Question

I was wondering if there is any difference in performance when you compare/contrast

A) Allocating objects on the heap, putting pointers to those objects in a container, operating on the container elsewhere in the code

Ex:

std::list<SomeObject*> someList;

// Somewhere else in the code
SomeObject* foo = new SomeObject(param1, param2);
someList.push_back(foo);

// Somewhere else in the code
while (itr != someList.end())
{
    (*itr)->DoStuff();
    //...
}

B) Creating an object, putting it in a container, operating on that container elsewhere in the code

Ex:

std::list<SomeObject> someList;

// Somewhere else in the code
SomeObject newObject(param1, param2);
someList.push_back(newObject);

// Somewhere else in the code
while (itr != someList.end())
{
    itr->DoStuff();
    ...
}

Assuming the pointers are all deallocated correctly and everything works fine, my question is…

If there is a difference, what would yield better performance, and how great would the difference be?

Answer 1

There is a performance hit when inserting objects instead of pointers to objects.

std::list as well as other std containers make a copy of the parameter that you store (for std::map both key and value is copied).

As your someList is a std::list the following line copies your object:

Foo foo;
someList.push_back(foo);           // copy foo object

It will get copied again when you retrieve it from list. So you are making of copies of the whole object compared to making copies of pointer when using:

Foo * foo = new Foo();
someList.push_back(foo);             // copy of foo*

You can double check by inserting print statements into Foo’s constructor, destructor, copy constructor.

EDIT: As mentioned in comments, pop_front does not return anything. You usually get reference to front element with front then you pop_front to remove the element from list:

Foo * fooB = someList.front();    // copy of foo*
someList.pop_front();

OR

Foo fooB = someList.front();  // front() returns reference to element but if you
someList.pop_front();         // are going to pop it from list you need to keep a
                              // copy so Foo fooB = someList.front() makes a copy