std::basic_ifstream::basic_ifstream

(1) 默认构造函数

Constructs a basic_ifstream object that is not associated with any file.
Internally, its basic_istream base constructor is passed a pointer to a newly constructed basic_filebuf object (the internal file stream buffer).

(2) initialization constructor

Constructs a basic_ifstream object, initially associated with the file identified by its first argument (filename), open with the mode specified by mode.
Internally, its basic_istream base constructor is passed a pointer to a newly constructed basic_filebuf object (the internal file stream buffer). Then, basic_filebuf::open is called with filename and mode as arguments.
如果文件无法打开，则设置流的failbit标志。

(3) 复制构造函数 (已删除)

已删除 (无复制构造函数)。

(4) move constructor

获取 x 的内容。
First, the function move-constructs both its base basic_istream class from x and a basic_filebuf object from x's internal basic_filebuf object, and then associates them by calling member set_rdbuf.
x 处于未指定但有效的状态。

参数

filename

A string representing the name of the file to open.
有关其格式和有效性的具体说明取决于库实现和运行环境。

mode

描述文件请求的 i/o 模式的标志。
这是一个位掩码成员类型 openmode 的对象，由以下成员常量的组合构成：

成员常量	代表	access
in *	输入	文件以读模式打开：内部流缓冲区支持输入操作。
下可用的类型	输出	文件以写模式打开：内部流缓冲区支持输出操作。
binary	binary	操作以二进制模式执行，而非文本模式。
ate	在末尾	输出位置从文件末尾开始。
app	append (追加)	所有输出操作都在文件末尾进行，追加到其现有内容。
trunc	truncate (截断)	文件打开前存在的任何内容都将被丢弃。

这些标志可以通过按位或运算符（|）组合。
* in is always set for basic_ifstream objects (even if explicitly not set in argument mode).
Note that even though basic_ifstream is an input stream, its internal basic_filebuf object may be set to also support output operations.

如果模式设置了 app，则打开操作失败。如果设置了 trunc 但未设置 out，操作也会失败。

如果模式同时设置了 trunc 和 app，则打开操作失败。如果设置了 trunc 但未设置 out，操作也会失败。

x

A basic_ifstream object of the same type (with the same class template parameters charT and traits), whose value is moved.

示例

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// ifstream constructor.
#include <iostream>     // std::cout
#include <fstream>      // std::ifstream

int main () {

  std::ifstream ifs ("test.txt", std::ifstream::in);

  char c = ifs.get();

  while (ifs.good()) {
    std::cout << c;
    c = ifs.get();
  }

  ifs.close();

  return 0;
}

数据竞争

异常安全

另见

basic_ifstream::open

打开文件 (公开成员函数)