what is the difference between defining a vector that as vector v(n) vs vector v[n]?

Question

when I use this code:

int main()
{
    unsigned int n;
    cin >> n;
    vector<int>number[n];
  
    return 0;
}

the compiler mark 'n' as an error:
"expression must have a constant value"
"the value of variable 'n' cannot be used as a constant"

but when I use vector<int> v1(n) instead, the error disappeared and worked well.

so, here is my questions:
what is the difference between defining a vector as vector<int> v1(n) and as vector<int> v2[n] ?
Do vectors use dynamic allocation?

and thanks in advance

Answer 1

type_name variable_name[array_size];

Is syntax for a default initialised array variable. vector<int>number[n]; is an array of n vectors. If the size is not compile time constant - such as it isn't in the example, then the program is ill-formed as evidenced by the error that you quoted.

type_name variable_name(args);

Is syntax for direct initialisation of a variable.

Do vectors use dynamic allocation?

As with most standard containers (all except std::array ), vector elements are dynamic objects. It acquires the storage using the allocator that has been provided to it.

Answer 2

• vector<int> v1(n) defines a single vector that contains n elements.

• vector<int> v2[n] defines a variable sized array of vectors, each of which will contain no elements. This is commonly called a VLA (Variable Length Array) and not officially part of C++ (it is part of C99 however). Some compilers like gcc do support it for c++ as well, but you shouldn't count on it.