Vectors of variables vs vector of pointers

Question

I was just curious about the differences of using vectors of variables vs a vector of pointers with dynamic memory, and I've found something that confused me. I have a simple main.cpp that looks like this,

#include <iostream>
#include <vector>

using namespace std;

class A
{
public:
    A() { x = 2;}
    virtual ~A() { cout << "I'm a dead A\n";}

public:
    int x;
};

class B : public A
{
public:
    B() {x = 4;}
    ~B() { cout << "I'm a dead B\n";}
};

class C : public A
{
public:
    C() { x = 6;}
    ~C() { cout << "I'm a dead C\n";}
};

int main()
{
    cout << "Starting variable list\n";
    std::vector<A> list;

    list.push_back( B() );
    list.push_back( A() );
    list.push_back( B() );
    list.push_back( C() );
    list.push_back( A() );


    for(std::vector<A>::iterator it = list.begin(); it != list.end(); it++)
    {
        cout << it->x << endl;
    }

    cout << "\n\nStarting pointer list\n";

    std::vector<A *> ptrList;

    ptrList.push_back( new B());
    ptrList.push_back( new A());
    ptrList.push_back( new B());
    ptrList.push_back( new C());
    ptrList.push_back( new A());

    for(std::vector<A *>::iterator it = ptrList.begin(); it != ptrList.end(); it++)
    {
        cout << (*it)->x << endl;
    }

    for(std::vector<A *>::iterator it = ptrList.begin(); it != ptrList.end(); it++)
    {
        delete *it;
    }

    system("PAUSE");
    return 0;
}

And I get a printout that looks like this:

Starting variable list
I'm a dead B
I'm a dead A
I'm a dead A
I'm a dead A
I'm a dead A
I'm a dead A
I'm a dead B
I'm a dead A
I'm a dead A
I'm a dead A
I'm a dead A
I'm a dead C
I'm a dead A
I'm a dead A
I'm a dead A
I'm a dead A
I'm a dead A
I'm a dead A
4
2
4
6
2


Starting pointer list
4
2
4
6
2
I'm a dead B
I'm a dead A
I'm a dead A
I'm a dead B
I'm a dead A
I'm a dead C
I'm a dead A
I'm a dead A
Press any key to continue . . .

What and why did all those destructions occurr in the normal variable list?

Answer 1

Before concentrating on the dynamics of contructing / destructing / copy (and eventual optimization), there is a consideration you seem not aware of: values are not polymorphic.

If B derives from A,

B b;
A a(b);

will not make a a copy of b. It will just copy in a the b's A subcomponent.

Unlike values, pointer and references are polymorphic:

B b;
B* pb = &b;
A* pa = pb;
B* pb2 = const_cast<B*>(pa);

will in fact result in pa pointing into b's A subcomponent, but pb and pb2 to point to the same b.

That said, a vector<A> contains A values, hence,

vecotr<A> v;
v.push_back(B());

will result in:

• Create an empty v;
• Create a temporary B();
• Make v large enough to contain an A
• Create at v.end() an A copied from the temporary B's A subcomponent.
• Destroy the temporary B

And - at the end of the function,

• destroy v (and thus destroy the A inside it)

The memory is now clean.

If using pointers:

vector<A*> v;
v.push_back(new B());

will result in:

• Create an empty v
• Create a B on the heap
• Enlarge v to contain an A*
• Convert the B's address into its A's subcomponent address (for single inheritance they will most likely be the same)
• Create at v.end() a A* copied from the B's converted address (note you are converting pointers, not objects).
• Destroy v
• Destroy the A* in it.
• The B on the heap is still there (memory leak, since there is no other way to access it to delete it)

To avoid the leak you should either:

• Create B on the stack, and get its address or...
• Use a std::unique_ptr<A> instead of A* in vector (so that, at vector destruction, the unique_ptr is destroyed and its destructor destroy the pointed A sub-object, that, having a virtual destructor will result in B's destruction.

A more effective demonstration on the above issue can be given by the following code:

// Compile as g++ -pedantic -Wall -std=c++11

#include <vector>
#include <list>
#include <iostream>

class A
{
public:
    A() { std::cout << "- creating A at " << this << std::endl; }
    A(const A& a) { std::cout << "- creating A at " << this << " from " << &a << std::endl; }
    A& operator=(const A& a) { std::cout << "- assigning A at " << this << " from " << &a << std::endl; return *this; }
    virtual ~A() { std::cout << "- destroying A at " << this << std::endl; }
    virtual void hello() const { std::cout << "- A's hello from " << this << std::endl; }
};

class B: public A
{
public:
    B() { std::cout << "- creating B at " << this << std::endl; }
    B(const B& a) { std::cout << "- creating B at " << this << " from " << &a << std::endl; }
    B& operator=(const B& a) { std::cout << "- assigning B at " << this << " from " << &a << std::endl; return *this; }
    virtual ~B() { std::cout << "- destroying B at " << this << std::endl; }
    virtual void hello() const { std::cout << "- B's hello from " << this << std::endl; }
};

class C: public A
{
public:
    C() { std::cout << "- creating C at " << this << std::endl; }
    C(const C& a) { std::cout << "- creating C at " << this << " from " << &a << std::endl; }
    C& operator=(const C& a) { std::cout << "- assigning C at " << this << " from " << &a << std::endl; return *this; }
    virtual ~C() { std::cout << "- destroying C at " << this << std::endl; }
    virtual void hello() const { std::cout << "- C's hello from " << this << std::endl; }
};

int main()
{
    std::cout << "creating some objects" << std::endl;
    A a1, a2;
    B b1, b2;
    C c1, c2;

    {
        std::cout << "operating with values" << std::endl;
        std::vector<A> valvect;
        valvect.push_back(a1);
        valvect.push_back(a1);
        valvect.push_back(b1);
        valvect.push_back(b1);
        valvect.push_back(c1);
        valvect.push_back(c1);
        valvect.push_back(a2);
        valvect.push_back(a2);
        valvect.push_back(b2);
        valvect.push_back(b2);
        valvect.push_back(c2);
        valvect.push_back(c2);
        for(const auto& x: valvect) x.hello();
        std::cout << "at '}' destroy the value vector" << std::endl;
    }


    {
        std::cout << "operating with pointers" << std::endl;
        std::vector<A*> ptrvect;
        ptrvect.push_back(&a1);
        ptrvect.push_back(&a1);
        ptrvect.push_back(&b1);
        ptrvect.push_back(&b1);
        ptrvect.push_back(&c1);
        ptrvect.push_back(&c1);
        ptrvect.push_back(&a2);
        ptrvect.push_back(&a2);
        ptrvect.push_back(&b2);
        ptrvect.push_back(&b2);
        ptrvect.push_back(&c2);
        ptrvect.push_back(&c2);
        for(const auto& x: ptrvect)
            x->hello();
        std::cout << "at '}' destroy the pointer's vector" << std::endl;
    }

    {
        std::cout << "operating with list of values" << std::endl;
        std::list<A> vallst;
        vallst.push_back(a1);
        vallst.push_back(a1);
        vallst.push_back(b1);
        vallst.push_back(b1);
        vallst.push_back(c1);
        vallst.push_back(c1);
        vallst.push_back(a2);
        vallst.push_back(a2);
        vallst.push_back(b2);
        vallst.push_back(b2);
        vallst.push_back(c2);
        vallst.push_back(c2);
        for(const auto& x: vallst)
            x.hello();
        std::cout << "at '}' destroy the value list" << std::endl;
    }


    {
        std::cout << "operating with list of pointers" << std::endl;
        std::list<A*> ptrlst;
        ptrlst.push_back(&a1);
        ptrlst.push_back(&a1);
        ptrlst.push_back(&b1);
        ptrlst.push_back(&b1);
        ptrlst.push_back(&c1);
        ptrlst.push_back(&c1);
        ptrlst.push_back(&a2);
        ptrlst.push_back(&a2);
        ptrlst.push_back(&b2);
        ptrlst.push_back(&b2);
        ptrlst.push_back(&c2);
        ptrlst.push_back(&c2);
        for(const auto& x: ptrlst)
            x->hello();
        std::cout << "at '}' destroy the pointer's list" << std::endl;
    }



    std::cout << "now finally at '};' destroy the objects created at the beginning" << std::endl;
    return 0;
}

It will output like

creating some objects
- creating A at 0x22febc
- creating A at 0x22feb8
- creating A at 0x22feb4
- creating B at 0x22feb4
- creating A at 0x22feb0
- creating B at 0x22feb0
- creating A at 0x22feac
- creating C at 0x22feac
- creating A at 0x22fea8
- creating C at 0x22fea8
operating with values
- creating A at 0x3e3eb8 from 0x22febc
- creating A at 0x3e2434 from 0x22febc
- creating A at 0x3e2430 from 0x3e3eb8
- destroying A at 0x3e3eb8
- creating A at 0x3e2448 from 0x22feb4
- creating A at 0x3e2440 from 0x3e2430
- creating A at 0x3e2444 from 0x3e2434
- destroying A at 0x3e2430
- destroying A at 0x3e2434
- creating A at 0x3e244c from 0x22feb4
- creating A at 0x3e2468 from 0x22feac
- creating A at 0x3e2458 from 0x3e2440
- creating A at 0x3e245c from 0x3e2444
- creating A at 0x3e2460 from 0x3e2448
- creating A at 0x3e2464 from 0x3e244c
- destroying A at 0x3e2440
- destroying A at 0x3e2444
- destroying A at 0x3e2448
- destroying A at 0x3e244c
- creating A at 0x3e246c from 0x22feac
- creating A at 0x3e2470 from 0x22feb8
- creating A at 0x3e2474 from 0x22feb8
- creating A at 0x3e24a0 from 0x22feb0
- creating A at 0x3e2480 from 0x3e2458
- creating A at 0x3e2484 from 0x3e245c
- creating A at 0x3e2488 from 0x3e2460
- creating A at 0x3e248c from 0x3e2464
- creating A at 0x3e2490 from 0x3e2468
- creating A at 0x3e2494 from 0x3e246c
- creating A at 0x3e2498 from 0x3e2470
- creating A at 0x3e249c from 0x3e2474
- destroying A at 0x3e2458
- destroying A at 0x3e245c
- destroying A at 0x3e2460
- destroying A at 0x3e2464
- destroying A at 0x3e2468
- destroying A at 0x3e246c
- destroying A at 0x3e2470
- destroying A at 0x3e2474
- creating A at 0x3e24a4 from 0x22feb0
- creating A at 0x3e24a8 from 0x22fea8
- creating A at 0x3e24ac from 0x22fea8
- A's hello from 0x3e2480
- A's hello from 0x3e2484
- A's hello from 0x3e2488
- A's hello from 0x3e248c
- A's hello from 0x3e2490
- A's hello from 0x3e2494
- A's hello from 0x3e2498
- A's hello from 0x3e249c
- A's hello from 0x3e24a0
- A's hello from 0x3e24a4
- A's hello from 0x3e24a8
- A's hello from 0x3e24ac
at '}' destroy the value vector
- destroying A at 0x3e2480
- destroying A at 0x3e2484
- destroying A at 0x3e2488
- destroying A at 0x3e248c
- destroying A at 0x3e2490
- destroying A at 0x3e2494
- destroying A at 0x3e2498
- destroying A at 0x3e249c
- destroying A at 0x3e24a0
- destroying A at 0x3e24a4
- destroying A at 0x3e24a8
- destroying A at 0x3e24ac
operating with pointers
- A's hello from 0x22febc
- A's hello from 0x22febc
- B's hello from 0x22feb4
- B's hello from 0x22feb4
- C's hello from 0x22feac
- C's hello from 0x22feac
- A's hello from 0x22feb8
- A's hello from 0x22feb8
- B's hello from 0x22feb0
- B's hello from 0x22feb0
- C's hello from 0x22fea8
- C's hello from 0x22fea8
at '}' destroy the pointer's vector
operating with list of values
- creating A at 0x3e2448 from 0x22febc
- creating A at 0x3e24d0 from 0x22febc
- creating A at 0x3e24e8 from 0x22feb4
- creating A at 0x3e2500 from 0x22feb4
- creating A at 0x3e2518 from 0x22feac
- creating A at 0x3e2530 from 0x22feac
- creating A at 0x3e2548 from 0x22feb8
- creating A at 0x3e2560 from 0x22feb8
- creating A at 0x3e2578 from 0x22feb0
- creating A at 0x3e2590 from 0x22feb0
- creating A at 0x3e25a8 from 0x22fea8
- creating A at 0x3e25c0 from 0x22fea8
- A's hello from 0x3e2448
- A's hello from 0x3e24d0
- A's hello from 0x3e24e8
- A's hello from 0x3e2500
- A's hello from 0x3e2518
- A's hello from 0x3e2530
- A's hello from 0x3e2548
- A's hello from 0x3e2560
- A's hello from 0x3e2578
- A's hello from 0x3e2590
- A's hello from 0x3e25a8
- A's hello from 0x3e25c0
at '}' destroy the value list
- destroying A at 0x3e2448
- destroying A at 0x3e24d0
- destroying A at 0x3e24e8
- destroying A at 0x3e2500
- destroying A at 0x3e2518
- destroying A at 0x3e2530
- destroying A at 0x3e2548
- destroying A at 0x3e2560
- destroying A at 0x3e2578
- destroying A at 0x3e2590
- destroying A at 0x3e25a8
- destroying A at 0x3e25c0
operating with list of pointers
- A's hello from 0x22febc
- A's hello from 0x22febc
- B's hello from 0x22feb4
- B's hello from 0x22feb4
- C's hello from 0x22feac
- C's hello from 0x22feac
- A's hello from 0x22feb8
- A's hello from 0x22feb8
- B's hello from 0x22feb0
- B's hello from 0x22feb0
- C's hello from 0x22fea8
- C's hello from 0x22fea8
at '}' destroy the pointer's list
now finally at '};' destroy the objects created at the beginning
- destroying C at 0x22fea8
- destroying A at 0x22fea8
- destroying C at 0x22feac
- destroying A at 0x22feac
- destroying B at 0x22feb0
- destroying A at 0x22feb0
- destroying B at 0x22feb4
- destroying A at 0x22feb4
- destroying A at 0x22feb8
- destroying A at 0x22febc

Answer 2

All those destructions occurred in the normal variable list because

    list.push_back( B() );

will allocate a new object inside the vector and use the assignment operator to copy the one in argument (see Does std::vector use the assignment operator of its value type to push_back elements?). The one you used as argument is temporary, so will be destroyed after being created.

Furthermore Destructing an object of type C or B will output two lines. in the case of B it will be

I'm a dead B
I'm a dead A

When you pass pointers it make the copy of the value of the pointer, the object pointed to is not modified.

Personally I think the overhead of using a vector of values is negligible if the copy constructor and assignment operator are lightweight and declared inline.

Answer 3

A few things:

1. You are using a vector of A object, all of them, not pointers so, you can't delete vector items. The good way to allow this y declaring a vector of A pointers such std::vector<A *> and push_back them with v.push_back(new B()); or v.push_back(new C());
2. If you add to this pointers vector A subclases, like B or C, you must ensure to delete all 'information'. If you do not declare a virtual destructor in A, you only will delete subclass information, not base clases one. Remember this for all inheritance situations.

Besides, have in mind all recomendations from UmNyobe and Luchian.