C++ 命名空间名称和 class 名称重复是否会导致冲突？

Question

我正在测试 C++ 的名称查找规则。我有一个包含 3 个文件的简单程序：

$cat testns01.h
struct S{
    static int f(){return 1;}
};

$cat testns02.h
namespace S{
    static int f(){return 2;}
}

$cat testns3.cpp
#include "testns02.h" // namespace
#include "testns01.h" // struct
#include<stdio.h>
int main()
{
    int i = S::f();
    printf("%d\n",i);
    return 0;
}

如果我编译运行，我得到：

$g++ testns3.cpp && ./a.out
1

好的，我有 3 个问题：

name "S" class name 和 namespace 之间的重复不冲突？
当两者都具有名称 "S" 时，似乎 "struct S" 具有更高的优先级

如果我注释掉#include "testns01.h"这一行，程序会打印2，还是可以的。所以我的第三个问题是：

c++ 标准是否讨论名称查找如何解决名称重复问题？

Answer 1

name "S" duplication between class name and namespace doesn't conflict?

他们有。

When both has name "S", seems "struct S" has higher priority

没有。（往下看）

Does c++ stardard talke about how name lookup resolves duplicated name duplication?

是的。引用N4140的相关部分：

§3.3.1 [basic.scope.declarative] / 4

Given a set of declarations in a single declarative region, each of which specifies the same unqualified name,

they shall all refer to the same entity, or all refer to functions and function templates; or

exactly one declaration shall declare a class name or enumeration name that is not a typedef name and the other declarations shall all refer to the same variable or enumerator, or all refer to functions and function templates; in this case the class name or enumeration name is hidden. [ Note: A namespace name or a class template name must be unique in its declarative region. —end note ]

我想你不小心让你的例子为你工作，因为你重复了包含守卫。我能够从问题中为 class S 重现 "preference"：

#ifndef FOO
#define FOO
struct S{
    static int f(){return 1;}
};
#endif

#ifndef FOO
#define FOO
namespace S{
    static int f(){return 2;}
}
#endif

#include<stdio.h>
int main()
{
    int i = S::f();
    printf("%d\n",i);
    return 0;
}

link

C++ 命名空间名称和 class 名称重复是否会导致冲突？

Does C++ namespace name and class name duplication lead to conflict?

c++

struct

symbols

namespaces

class