Why is including "using namespace" into a header file a bad idea in C++?

While reading from Bruce Eckel's "Thinking in C++" about namespaces, I encountered the following statement:

However you'll virtually never see a using directive in a header file (at least not outside of scope). The reason is that using directive eliminate the protection of that particular namespace, and the effect last until the end of current compilation unit. If you put a using directive (outside of a scope) in a header file, it means that this loss of "namespace protection" will occur within any file that include this header, which often mean other header files.

Would you please like to help me to comprehend the above statement with some easy example?

Consider this program:

line#
    1 #include <string>                                                               
    2                                                                                 
    3 using namespace std;                                                            
    4                                                                                 
    5 struct string { const char* p; };  // Beware: another string!
    6                                                                                 
    7 int main()                                                                      
    8 {                                                                               
    9     string x; // Error: ambiguous - which string is wanted?
   10 }

If you try to compile it, you'll see errors:

g++     using.cc   -o using
using.cc: In function `int main()':
using.cc:9: error: use of `string' is ambiguous
using.cc:5: error:   first declared as `struct string' here
/usr/lib/gcc/i386-redhat-linux/3.4.6/../../../../include/c++/3.4.6/bits/stringfwd.h:60: error:
   also declared as `typedef struct std::basic_string<char, std::char_traits<char>, std::allocator<char> > std::string' here
using.cc:9: error: `string' was not declared in this scope
using.cc:9: error: expected `;' before "x"

The problem here is that when main() specifies string x;, the compiler's not sure whether the user-defined ::string or included std::string is wanted.

Now imagine you take the top part of the program... lines 1 through 5 - up to and including the struct string... and put it into a header file which you then #include before main(). Nothing changes: you still have an error. So, just as for standalone programs, header files with using statements in them can cause trouble for other code that includes them, making some of their statements ambiguous.

It can be a bigger pain though, as headers can be included - directly or indirectly - by arbitrarily huge amounts of dependent code, and...

• removing the using statement from the header, or
• a change to the contents of <string>, or any other header affecting std::

...might break code including the problematic header. Either problem may render dependent code uncompilable, and issues may not even be noticed until another compilation is attempted. Further, the person suffering due to the using statement may not have filesystem/code-repository permissions, corporate authority etc. to remove the using statement from the header, nor fix other affected client code.

That said, if a header only has "using" inside a class or function, then there's no affect on code beyond that scope, so the potential impact of changes to std:: is dramatically reduced.