Getting error while declaring size of 2D vector - c++

Question

I want to declare my 2D vector first, then give it a size.
But why I am getting error?
Can anyone explain me?
int main() {
vector<vector<int>> a;
a = vector<int>(16, vector<int>(15));
cout << a.size() << a[0].size();
}
The reason for doing it is that I don't know the size before but after getting the input from the user, I want to give it the size.
Error:
Char 9: error: no matching constructor for initialization of 'vector<int>'
a = vector<int>(16, vector<int>(15));
^ ~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:507:7: note: candidate constructor not viable: no known conversion from 'vector<int>' to 'const std::vector<int, std::allocator<int>>::allocator_type' (aka 'const std::allocator<int>') for 2nd argument
vector(size_type __n, const allocator_type& __a = allocator_type())
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:519:7: note: candidate constructor not viable: no known conversion from 'vector<int>' to 'const std::vector<int, std::allocator<int>>::value_type' (aka 'const int') for 2nd argument
vector(size_type __n, const value_type& __value,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:572:7: note: candidate constructor not viable: no known conversion from 'int' to 'const std::vector<int, std::allocator<int>>' for 1st argument
vector(const vector& __x, const allocator_type& __a)
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:604:7: note: candidate constructor not viable: no known conversion from 'int' to 'std::vector<int, std::allocator<int>>' for 1st argument
vector(vector&& __rv, const allocator_type& __m)
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:622:7: note: candidate constructor not viable: no known conversion from 'int' to 'initializer_list<std::vector<int, std::allocator<int>>::value_type>' (aka 'initializer_list<int>') for 1st argument
vector(initializer_list<value_type> __l,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:650:2: note: candidate template ignored: deduced conflicting types for parameter '_InputIterator' ('int' vs. 'std::vector<int, std::allocator<int>>')
vector(_InputIterator __first, _InputIterator __last,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:494:7: note: candidate constructor not viable: requires single argument '__a', but 2 arguments were provided
vector(const allocator_type& __a) _GLIBCXX_NOEXCEPT
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:550:7: note: candidate constructor not viable: requires single argument '__x', but 2 arguments were provided
vector(const vector& __x)
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:569:7: note: candidate constructor not viable: requires 1 argument, but 2 were provided
vector(vector&&) noexcept = default;
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:582:7: note: candidate constructor not viable: requires 3 arguments, but 2 were provided
vector(vector&& __rv, const allocator_type& __m, true_type) noexcept
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:586:7: note: candidate constructor not viable: requires 3 arguments, but 2 were provided
vector(vector&& __rv, const allocator_type& __m, false_type)
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:484:7: note: candidate constructor not viable: requires 0 arguments, but 2 were provided
vector() = default;
^
1 error generated.

Answer 1

vector<int> is not a 2D vector.
Instead of this:
a = vector<int>(16, vector<int>(15));
You should use this:
a = vector<vector<int>>(16, vector<int>(15));