什么可以和我不能专门在std命名空间？ [英] What can and can't I specialize in the std namespace?

问题描述

用户可以向 std 命名空间添加显式专门化。

有什么模板可以和我不能专业化吗？

解决方案

从标准中松散：

• numeric_limits 不适用于非算术标准类型（例如 complex< T> ）




• shared_ptr 的属性应为CopyConstructible，CopyAssignable和LessThanComparable [and]可转换为 bool 。




• weak_ptr




• [T] emplate specializations [of std :: hash ]必须满足类模板哈希




• < type_traits> 中的任何内容：对于本子条款中定义的任何类模板，除非另有说明，否则为未定义。 （仅明确允许 common_type 的一些专业化）






• istreambuf_iterator 的所有专业化应具有一个简单的复制构造函数，一个constexpr默认构造函数




• 对任何类型的其他类型实例化类模板 complex 的效果比float，double或long double都未指定。




• atomic




• 类模板模板必须具有已删除的副本构造函数，已删除的副本赋值运算符和constexpr值构造函数。 unary_function 和 binary_function 不推荐使用。程序不应声明这些模板的特殊化。

当然，总体条款17.6.4.2.1，句子1（感谢@sehe和@curiousguy）：

< blockquote>

除非另有说明，否则如果C ++程序将声明或定义添加到命名空间std或命名空间std中的
命名空间，那么它的行为是未定义的。程序可以将任何标准库模板的模板专用化
添加到命名空间std中，只要声明依赖于用户定义的类型
，并且专业化满足原始模板的标准库要求，并且不明确
禁止。

而句子2：

如果声明了

• ，那么C ++程序的行为是未定义的。标准库类模板或




• 标准库类或类模板的任何成员函数模板的显式特化，

  
  

• 标准库类或类模板的任何成员类模板的显式或部分专门化。

程序可以仅在声明
取决于用户定义类型的名称并且实例化满足标准库要求
时才显式实例化在标准库中定义的模板原始模板。

Users are allowed to add explicit specializations to the std namespace. However, there are a few templates that I am explicitly forbidden from specializing.

What templates can and can't I specialize?

解决方案

Quoting loosely from the standard:

• numeric_limits shall not be specialized for non-arithmetic standard types (e.g. complex<T>)

• "[S]pecializations of shared_ptr shall be CopyConstructible, CopyAssignable, and LessThanComparable [and] convertible to bool."

• "Specializations of weak_ptr shall be CopyConstructible and CopyAssignable."

• "[T]emplate specializations [of std::hash] shall meet the requirements of class template hash."

• Anything in <type_traits>: "The behavior of a program that adds specializations for any of the class templates defined in this subclause is undefined unless otherwise specified." (only some specializations of common_type are explicitly allowed)

• Locales have certain required specializations.

• All specializations of istreambuf_iterator shall have a trivial copy constructor, a constexpr default constructor, and a trivial destructor.

• "The effect of instantiating the class template complex for any type other than float, double, or long double is unspecified." I take it that means that defining such other specializations is pointless.

• "Specializations and instantiations of the atomic template shall have a deleted copy constructor, a deleted copy assignment operator, and a constexpr value constructor."

• "The class templates unary_function and binary_function are deprecated. A program shall not declare specializations of these templates."

And of course the overarching clause 17.6.4.2.1, sentence 1 (thanks @sehe and @curiousguy):

The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std unless otherwise specified. A program may add a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.

And sentence 2:

The behavior of a C++ program is undefined if it declares

• an explicit specialization of any member function of a standard library class template, or

• an explicit specialization of any member function template of a standard library class or class template, or

• an explicit or partial specialization of any member class template of a standard library class or class template.

A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a user-defined type and the instantiation meets the standard library requirements for the original template.

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