Correct use of std::map as class member

Question

In the past I always created a map like this:

class TestClass
{
    private:
        std::map<int,int> *mapA;
};

TestClass::TestClass
{
    mapA = new std::map<int,int>();
}

TestClass::~TestClass
{
    mapA->clear(); // not necessary
    delete mapA;
}

So, now I read all over the place at Stackoverflow: avoid pointers as often as possible

Currently I want to create the map without pointer and new (no need to delete the object by myself and less danger of getting some memory leak)!

class TestClass
{
    public:
        TestClass() : mapA() // this is also needed?
        {};
    private:
        std::map<int,int> mapA;
};

Any further steps for correct creation of the map necessary?

Thanks for any help and/or clarification!

I just noticed you edited in a mapA->clear() to your original destructor. This is not necessary - the map will clear itself during deconstruction. — zennehoy
– zennehoy, Commented Sep 27, 2013 at 8:20

zennehoy · Accepted Answer · 2013-09-27 08:16:54Z

9

Nope that's it, and you don't need to explicitly initialize it in the constructor.

answered Sep 27, 2013 at 8:16

zennehoy

7,00631 silver badges59 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

john Over a year ago

I'm always surpised that although the pointerless code is much simpler than the pointer code, newbies always seem to go for the pointer version.

Gorpik Over a year ago

@john Maybe this is not so; it is only that those who go for the pointer get in trouble and we notice them.

Andre Kostur Over a year ago

@john Frequently people come over from Java, and since you have to new everything over in Java, they do the same in C++.

Sz. Over a year ago

Andre, john: and from C, or course, the original home hell of pointers. @Gorpik: :-))

Autechre Over a year ago

Why don't we need to initialize the map in the member initializer list ?

|

ChronoTrigger · Accepted Answer · 2013-09-27 08:27:39Z

As zennehoy says, it is not necessary to initialize the map in the TestClass constructor. Let me note a difference between the two implementations:

In the first one, the TestClass, as it is currently written, is not copyable without undesirable effects because the raw pointer to the dynamically allocated map is copied:

TestClass *A = new TestClass;      // A has a map
TestClass *B = new TestClass(A);   // B shares the map with A!

delete A; // this deletes A's map (in destructor)
delete B; // this deletes A's map again! wrong

In your second implementation, that does not happen because the map, and not just its address, is copied completely.

To solve that issue in your first implementation, you should use a shared pointer, or do the work yourself by implementing the operator= and the copy constructor. Or, if you want to really share the map between copied instances, you should implement a reference counting mechanism.

Collectives™ on Stack Overflow

Correct use of std::map as class member

2 Answers 2

6 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related