Hi！这里是山幺幺的c++ primer系列。写这个系列的初衷是，虽然在学校学习了c++，但总觉得对这门语言了解不深，因此我想来啃啃著名的c++ primer，并在这里同步记录我的学习笔记。由于我啃的是英文版，所以笔记是中英文夹杂的那种。另外由于已有一定的编程基础，所以这个系列不会含有太基础的知识，想入门的朋友最好自己啃书嘻嘻~

I/O Classes

I/O library types

PS：w开头的类：To support languages that use wide characters, the library defines a set of types and objects that manipulate wchar_t data

ifstream and istringstream inherit from istream
ofstream and ostringstream inherit from ostream

I/O Objects的限制

No Copy or Assign for I/O Objects，因此parameter or return type都不能是I/O type
reading or writing an I/O object changes its state, so the reference to I/O objects must not be const

I/O Library Condition State

I/O objects will be in an error state if 输入的对象类型与期待的类型不匹配等
Once an error has occurred, subsequent IO operations on that stream will fail. We can read from or write to a stream only when it is in a non-error state.
The IO library defines a machine-dependent integral type named iostate，该类型有四个值：
- badbit：indicates a system-level failure, such as an unrecoverable read or write error.
- failbit：is set after a recoverable error, such as reading a character when numeric data was expected.
- eofbit：reaching end-of -file sets both eofbit and failbit.
- goodbit：guaranteed to have the value 0, indicates no failures on the stream.

PS：If any of badbit, failbit, or eofbit are set, then a condition that evaluates that stream will fail.

栗子

// turns off failbit and badbit but all other bits unchanged
cin.clear(cin.rdstate() & ~cin.failbit & ~cin.badbit);

output buffer

buffer的作用：Using a buffer allows the operating system to combine several output operations from our program into a single system-level write，从而提升性能

flush buffer（即buffer中内容全部输出）的时机：

The program completes normally. All output buffers are flushed as part of the return from main
buffer满了之后will be flushed before writing the next value
We can flush the buffer explicitly using a manipulator

cout << "hi!" << endl; // writes hi and a newline, then flushes the buffer
cout << "hi!" << flush; // writes hi, then flushes the buffer; adds no data
cout << "hi!" << ends; // writes hi and a null, then flushes the buffer

可以用unitbuf把stream的state设置为empty the buffer after each output operation：By default, unitbuf is set for cerr, so that writes to cerr are flushed immediately

cout << unitbuf; // all writes will be flushed immediately
cout << nounitbuf; // returns to normal buffering

the buffer of the tied stream is flushed whenever the tied stream is read or written.

By default, cin and cerr are both tied to cout. Hence, reading cin or writing to cerr flushes the buffer in cout.

cin >> ival; // causes the buffer associated with cout to be flushed.

We can tie either an istream or an ostream object to another ostream；一个istream/ostream只能 be tied to 一个ostream

ostream *p = cin.tie(); // p points to the stream (if any) currently tied to cin, 若无则p = nullptr
cin.tie(&cout); // illustration only: the library ties cin and cout for us
// old_tie points to the stream (if any) currently tied to cin
ostream *old_tie = cin.tie(nullptr); // cin is no longer tied
cin.tie(&cerr); // reading cin flushes cerr, not cout
cin.tie(old_tie); // reestablish normal tie between cin and cout

PS：Output buffers are not flushed if the program terminates abnormally.

文件I/O

file I/O types

ifstream：read from a given file
ofstream：write to a given file
fstream：reads and writes a given file
PS：这些类型继承了iostream types的操作，比如<< >> getline等；所有可以使用iostream types的地方都可以使用对应的file I/O types

fstream-Specific Operations

除了从iostream继承的操作，file I/O types还支持以下操作：

定义file I/O对象

栗子（filename可以是string or C-style character array）

ifstream in(ifile); // construct an ifstream and open the given file
ofstream out; // output file stream that is not associated with any file

open和close

open
- 用法
```
ofstream out; // output file stream that is not associated with any file
out.open(ifile + ".copy"); // open the specified file
if (out) { ... }
```
- Once a file stream has been opened, it remains associated with the specified file
- calling open on a file stream that is already open will fail and set failbit
close
- 用法
```
in.close(); // close the file
in.open(ifile + "2"); // open another file
```
- To associate a file stream with a different file, we must first close the existing file. Once the file is closed, we can open a new one
- When an fstream object is destroyed, close is called automatically

File Mode

out may be set only for an ofstream or fstream object
in may be set only for an ifstream or fstream object
trunc may be set only when out is also specified
app mode may be specified so long as trunc is not. If app is specified, the file is always opened in output mode, even if out was not explicitly specified
By default, a file opened in out mode is truncated even if we do not specify
trunc. By default, when we open an ofstream, the contents of the file are discarded
To preserve the contents of a file opened with out, either we must also specify app, in which case we can write only at the end of the file, or we must also specify in, in which case the file is open for both input and output
The ate and binary modes may be specified on any file stream object type
and in combination with any other file modes
默认模式：Files associated with an ifstream are opened in in mode; files associated with an ofstream are opened in out mode; and files associated with an fstream are opened with both in and out modes
栗子

// file1 is truncated in each of these cases
ofstream out("file1"); // out and trunc are implicit
ofstream out2("file1", ofstream::out); // trunc is implicit
ofstream out3("file1", ofstream::out | ofstream::trunc);
// to preserve the file's contents, we must explicitly specify app mode
ofstream app("file2", ofstream::app); // out is implicit
ofstream app2("file2", ofstream::out | ofstream::app);

string Streams

特点

in-memory IO
read from or write to a string as if the string were an IO stream

类型

istringstream：reads a string
ostringstream：writes a string
stringstream：reads and writes the string
PS：这些类型继承了iostream types的操作，比如<< >> getline等；所有可以使用iostream types的地方都可以使用对应的string streams types

sstream-Specific Operations

除了从iostream继承的操作，string streams types还支持以下操作：

栗子

istringstream
要读的文件如下
morgan 2015552368 8625550123
drew 9735550130
lee 6095550132 2015550175 8005550000

struct PersonInfo {
  string name;
  vector<string> phones;
};

string line, word;
vector<PersonInfo> people; // will hold all the records from the input
// read the input a line at a time until cin hits end-of-file (or another error)
while (getline(cin, line)) {
  PersonInfo info; // create an object to hold this record's data
  istringstream record(line); // bind record to the line we just read
  record >> info.name; // read the name
  while (record >> word) // read the phone numbers
    info.phones.push_back(word); // and store them
  people.push_back(info); // append this record to people
  }

ostringstream

for (const auto &entry : people) { // for each entry in people
  ostringstream formatted, badNums; // objects created on each loop
  for (const auto &nums : entry.phones) { // for each number
    if (!valid(nums)) {
      badNums << " " << nums; // string in badNums
    } else
    // ''writes'' to formatted's string
    formatted << " " << format(nums);
  }
  if (badNums.str().empty()) // there were no bad numbers
    os << entry.name << " " // print the name
        << formatted.str() << endl; // and reformatted numbers
  else // otherwise, print the name and bad numbers
    cerr << "input error: " << entry.name
            << " invalid number(s) " << badNums.str() << endl;
}

C++ primer 第八章-I/O library