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[C++] Memory allocation question

Started by sheep19 Nov 24, 2011 at 11:38 PM 3 replies 1k views

Original Post

sheep19

105

sheep19

Nov 24, 2011 at 11:38 PM

Hi. I have a structure that represents a node in a linked list:

template <class T>
 
 struct Foo
 {
 public:
 Foo()
 {
 name = new string;
 age = 10;
 }
 
 ~Foo()
 {
 delete name;
 }
 
 private:
 Foo(const Foo&);
 Foo &operator = (const Foo &);
 
 string *name;
 int age;
 
 };
 
 struct Node
 {
 T data;
 Node<T> *next;
 };

Node *n = new Node;

Now the space has been allocated on the heap.The name(string name) of the Foo object is allocated on the heap. Even if n was created on the stack, Foo::name would be still allocated on the heap.
But what is the case of Foo::age when n is allocated on the heap? Is it allocated on the heap or on the stack?

Hodgman

Mod

5,991

Hodgman

Nov 24, 2011 at 11:53 PM

age is a part of the Foo object, so when you allocate a Foo object, you're allocating age.

kilah

Nov 24, 2011 at 11:57 PM

The whole object is allocated in the heap, Foo::age too. Think that the size of your object is what the allocation request is about, that includes Foo::age. What it is going to happen is that you have 2 allocations on heap per object in this case. That means first allocation is sizeof( Foo) = sizeof(age) + sizeof(name), and second allocation sizeof(string) for name pointer. (In fact there is going to be even more allocations due to std::string but that's another story)

alvaro

1,980

alvaro

Nov 25, 2011 at 12:57 AM

Why is name a pointer? And how come "template" appears before a class definition that doesn't use T?

dpadam450

523

dpadam450

Nov 25, 2011 at 1:34 AM

The variable n by itself is just a pointer (32/64bit address). The pointer is on the stack, the object it points to is on the heap.

Node *n = new Node;
n = new Node;

Here you would have just changed the variable n (on the stack) to hold a different address. The two Foo objects are still on the heap.

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