Sunday, July 31, 2011

vector insertion -- implicit copy-ctor frequently

When you insert your object into a vector, the original object (B) is passed by value. Copy-ctor called exactly once for each insert.

    vector<B> v(3); // default ctor -- 3 calls.
// update -- now I see one default ctor call, and 3 copy ctor calls.


    cout<<v.capacity()<<endl; // capacity = 3. Next insert would reallocate to 6

    v.push_back(B()); // temp B instance created, copied in, then destroyed. The reallocation calls copy ctor 3 times on the existing elements. 3 original objects destroyed

    cout<<v.capacity()<<endl; // Capacity is now 6

    v.push_back(B()); // temp B instance created, copied in, then destroyed. No reallocation

    cout<<v.capacity()<<endl; // Capacity remains 6

Here are some useful instrumentation code --

struct B {
    static int count;
    int id;
    B() {
        ++count;
        cout << count << " B()\n";
        id = count;
    }
    B(B const & rhs) :
        id(rhs.id + 10) {
        cout <<id << " copied\n";
    }
    ~B(){
        cout<<id<<" ~B\n";
    }
};
int B::count = 0;
ostream & operator<<(ostream & os, B const & b){
    os<<b.id<<endl;
}

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