Table of content#

• Operator overload
  • ostream
  • istream
  • Overload >>
  • Overload <<
• Stream
  • stream
  • String stream
  • File stream

Streams#

• Streams are a very central and powerful concept in C++
• 流是 C++中一个非常核心且强大的概念
• Screen/keyboard, files, network communication, or just memory
• 屏幕/键盘，文件，网络通信，或者仅仅是内存
• Standard library provides three essential parts
• 标准库提供三个基本部分
• Generic interfaces / templates
• 通用接口/模板
• Standard implementations
• 标准实现
• Common operators that can operate on all streams
• 所有流都可以操作的常见运算符

• You know cin and cout already
• 您已经知道 cin 和 cout
• They implement the basic input-stream and output-stream
• 它们实现了基本的输入流和输出流

• The operator << is one of the general manipulator for streams
• 运算符 << 是流的一般操纵器之一

<<：向左移位符

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#include <iostream>
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int main() {
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    std::cout<< "Hello, World!" << std::endl;
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    return 0;
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}

This one has the signature
ostream& operator<< (ostream& os, const string& str);
It is defined in <string>

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std::cout << "Hello, World!" << std::endl;

↓

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ostream& operator<< (ostream& os, const string& str)

First parameter: Represent the output stream object which exist at the left side of operator <<
第一个参数：代表位于操作符 << 左侧的输出流对象
second parameter: Represent the const string which exist at the right side of operator <<
第二个参数：表示位于操作符 << 右侧的 const 字符串
how to use operator <<
如何使用操作符 <<
Return type: Output stream object reference
返回类型：输出流对象引用

TIP

argument: 实参
parameter: 形参

NOTE

双目运算符
三目运算符 condition ? expression1 : expression2;

• The operator >> is another one of the general manipulator for streams
• 操作符 >> 是流的一般操作符之一

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double x;
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int n;
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std::cin >> x >> n;

↓

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istream& operator>> (istream& is, double& val);
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istream& operator>> (istream& is, int& val);

Return type: Output stream object reference
返回类型：输出流对象引用
how to use operator >>
如何使用操作符 >> second parameter: Represent the const string which exist at the right side of operator >>
第二个参数：表示操作符 >> 右侧存在的常量字符串

Operator overloading#

• cin is the object of istream class
• cin 是 istream 类的对象
• cout is the object of ostream class
• cout 是 ostream 类的对象

• Overload one operator by using following format
• 使用以下格式重载一个运算符

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operator <op> (……)

The following operators can be given custom definitions:
以下运算符可以赋予自定义定义：

• + - * / % ? ++ -- += -= *= /= %=
• && || & | ~ ! &= |=
• < > << >> >>= <<= == != <= >=
• ->* -> ( ) [ ]
• = new delete

WARNING

Don’t overload ->* -> new delete

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#include <iostream>
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using namespace std;
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class MyInteger{
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public:
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    MyInteger(int64_t iP) : i(iP) {}
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    int64_t i;
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};
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ostream& operator<<(ostream& os, const MyInteger &num){
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    os << num.i;
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    return os;
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}
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int main () {
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    MyInteger mi(6);
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    cout << mi;
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    return 0;
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}

Back to Stream Part#

• istream means in→stream
• ostream means out→stream
• iostream means in and out →stream

String Stream#

• Another stream is the stringstream
• 另一个流是 stringstream
• A stringstream represents a string
• 一个 stringstream 表示一个字符串
• Can be modified by << and >>
• 可以通过 << 和 >> 进行修改

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#include <string> // std::string
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#include <iostream> // std::cout
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#include <sstream> // std::stringstream, std::stringbuf
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int main () {
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    std::stringstream ss("Example string");
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    std::string s = ss.str(); // get current string
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    std::cout << s << std::endl; // Example string
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    return 0;
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}

Example: stringstream1

• A stringstream represents a string
• Can be modified by << and >>

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#include <string> // std::string
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#include <iostream> // std::cout
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#include <sstream> // std::stringstream, std::stringbuf
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int main () {
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    std::stringstream ss("120*3.5");
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    int n;
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    char c;
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    double x;
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    ss >> n >> c >> x;
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    std::cout << "(int)n=" << n << std::endl; // 120
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    std::cout << "(char)c=" << c << std::endl; // *
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    std::cout << "(double)x=" << x << std::endl; // 3.5
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    return 0;
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}

Example: stringstream2

File Stream#

• Files, too, are represented and manipulated by such streams
• 同样地，文件也通过此类流进行表示和处理
• ofstream: Writable file (output-file-stream)
• ofstream：可写文件（输出文件流）

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#include <fstream> // std::fstream
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int main () {
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    std::ofstream myFile("myfile.txt");
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//create one myfile.txt file as object myFile
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    myFile << "Hello, World!";
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    myFile.close();
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    return 0;
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}

Example: filestream1

• Input file streams (ifstream)
• 输入文件流（ifstream）
• Can read formatted input (int, double) with operator, but other methods possible
• 可以使用操作符读取格式化输入（int，double），但还有其他方法可行

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#include <iostream>
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#include <fstream> // std::fstream
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int main () {
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    std::ifstream myFile("myfile.txt");
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//myfile.txt already existed in folder,it be input to myFile object
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    std::string content;
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    myFile >> content;
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    std::cout << content;
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    myFile.close();
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    return 0;
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}

Example: filestream2

• Input file streams (ifstream)

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#include <iostream>
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#include <fstream> // std::fstream
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int main () {
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    std::ifstream myFile("myfile.txt");
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    std::string content;
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    getline(myFile, content);
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    std::cout << content;
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    myFile.close();
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    return 0;
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}

Example: filestream3

• Structured input can go wrong!
• 结构化输入可能会出错！

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#include <iostream>
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#include <fstream> // std::fstream
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int main () {
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    std::ifstream myFile("myfile.txt");
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    double x;
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    myFile >> x;
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    std::cout << "Read number " << x;
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    myFile.close();
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    return 0;
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}

Example: filestream4

• Errors due to content / logical errors
• 由于内容/逻辑错误导致的错误
• Test with fail()
• 使用 fail() 进行测试

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#include <iostream>
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#include <fstream> // std::fstream
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int main () {
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    std::ifstream myFile("myfile.txt");
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    double x;
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    myFile >> x;
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    if(myFile.fail()){
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        std::cout << "Could not read number" << std::endl;
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    }
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    std::cout << "Number is " << x;
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    myFile.close();
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    return 0;
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}

Example: filestream5

Stream states#

• Streams can be in various states
• 流可以处于各种状态
• Any error flags can be cleared with clear()
• 任何错误标志都可以通过 clear() 清除
• Since fail() depends on application logic, it is good practice to set it in custom operators if an unexpected input occurs
• 由于 fail() 依赖于应用程序逻辑，如果出现意外输入，在自定义算子中设置它是良好的实践
• Always watch this flag(s) when reading structured input
• 始终在读取结构化输入时关注此标志（或标志组）

Stream access#

• Many different (unstructured) I/O methods commonly available
• 许多不同的（非结构化）I/O 方法通常可用
• Common for output streams:
• 对于输出流：

Common for input streams:
对于输入流：

Summary#

• Can encode to overload operator
• Can use
  • stringstream object
  • ofstream object
  • ifstream object