I've been told it's bad to have "using namespace ns123" in a header file, but I can't remember what the reason given was. Is it in fact a bad thing to do, and why?
Using ‘using’ Keyword in C++ Header Files – Best Practices
c++namespaces
Related Solutions
Could you do typedef gee::whiz::abc::def::Hello Hello
?
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 affectingstd::
...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.
Best Answer
It's a bad practice, in general, because it defeats the purpose of namespaces. By defining in a header you're not enforcing strict control over the scope of the using declaration, meaning that you can run into name clashes in unexpected places.