cin and cout are defined in the header iostream and in the namespace std. These concepts are orthogonal. iostream is a file name and std is a namespace used by the source code of that file.
As a way of keeping things organized, c++ provides namespaces. All of the standard library is defined within the namespace called std. Other libraries you might write or include may use their own namespace. For example, you might include a physics library in your project which wants to define the concept of algebraic vectors. By using it's own namespace (let's called if physlib) it can differentiate between it's vector (physlib::vector) and the standard vector (std::vector). Namespaces can also be nested to help organize large projects. For example, time keeping parts of the standard library are in std::chrono and file system related components are in std::filesystem.
The preferred way of using cin and cout is as following. :
#include
int main()
{
std::cout << "Hello, World!\n";
return 0;
}
The statement using namespace std is simply an instruction to look in the namespace std by default. It allows you to omit the std:: part of using standard library components. It's generally regarded as a bad idea to used using namespace std.
