Question

I’m trying to understand how much memory is taken up by using a vector of objects versus vector of pointers. I wrote a small test code:

class Test {
public: 
   Test()
   { v.resize(2000); }

private:
    std::vector<int> v;
};

int main() 
{
   std::vector<Test> OCtr;
   std::vector<Test*> PCtr;
   Octr.resize(10);
   PCtr.resize(10, new Test);
   cout << OCtr.size()*sizeof(Test) << endl;
   cout << PCtr.size()*sizeof(Test*) << endl; 
}

The output is 120 and 40. I can understand 40 – as it is 10* 4 bytes, but surprised by 120. Is that the real size of the container – OCtr? It looks like sizeof(Test) is not the right way to get complete size of the object.

• Instead of providing a member function inside Test class to sum up memory of all Test data members, is there another to get size of any class object.
• I have read answers on when to use vector of objects versus vector of object pointers. But in a case where I’m creating a container of simple class (no polymorphic behavior), where memory consumption is primary criteria and I can handle deletion of object pointers, which container should I choose?

Answer 1

sizeof is a compile-time operator. That is, sizeof(X) will be replaced during compilation with a constant number. This means that the sizeof an empty vector<T> is always the same as the sizeof a vector<T> with thousands of elements inside of it. Remember, sizeof does not do anything during runtime, so the result cannot possibly reflect the current state of the vector.