What is the difference between vector a , vector a[n] and vector a(n)?

Question

I just learnt about vectors and I'm confused about their use.

Please tell me what is the difference between:

vector a;

,

vector a[n];

and

vector a(n);

user_3425536 · Answer

vector a; declares an empty vector.

vector a[n]; declares an array containing n empty vectors.

vector a(n); declares a vector containing n zeroes.

Bonus:

vector a{n}; declares a vector containing the single element n.

user_6865932 · Answer

vector a;

Declaration of a vector of ints named a

vector a[n];

Declaration of an array of vectors of ints named a with n elements.

vector a(n);

Declaration of a vector of ints a initialized to n number of 0s.

What is the difference between vector<int> a , vector<int> a[n] and vector<int> a(n)?