c++ - c++ using 指令的范围

Question

来自 c++11 标准的第 7.3.4.2 节:

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. [ Note: In this context, “contains” means “contains directly or indirectly”. —end note ]

第二句和第三句到底是什么意思？请举例说明。

这是我试图理解的代码:

namespace A
{
    int i = 7;
}
namespace B
{
    using namespace A;
    int i = i + 11;
}
int main(int argc, char * argv[])
{
    std::cout << A::i << " " << B::i << std::endl;
    return 0;
}

它打印“7 7”而不是我期望的“7 18”。

抱歉打错了，程序实际上打印了“7 11”。

Answer 1

代码中的 using 语句是无关紧要的。当评估 B::i 的初始化程序时，B::i 已经在范围内。您可以通过删除 using 语句来简单地证明这一点；您的代码应该可以正常编译和运行。在任何情况下，B::i 的值最终都是未定义的，因为它依赖于一个未初始化的值(即 B::i 在初始化器被初始化时的值)评估)。