CODEWITHSUNDEEP
c++liststlcontainers
user2419831
user2419831

Reputation: 181

creating a List in c++ with initial capacity

I am creating a template class with a list object as a private member:

 .
 .
 .
 private:

list<E> listObject;

 };

The first constructor has to create a list object with capacity of 10. How would you do that?

 template<class T, class E>
 Queue<T,E>::Queue()
 {
listObject.resize(10); 

 }

or 

 template<class T, class E>
 Queue<T,E>::Queue()
 {
listObject = new list<E>(10); 

 }

or any other idea?

Upvotes: 0

Views: 3926

Answers (3)

Arthur
Arthur

Reputation: 1

Currently, no setting capacity api is provided. Its max-size() returned is a super large integer, just a theorial value. api resize will actually allocate that specified N size elements with default value (or object constructor), not just reserve N elements space.

If u want to support a capacity, maybe you should inherit std::list, maintain a capacity value in your implemented child template class.

Upvotes: 0

juanchopanza
juanchopanza

Reputation: 227438

The most idiomatic option would be to initialize the list with the desired size. This is done in the constructor initialization list:

Queue<T,E>::Queue() : listObject(10) 
{
  ....
}

This will leave you with a list of 10 default constructed objects (whether you actually need that is a different matter).

Note that in C++11 you can initialize data members at the point of declaration. So you could also do this:

template <typename T, typename E> Queue
{
  ....
  list<E> listObject = list<E>{10};
};

More on std::list here.

Upvotes: 1

NicholasM
NicholasM

Reputation: 4673

There is a constructor for list that takes a size argument. You can call that constructor in your class's constructor using an initializer list

template<class T, class E>
Queue<T,E>::Queue()
:  listObject(10)
{}

If you need information, search and learn about initializer lists. You can read more about the std::list class's constructors on a reference website -- that's what I did here -- although I admit the long series of constructors is a little bit much when you're first getting started.

That page says the following about this constructor, with only a count specified (assuming C++11): "Constructs the container with count value-initialized (default constructed, for classes) instances of T."

Upvotes: 0

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