C++: Introduction to iostream

April 15, 2015 6 minute read

整理自：Thinking in C++, Volume 2 & C++ Primer, 5th Edition

1. Getting Started

1.1 The iostream library

Fundamental to the <iostream> library are two types named istream and ostream, which represent input and output streams, respectively. A stream is a sequence of characters read from or written to an IO device. The term stream is intended to suggest that the characters are generated, or consumed, sequentially over time.

The <iostream> library defines four IO objects.

To handle input, we use an object of type istream named cin (pronounced see-in). This object is also referred to as the standard input.
For output, we use an ostream object named cout (pronounced see-out；short for “console output”). This object is also known as the standard output.
The library also defines two other ostream objects, named cerr and clog (pronounced see-err and see-log, respectively). We typically use cerr, referred to as the standard error, for warning and error messages and clog for general information about the execution of the program.

Ordinarily, the system associates each of these objects with the window in which the program is executed. So, when we read from cin, data are read from the window in which the program is executing, and when we write to cout, cerr, or clog, the output is written to the same window.

1.2 Operator Precedence

std::cout << "Enter two numbers:" << std::endl;

The output operator << takes two operands: The left-hand operand must be an ostream object; the right-hand operand is a value to print. The operator writes the given value on the given ostream. The result of the output operator is its left-hand operand. That is, the result is the ostream on which we wrote the given value.

Thus, our expression is equivalent to

(std::cout << "Enter two numbers:") << std::endl;

std::cout << "Enter two numbers:";
std::cout << std::endl;

The input operator >> behaves analogously to the output operator.

1.3 Flushing

Writing endl has the effect of ending the current line and flushing the buffer associated with that device. Flushing the buffer ensures that all the output the program has generated so far is actually written to the output stream, rather than sitting in memory waiting to be written.

Programmers often add print statements during debugging. Such statements should always flush the stream. Otherwise, if the program crashes, output may be left in the buffer, leading to incorrect inferences about where the program crashed.

By default, reading cin flushes cout; cout is also flushed when the program ends normally.

1.4 Stream as a condition

// read until end-of-file, calculating a running total of all values read
while (std::cin >> value)
	sum += value;

When we use an istream as a condition, the effect is to test the state of the stream. If the stream is valid—that is, if the stream hasn’t encountered an error—then the test succeeds. An istream becomes invalid when we hit end-of-file or encounter an invalid input, such as reading a value that is not an integer. An istream that is in an invalid state will cause the condition to yield false.

On Windows systems we enter an end-of-file by typing a “ctrl + z” followed by hitting either the Enter or Return key. On UNIX systems, including on Mac OS X machines, end-of-file is usually “ctrl + d”.

1.5 Redirection Command

> addItems <infile >outfile

Assuming our addition program has been compiled into an executable file named “addItems.exe” (or “addItems” on UNIX systems), this command will (force std::cin to) read transactions from a file named infile and (force std::cout to) write its output to a file named outfile in the current directory.

2. Inserters & extractors

ostream::operator<< is often referred to as an inserter, and istream::operator>> is often referred to as an extractor.

Consider how to output the representation of a Date object in MM-DD-YYYY format:

ostream& operator<<(ostream& os, const Date& d) {
	// 设置 fill 模式为首位填 0
	char fillc = os.fill('0'); 
	os << setw(2) << d.getMonth() << '-'
		<< setw(2) << d.getDay() << '-'
		<< setw(4) << setfill(fillc) << d.getYear();		
		// 对年份不需要做填充，setfill(fillc) 恢复原 fill 模式
		// setfill(fillc) 返回一个 manipulator，类似 endl，所有可以直接接到 << 后面
	
	return os;
}

Extractors require a little more care because things can go wrong with input data. The way to signal a stream error is to set the stream’s fail bit, as follows:

istream& operator>>(istream& is, Date& d) {
	is >> d.month;
	
	char dash;
	is >> dash;
	if(dash != '-')
		is.setstate(ios::failbit);
	
	is >> d.day;
	
	is >> dash;
	if(dash != '-')
		is.setstate(ios::failbit);
	
	is >> d.year;
	
	return is;
}

When an error bit is set in a stream, all further streams operations are ignored until the stream is restored to a good state. 所以这里不需要用异常或是用 if-else 使得出错后跳过后续的某些操作，你正常写就好了. This implementation is somewhat forgiving in that it allows white space between the numbers and dashes in a date string because operator>> skips white space by default when reading built-in types.

In addition, formatted input defaults to white space delimiters. 比如 int i; char c; cin >> i; cin >> c;，你输入 47 x，那就是 i == 47 和 c == 'x'；如果你输入 4 7 x，那就是 i == 4、c == '7'，x 会被忽略。

Digress:

如果不想 include 整个 <iostream>、要用 forward declaration 的话，可以用 #include <iosfwd>。

3. Handling stream errors

3.1 Stream state

Stream state	Meaning
badbit	Some fatal (perhaps physical) error occurred. The stream should be considered unusable.
eofbit	End-of-input has occurred (either by encountering the physical end of a file stream or by the user terminating a console stream, such as with Ctrl-Z or Ctrl!D).
failbit	An I/O operation failed, most likely because of invalid data (e.g., letters were found when trying to read a number). The stream is still usable. The failbit flag is also set when end-of-input occurs.
goodbit	All is well; no errors. End-of-input has not yet occurred.

对应有 stream::bad()、stream::eof()、stream::fail()、stream::good() 四个方法，都是返回 true/false 表示是否处于该状态。但是需要额外注意 stream::fail() 是 returns true if either failbit or badbit is set.

Once any of the error bits in a stream’s state are set, they remain set. 如果要清除的话，需要用 stream::clear()：

myStream.clear(); 							// clear all error bits
myStream.clear(ios::failbit | ios::eofbit); // clear failbit and eofbit

3.2 Streams and exceptions

可以使用 stream::exception() 方法来注册一个 exception state，当出现这个状态的时候就抛一个异常，一般为 std::ios_base::failure。比如：

std::ifstream file;
file.exceptions(std::ifstream::failbit | std::ifstream::badbit);

The exceptions() function returns a bitmask of type iostate (which is a compiler-dependent type convertible to int) indicating which stream states will cause exceptions. If those states have already been set, an exception is thrown immediately.

不过实践中，貌似大家都不喜欢对读写操作写 try-catch（Java IO 一写就是一坨 try-catch 也是蛮烦的），所以一般的做法是手动检查 stream state。

4. Manipulators

Manipulators with no arguments are provided in <iostream>, including:

There are six standard manipulators that take arguments. These are defined in the header file <iomanip>:

Manipulator	Effect
setiosflags(fmtflags n)	等价于 `ios::setf(n)`
resetiosflags(fmtflags n)	等价于 `ios::unsetf(n)`
setbase(base n)	Changes base to n, where n is 10, 8, or 16. (Anything else results in 10.)
setfill(char n)	等价于 `ios::fill(n)`，设置填充字符
setprecision(int n)	等价于 `ios::precision(n)`，设置精度
setw(int n)	等价于 `ios::width(n)`，设置字符串宽度

Creating manipulators

我们先看下 endl 的构造：

ostream& endl(ostream&);

然后 cout << endl; 里实际是传的这个 function name，也就是 function 的地址。然后 operator<< 的逻辑大概是（The actual definition is a little more complicated since it involves templates）：

ostream& ostream::operator<<(ostream& (*pf)(ostream&)) {
	return pf(*this);
}

所以我们只要参照 endl 函数的签名，自己做一个 manipulator 也不是很难，比如：

#include <iostream>
using namespace std;

ostream& nl(ostream& os) { // 'nl' for 'newline'
	return os << '\n';
}

int main() {
	cout << "newlines" << nl;
}

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