Ordering of using namespace std; and includes?

A perhaps interesting data point. When I compile the following:

using namespace std;
using namespace no_such_namespace;

with g++ 4.5.2, I get:

c.cpp:2:17: error: ‘no_such_namespace’ is not a namespace-name
c.cpp:2:34: error: expected namespace-name before ‘;’ token

To be clear, those two lines are the entire source file I compiled.

Neither std nor no_such_namespace has been defined as a namespace at that point, but g++ complains only about the second. I don't think there's anything special about the identifier std in the absence of a declaration of it. I think @James Kanze is right that this is a bug in g++.

EDIT: And it's been reported. (5 years ago!)

UPDATE: Now it's more than 8 years, and still hasn't been assigned to anyone, much less fixed. g++ 4.9.2 exhibits the problem. clang++ 3.5 doesn't, but it issues a warning for std and a fatal error for no_such_namespace:

c.cpp:1:17: warning: using directive refers to implicitly-defined namespace 'std'
using namespace std;
                ^
c.cpp:2:17: error: expected namespace name
using namespace no_such_namespace;
                ^
1 warning and 1 error generated.

UPDATE: As of 2021-09-24, the bug report is still open and the bug exists in g++ 11.2.0. A comment posted 2021-07-24 suggests that g++ should warn about this.

I don't think it's legal, but the standard isn't 100% clear about it. Basically, name lookup (as defined in §3.4) can't find a previous declaration of the namespace, because there isn't one. Everything hinges on whether:

using namespace std;

is a declaration of the namespace or not. And I don't see any text in §7.3.4 which says that a using-directive declares the nominated namespace. G++ allows your code, but IMHO, this is a bug.

From SO/IEC 14882:2003

[7.3.3.9] The entity declared by a using-declaration shall be known in the context using it according to its definition at the point of the using-declaration. Definitions added to the namespace after the using-declaration are not considered when a use of the name is made.

[3.4.3.2.2] Given X::m (where X is a user-declared namespace), or given ::m (where X is the global namespace), let S be the set of all declarations of m in X and in the transitive closure of all namespaces nominated by using-directives in X and its used namespaces, except that using-directives are ignored in any namespace, including X, directly containing one or more declarations of m. No namespace is searched more than once in the lookup of a name. If S is the empty set, the program is ill-formed. Otherwise, if S has exactly one member, or if the context of the reference is a using-declaration (7.3.3), S is the required set of declarations of m. Otherwise if the use of m is not one that allows a unique declaration to be chosen from S, the program is ill-formed

So if it happens to work, it's a fluke and not portable.

This code is undefined behavior [lib.using.headers]:

A translation unit shall include a header only outside of any external declaration or definition, and shall include the header lexically before the first reference to any of the entities it declares or first defines in that translation unit.

You reference std and then include a header that declares it. Even this is still undefined behavior:

#include <string>
using namespace std;
#include <iostream>