使用声明解决歧义 [英] Resolving ambiguity with using declaration

问题描述

看看这个片段:

namespace A {
int fn();
}
namespace B {
int fn();
}

// namespace Ns {

using namespace A;
using namespace B;
using A::fn;

int z = fn();

// }

此代码无法编译，因为 fn()在 int z = fn();

This code doesn't compile, as fn() is ambiguous at int z = fn();

如果我将使用，和 z 放入命名空间(删除两个//)，则代码会编译.这是为什么?全局名称空间有什么特别之处?

If I put using's and z into a namespace (remove the two //), the code compiles. Why is that? What is special about the global namespace?

推荐答案

请参阅[namespace.udir]/2

See [namespace.udir]/2

using-directive (使用指令)指定可以在以下范围内使用指定名称空间中的名称: using-directive 出现在 using-directive 之后.在不合格的名称查找(3.4.1)期间，出现名称就像它们是在包含 using-directive 和提名的名称空间.

A using-directive specifies that the names in the nominated namespace can be used in the scope in which the using-directive appears after the using-directive. During unqualified name lookup (3.4.1), the names appear as if they were declared in the nearest enclosing namespace which contains both the using-directive and the nominated namespace.

因此，当您拥有名称空间 Ns 时，指令使用名称空间A; 和使用名称空间B 会使 A ::fn 和 B :: fn 出现在全局名称空间中，而使用A :: fn; 会使 fn 出现在 Ns .后者声明在名称查找期间获胜".

Thus, when you have the namespace Ns, the directives using namespace A; and using namespace B make A::fn and B::fn appear in the global namespace, whereas using A::fn; makes fn appear in Ns. The latter declaration "wins" during name lookup.

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