std :: ios_base :: ate和std :: ios_base :: trunc [英] std::ios_base::ate and std::ios_base::trunc

问题描述

std :: ios_base :: ate （除std :: ios_base :: app ，例如）和 std :: ios_base :: trunc （ std :: ios_base :: out 例如）？

我应该优先写 std :: ios_base :: smth ，而不是 std :: ios :: smth ？

解决方案

std :: ios_base :: ate position光标在文本的末尾，而std :: ios_base_app在末尾附加文本（带有写操作），尽管你仍然可以从头开始读）：

std :: ios_base :: trunc截断文件，因此它被清空，而std :: ios_base :: out只是指定要写入流。

不能引用标准（在我的平板电脑上，Acrobat Reader不会允许复制），但是从ISO 14882：1998的第27.4.2.1.4节，您可以在链接上看到的信息几乎是准确的：http://cplusplus.com/reference/iostream/ios_base/openmode/

总结：

  std :: ios_base :: app = append 
  

通过在每次写入之前seek [ing]结束，在流末尾添加

  std :: ios_base :: ate =结束时
  

打开并在打开后立即查找结束

  std :: ios_base :: binary = binary 
  
 
 
 
以文本形式执行二进制操作
  std :: ios_base :: in = input 
  
以读取模式打开
  std :: ios_base :: out = output 
  
以书写模式打开
  std :: ios_base :: trunc = truncate 
  
开启时截断讯息串。
 
 
 
标志，所以你可以在读/写二进制流在最后打开流：
  std :: ios_base :: in | std :: ios_base :: out | std :: ios_base :: ate | std :: ios_base :: binary 
  
 
 
 
 
 
 
使用这些值，它是你想要的。它们在 std :: ios_base 类中被声明为公共静态字段（见27.4.2），因此可以使用 std :: ios :: ate ，甚至是 cout.binary ！
 
 
 
 
 
 
你必须注意的一点是 std :: ios_base :: ate 不表示 std :: ios_base： ：app ，也不会 std :: ios_base :: out 也不表示 std :: ios_base :: trunc 。每个字段有不同的含义，不同的使用情况，虽然大多数不能单独使用：）
 
What the point of the std::ios_base::ate (other than std::ios_base::app, for example) and std::ios_base::trunc (other than std::ios_base::out, for example)?

And should i preferly write std::ios_base::smth instead of std::ios::smth?
解决方案
std::ios_base::ate position the cursor at the end of the text whereas std::ios_base_app appends text (with a write operation) at the end, though you can still read from the beginning :)

std::ios_base::trunc truncates the file so it is emptied, whereas std::ios_base::out just specify you want to write to the stream.

I currently can't quote the standard (on my tablet and Acrobat Reader won't let met copy) but from paragraph 27.4.2.1.4 from ISO 14882:1998 the information you can see on the link is almost exact: http://cplusplus.com/reference/iostream/ios_base/openmode/

To sum up:
std::ios_base::app = append
Append at the end of the stream by "seek[ing] to end before each write"
std::ios_base::ate = At The End
Open and seek immediately at the end after opening
std::ios_base::binary = binary
Perform operation in binary as opposed to text
std::ios_base::in = input
Open in read mode
std::ios_base::out = output
Open in write mode
std::ios_base::trunc = truncate
Truncate the stream on opening.

These values are just flags, so you can open a stream in read/write binary at the end with :
std::ios_base::in | std::ios_base::out | std::ios_base::ate | std::ios_base::binary




Concerning the way of using those values, it is as you wish. They are declared as public static fields in std::ios_base class (see 27.4.2) thus it is possible to access them using std::ios::ate or even something like cout.binary !



The points where you must take attention is that std::ios_base::ate does NOT imply std::ios_base::app nor does std::ios_base::out implies std::ios_base::trunc. Each field has a different meaning, and a different case of use, though most of them can't be used alone :)

                        
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