爲什麼不把std :: fstream寫入文件？

Question

我得到了fstream與oftream之間的不同行爲，我無法解釋。

當我使用fstream，什麼也沒有發生，即沒有文件被創建：

int main() 
{ 
    std::fstream file("myfile.txt"); 
    file << "some text" << std::endl; 
    return 0; 
} 

，但是當我改變fstream到oftream，它的工作原理。

爲什麼？

fstream CTOR的第二個參數是ios_base::openmode mode = ios_base::in | ios_base::out這使我認爲該文件是以讀寫模式打開的，對吧？

Answer 1

ios_base::in requires the file to exist。

如果您只提供ios_base::out，那麼只有當文件不存在時纔會創建該文件。

+--------------------+-------------------------------+-------------------------------+ 
| openmode   | Action if file already exists | Action if file does not exist | 
+--------------------+-------------------------------+-------------------------------+ 
| in     | Read from start    | Failure to open    | 
+--------------------+-------------------------------+-------------------------------+ 
| out, out|trunc  | Destroy contents    | Create new     | 
+--------------------+-------------------------------+-------------------------------+ 
| app, out|app  | Append to file    | Create new     | 
+--------------------+-------------------------------+-------------------------------+ 
| out|in    | Read from start    | Error       | 
+--------------------+-------------------------------+-------------------------------+ 
| out|in|trunc  | Destroy contents    | Create new     | 
+--------------------+-------------------------------+-------------------------------+ 
| out|in|app, in|app | Write to end     | Create new     | 
+--------------------+-------------------------------+-------------------------------+ 

---

PS：

一些基本的錯誤處理也被證明用於理解這是怎麼回事：

#include <iostream> 
#include <fstream> 

int main() 
{ 
    std::fstream file("triangle.txt"); 
    if (!file) { 
    std::cerr << "file open failed: " << std::strerror(errno) << "\n"; 
    return 1; 
    } 
    file << "Some text " << std::endl; 
} 

輸出：

C:\temp> mytest.exe 
file open failed: No such file or directory 

C:\temp>