For using a standard library function in C++, what exactly is the relationship between its header file and std namespace?

Question

In C++, since the namespace std contains the declaration of all standard library functions, then why do we need to include iostream header to use I/O functions. Can we not just write 'using namespace std' so that the compiler knows where to get the definition? I want to know the need for including a header file for using any feature like strings too if that function declaration is already present in std namespace.

Answer 1

The compiler finds the declared namespace std in the included headers. It cannot know where to get the definition from unless you don't specify it by including libraries.