使用命名空间std;并包括？ [英] Ordering of using namespace std; and includes?

问题描述

我最近看到这个代码正在C ++项目的源文件中使用：

I recently saw this code being used in a source file in a C++ project:

using namespace std;
#include <iostream>

忽略所有问题：是否使用命名空间std 根本就是上面的代码甚至是合法的吗？在这两行之前，文件中没有代码。

Ignoring all issues of whether it's a good idea to have using namespace std at all, is the above code even legal? There is no code in the file before these two lines.

我认为这不会编译，因为 namespace std 尚未在范围内声明，直到 #include 指令将它包含到文件中，但使用编译系统为该项目编译正好。如果有人有一个指向相关部分的链接，这将是最感激。

I would have thought that this wouldn't compile, since namespace std hasn't been declared in scope until the #include <iostream> directive includes it into the file, but using the build system for the project this was compiling just fine. If someone has a link to a relevant part of the spec, that would be most appreciated.

推荐答案

一个可能有趣的数据点。当我编译以下：

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

using namespace std;
using namespace no_such_namespace;

使用g ++ 4.5.2，我得到：

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

std code> no_such_namespace 在这一点上已经被定义为命名空间，但是g ++只抱怨第二个。我不会认为在没有声明的情况下，标识符 std 有什么特别的。我认为@James Kanze是对的，这是g ++中的错误。

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++.

编辑：并且已经被报告。（5年前！）

更新：现在超过8年，还没有被分配给任何人，更不用说固定。 g ++ 4.9.2会出现问题。 clang ++ 3.5没有，但它对 std 发出警告，并为 no_such_namespace ：

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.

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