c++ using stl vector

Question

I am a little confused about the use of vector. We usually type

#include <vector>

first. Then why we still need to attach the name space of vector when using it, like:

std::vector<int> a;

Why not just

vector<int> a;

Answer 1

Every #include directive including something from the C++ standard library "loads" the entities into the std namespace (or some other namespace like this).

The namespace helps preventing global namespace pollution - by keeping everything in a seperate namespace, identifier collisions are rendered impossible.

In the <vector> file, then there is something like

namespace std {
    template<typename T> class vector {
        ...
    };
}

As you see, the vector template is still in the std namespace.

In summary, you use an #include preprocessor directive to use some facility provided in a header file. The file's contents textually replace the #include directive.
Still, these facilities are in a different namespace to prevent name clashing.

Answer 2

Namespaces were created to avoid collisions in naming. You might have something else as vector in your code. However you can use using namespace std; - it would allow you to use it like this (and anything else in std namespace): vector a; (of course if there is no name collision)