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Iterator Invalidation

CS253 Iterator Invalidation

Invalidation

Consider this poor code:

int main() {
    int *p = new int(42);
    cout << "Before: " << *p << '\n';
    delete p;
    cout << "After:  " << *p << '\n';
}

Before: 42
After:  9658

Nothing mysterious here. p was a valid pointer, then it became invalid.

Iterator invalidation

vector<int> bunch = {253};
vector<int>::iterator it = bunch.begin();

cout << "Before: " << *it << '\n';

for (int i=1; i<1000; i++)
    bunch.push_back(i);

cout << "After:  " << *it << '\n';

Before: 253
After:  6517974

Lipstick on a pig

Using auto makes the code prettier, but no better:

vector<int> bunch = {253};
auto it = bunch.begin();

cout << "Before: " << *it << '\n';

for (int i=1; i<1000; i++)
    bunch.push_back(i);

cout << "After: " << *it << '\n';

Before: 253
After: 29793927

Reservation

Using .reserve() pre-allocates memory:

vector<int> bunch = {253};
bunch.reserve(1005);
auto it = bunch.begin();

cout << "Before: " << *it << '\n';

for (int i=1; i<1000; i++)
    bunch.push_back(i);

cout << "After:  " << *it << '\n';

Before: 253
After:  253

How often?

How often does re-allocation happen? We can find out, for any particular implemention:

vector<int> bunch;

for (int i=1; i<1000; i++) {
    auto before = bunch.capacity();
    bunch.push_back(i);
    auto after = bunch.capacity();
    if (before != after)
        cout << i << ' ' << after << '\n';
}

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65 128
129 256
257 512
513 1024

Order Calculation

• All that reallocation looks bad.
• However, science is better than first impressions.
• What is the big-O time cost for adding n items to a vector?

Pre-allocation helps

Again, if we know how many items we’re going to add, we can .reserve() the space:

vector<int> bunch;
bunch.reserve(900);

for (int i=1; i<1000; i++) {
    auto before = bunch.capacity();
    bunch.push_back(i);
    auto after = bunch.capacity();
    if (before != after)
        cout << i << ' ' << after << '\n';
}

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