SF.7: Don't write using namespace at global scope in a header file
SF.7:不要在头文件中的全局作用域中使用using namespace指令
Reason(原因)
Doing so takes away an #includer's ability to effectively disambiguate and to use alternatives. It also makes #included headers order-dependent as they might have different meaning when included in different orders.
这么做去除了include操作有效消除歧义和使用其他选项的能力。另外,文件以不同次序被包含时的含义可能会随之不同,导致产生包含顺序依赖性。
Example(原因)
// bad.h
#include <iostream>
using namespace std; // bad
// user.cpp
#include "bad.h"
bool copy(/*... some parameters ...*/); // some function that happens to be named copy
int main()
{
copy(/*...*/); // now overloads local ::copy and std::copy, could be ambiguous
}
An exception is using namespace std::literals;. This is necessary to use string literals in header files and given the rules - users are required to name their own UDLs operator""_x - they will not collide with the standard library.
有一个例外是using namspace std::literals;。如果需要在头文件中使用字符串字面值而且满足这样的条件:用户被要求为他们自己的UDL运算符“”_x命名而且他们不会和标准库相冲突,使用using namespace std::literals是就可以认为是必要的。
Enforcement(实施建议)
Flag using namespace at global scope in a header file.
标记在头文件的全局作用域中使用using namspace指令的情况。
原文链接
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#sf7-dont-write-using-namespace-at-global-scope-in-a-header-file