streampos和pos_type，streamoff和off_type之间有什么区别？

问题描述

streampos 与 pos_type ， streamoff 和 off_type ，但它们的定义不同。 basic_stream<> :: seek 的函数应该如何使用？

解决方案

std :: basic_istream 和 std :: basic_ostream 都采用两种模板类型： CharT 和 Traits 。给定从基本流之一派生的类A， Traits 数据类型可以检索为

  A :: traits_type 
  

根据C ++的§21.2标准，此数据类型必须提供以下成员类型：

  char_type //必须与basic-stream $的CharT相同b $ b off_type 
 pos_type 
  

（以及与当前问题无关的一些其他数据类型） 。考虑到， off_type 和 pos_type 的预期含义是： p>

• pos_type 用于流中的绝对位置


• off_type 用于相对位置

使用绝对版本的 seekg（），你应该声明的数据类型是 A :: pos_type 与 A :: traits_type :: pos_type 相同）。对于相对版本，它是 A :: off_type 。

关于 std :: streampos 和 std :: streamoff ：这些也被标准定义为用于 traits_type 的默认版本。换句话说，如果你没有明确指定 Traits 模板参数， A :: pos_type 实际上是 std :: streampos 和 A :: off_type / em> std :: streamoff 。

如果您创建自己的版本 Traits ，并希望使用标准库模板如 std :: basic_istream<> 包括 pos_type 和 off_type （以及许多其他数据类型）的typedef，并确保它们符合§27.2。 2和§2​​7.3。

What are the differences between streampos and pos_type, streamoff and off_type, except they are defined differently. What should I use with the basic_stream<>::seek's functions?

解决方案

std::basic_istream and std::basic_ostream both take two template types, CharT and Traits. Given a class A that is derived from one of the basic-streams, the Traits data type can be retrieved as

A::traits_type

According to §21.2 of the C++ standard, this data type must provide the following member types:

char_type // must be identical to CharT of the basic-stream
off_type
pos_type

(and some further data types irrelevant to the present question). Given the way the std::basic_istream<>::seekg() method is defined, the intended meaning of off_type and pos_type is:

• pos_type is used for absolute positions in the stream
• off_type is used for relative positions

So if you want to use the absolute version of seekg(), the data type you should declare is A::pos_type (which is the same as A::traits_type::pos_type). For the relative version it is A::off_type.

Regarding std::streampos and std::streamoff: These are defined, too, by the standard as the data types that are used for the default version of the traits_type. In other words, if you do not explicitly specify the Traits template parameter, the A::pos_type will in fact be std::streampos, and A::off_type will in fact be std::streamoff.

If you create your own version of Traits and want to use it with standard library templates like std::basic_istream<> etc., you must include typedefs for pos_type and off_type (and a lot of other data types), and ensure they comply with §27.2.2 and §27.3 of the standard.

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