Cout header file Problems

Today I made a small mistake and forgot to add the header file.

In the console project under vc6, The cout output number is applied. The initial error "cout is not defined", and then add # include <iostream. h>:

Result: The program can be compiled normally, but no output is displayed during running ....

After adding # include <stdio. h>

It can run normally.

Supplement: # include <stdio. h> stdio. h indicates "standard input & output"

This indicates the standard input and output header file! This header file must be called when a standard input/output function is used!
Question: or add using namespace STD; to apply the standard namespace, is the reason why the compiler adds a program? Puzzled...