C.5: Place helper functions in the same namespace as the class they support
C.5:将帮助函数和它们支持的类放在同一命名空间
Reason(原因)
A helper function is a function (usually supplied by the writer of a class) that does not need direct access to the representation of the class, yet is seen as part of the useful interface to the class. Placing them in the same namespace as the class makes their relationship to the class obvious and allows them to be found by argument dependent lookup.
帮助函数是那种不需要直接访问类的表达（私有数据），而又可以看做类的有用接口的函数（通常有类的作者提供）。
Example（示例）
namespace Chrono { // here we keep time-related services
class Time { /* ... */ };
class Date { /* ... */ };
// helper functions:
bool operator==(Date, Date);
Date next_weekday(Date);
// ...
}Note（注意）
This is especially important for overloaded operators.
这一点（和支持对象的类放在同一个命名空间）对于预算符重载尤为重要。
Enforcement（实施建议）
- Flag global functions taking argument types from a single namespace.
- 如果发现有全局函数只使用单一的命名空间的参数类型，进行提示。
原文链接：
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c5-place-helper-functions-in-the-same-namespace-as-the-class-they-support
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