CODEWITHSUNDEEP
c++arrayspointersvector
Giannandrea Vicidomini
Giannandrea Vicidomini

Reputation: 35

C++ pointers to vectors behavior

I know that using raw pointers on vectors is not the best way because of the smart ones, but mine is just a question to understand something that I can't wrap my head around... If a I create a vector and then a pointer to it:

    vector<int> vec = {1,2,3,4,5};
    vector<int> *ptr = &vec;
  1. Why do i need to use the reference operator & whereas an array doesn't need it?? (aren't vectors dynamic arrays)
  2. Why if i write cout<<*ptr<<endl;, compiler gives error and I have to write cout<<(*ptr)[1]<<endl; or cout<< ptr->at(1)??

Usually if you create a pointer to an array that pointer points to the first element.

Can you help me understand what kind of animal vectors are?

Upvotes: 0

Views: 784

Answers (2)

user2100815
user2100815

Reputation:

aren't vectors dynamic arrays)

Conceptually yes, implementation wise, no. A vector instance is an object, of type std::vector <sometype>. If you want to take the address of an object like a vector instance, you need to use the address-of, aka &, operator. A vector is not the same kind of thing as a c++ array.

Why if i write cout<<*ptr<<endl;, compiler gives error

Because std::vector doesn't support streaming via operator<<.

Edit:

If you actually wanted to access the internal array, you would do:

vector <int> v = // whatever
int * ip = &v[0];

or:

int * ip = v.data();

Upvotes: 1

smac89
smac89

Reputation: 43234

Why do i need to use the reference operator(&) whereas an array doesn't need it??(aren't vectors dynamic arrays)

The equivalent of what you've done, with arrays is this:

int array[] = {1,2,3,4,5};
int (*ptr)[5] = &array; // less constrained to just 5 elements with vectors

So yea you still need the & operator;

Why if i write cout<<*ptr<<endl;, compiler gives error and I have to write cout<<(*ptr)[1]<<endl; or cout<< ptr->at(1)??

Because vectors are objects and the * operator was not overloaded to return anything useful apart from the vector itself.

What you probably want to do is to use the std::vector::data function of vectors which gives you a pointer to the underlying array used by the vector.

Upvotes: 2

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