Alternatives to ios::nocreate and ios::noreplace

最新推荐文章于 2024-05-31 10:06:55 发布

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最新推荐文章于 2024-05-31 10:06:55 发布

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分类专栏：语言tips 文章标签： ios file deprecated library c++

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博客介绍了在标准前C++中，<fstream.h>的ios::nocreate和ios::noreplace标志用于控制文件创建，但因平台特定未进入标准<fstream>库。不过可轻松实现这些过时标志的功能，还给出模仿ios::nocreate标志的示例，以及ios::noreplace的相反操作思路。

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Alternatives to ios::nocreate and ios::noreplace

In pre-standard C++, certain implementations of <fstream.h> offered the flags ios::nocreate and ios::noreplace for controlling file creation. These flags were too platform-specific and never made it into the standard <fstream> library, which supersedes the deprecated, pre-standard <fstream.h> header. However, you can achieve the functionality of these obsolete flags rather easily. The following example imitates the ios::nocreate flag. First, it attempts to open a file for read. If the file doesn't exist, no new file is created. If the file exists, the program closes it and reopens it in write mode:

 
fstream fs(“fname”, ios_base::in);// attempt open for read
if (!fs)
{
  // file doesn't exist; don't create a new one
}
else //ok, file exists. close and reopen in write mode
{
  fs.close();
  fs.open(“fname”, ios_base::out); // reopen for write
}

You can just do the opposite for ios::noreplace:

 
fstream fs(“fname”, ios_base::in);// attempt open for read
if (!fs)
{
 // file doesn't exist; create a new one
 fs.open(“fname”, ios_base::out); 
}
else //ok, file exists; close and reopen in write mode
{
 fs.close()
 fs.open(“fname”, ios_base::out); // reopen for write
}

// however, it seems that MS VC6.0 can not help implement fstream perfectly