floor_dimensions = b.floor_dimensions;
(gdb) p b.floors[0]
$4 = (Floor *) 0x610070
(gdb) p b.floors[1]
$5 = (Floor *) 0x6103f0
(gdb) p b.floors[2]
$6 = (Floor *) 0x610770
(gdb) n
30      num_floors = b.num_floors;
(gdb) p b.floors[0]
$7 = (Floor *) 0x610070
(gdb) p b.floors[1]
$8 = (Floor *) 0x8006103f0
(gdb) p b.floors[2]
$9 = (Floor *) 0x610770
(gdb) n
32      cout << floor_dimensions << endl;
(gdb) p b.floors[0]
$10 = (Floor *) 0x610070
(gdb) p b.floors[1]
$11 = (Floor *) 0x800000003
(gdb) p b.floors[2]
$12 = (Floor *) 0x610770

Does anyone have any idea what’s going on?

Answer 1

The problem with your code is most likely the fact that you are attempting to access an uninitialized pointer (b.floors). Assuming b was a result of this copy constructor, then b.floors will be pointing to random memory. b.floors[i] will then be pointing to random memory some bytes ahead of what b.floors is pointing to, and eventually you use -> to dereference this random address, which causes a segmentation fault.

The solution to your problem is to properly allocate and copy the floors array:

floors = new Floor* [num_floors];
for (int i=0; i<num_floors; ++i)
  floors[i] = b.floors[i];

You may also want to copy the pointed object Floor, depending on the details of your implementation.