Lecture 1&2:Streams

The reason why we use streams is we often want our programs to interact with external devices.

Stringstream¶

一、stringstream 是什么？

stringstream 是 C++ 标准库 <sstream> 中的一个类， 它是一个“基于字符串的流（stream）”。

换句话说：

它把字符串封装成一个 虚拟的输入/输出设备，

让你能像操作 cin、cout 一样去“读写”这个字符串。

ostringstream¶

#include<iostream> // for cout
#include<sstream> // for ostringstream

using namespace std;

int  main () {
    ostringstream oss("Ito En Green Tee");
    cout << oss.str() << endl;
    return 0;
}
// Ito En Green Tee

#include<iostream>
#include<sstream>

using namespace std;

int  main () {
    ostringstream oss("Ito En Green Tee");
    oss << 16.9 << "Out ";
    cout << oss.str() << endl;
    return 0;
}

// 16.9Out reen Tee

#include<iostream>
#include<sstream>

using namespace std;

int  main () {
    ostringstream oss("Ito En Green Tee", stringstream::ate);
    oss << 16.9 << " Out ";
    cout << oss.str() << endl;
    return 0;
}
// Ito En Green Tee16.9 Out

ostringstream is output string stream, which can write the given thing into a string just like writing into a file.

oss is an object in ostringstrea. Like cout, we can use << to add thing into oss. But the target of oss is a string buffer in memory but cout's target is the console.

oss.str() return the string in the buffer.

istringstream¶

#include <iostream>
#include <sstream>
#include <string>

using namespace std;

int main() {
    ostringstream oss("420 3.14", ostringstream::ate);
    oss << "Hello";

    istringstream iss(oss.str());

    int a;
    double b;
    string c;

    iss >> a >> b >> c;

    cout << "a = " << a << endl;
    cout << "b = " << b << endl;
    cout << "c = " << c << endl;

    return 0;
}

>> will automatically parse the data in the string according to the variable type

iss is just like cin

stringstream positioning functions¶

#include <iostream>
#include <sstream>

using namespace std;

int main() {
    ostringstream oss("Ount str 4200 3.14");
    oss << "Hello";

    fpos pos = oss.tellp() + streamoff(4);
    oss.seekp(pos);
    oss << "test";

    cout << oss.str() << endl;
    return 0;
}

fpos is a kind representing the position. and pos is a varible

Must use streamoff(4), instead of directly 4 because it is not int

We can put negtive number in streamoff() like streamoff(-4), just try it!

State bits¶

Indicate the state of the stream.

Check the buffer state before and after the After the function, if not Fail or EOF, the buffer is cleared after the program.

good(), bad(), fail(), eof(): functions of stream object whose return value is boolean. Like cin.good(), iss.bad(), oss.fail(), cout.eof().

#include <iostream>
#include <sstream>
#include <string>
#include <stack>
using namespace std;

string simplifyPath(const string& path) {
    stringstream ss(path);
    string token;
    stack<string> st;

    while (getline(ss, token, '/')) {  // 以 '/' 分割
        if (token == "" || token == ".") continue;
        if (token == ".." && !st.empty()) st.pop();
        else if (token != "..") st.push(token);
    }

    string result = "/";
    while (!st.empty()) {
        result = "/" + st.top() + result;
        st.pop();
    }
    return result;
}

int main() {
    cout << simplifyPath("/./a/b/..") << endl;  // 输出：/a
}

#include <iostream>
#include <sstream>
#include <iomanip>  // manipulators
using namespace std;

int main() {
    stringstream ss;
    ss << uppercase << hex << 255; // 转成十六进制大写格式
    cout << ss.str() << endl;      // 输出 "FF"
}

// ❌ 不推荐
stringstream ss;
ss << "Hello " << name << "!";
auto s = ss.str();

// ✅ 更快
string s = "Hello " + name + "!";

#include <iostream>
#include <sstream>
#include <string>
using namespace std;

int stringToInteger(const string& str) {
    stringstream ss(str);
    int value;
    ss >> value;
    return value;
}

int main() {
    cout << stringToInteger("123") + 10 << endl; // 输出 133
}

iostream¶

Including cin, cout, cerr, clog.

The last three are all output stream.

cout¶

It does not directly return the thing into the console, instead, it return it into a internal buffer which need to be flushed into the console , and, the endl we use equals to \n plus a flush.

cin¶

18 Alice⏎

[ eback() ........ gptr() ........ egptr() ]

eback()
|
v
[ '1' ][ '8' ][ ' ' ][ 'A' ][ 'l' ][ 'i' ][ 'c' ][ 'e' ][ '\n' ]
        ^
        gptr()
                            ^
                            egptr()

[ '1' ][ '8' ][ ' ' ][ 'A' ][ 'l' ][ 'i' ][ 'c' ][ 'e' ][ '\n' ]
                ^
                gptr()

[ '1' ][ '8' ][ ' ' ][ 'A' ][ 'l' ][ 'i' ][ 'c' ][ 'e' ][ '\n' ]
                                                    ^
                                                    gptr()

int x;
cin >> x;   // 期待数字

abc⏎

cin.fail()   // true
cin.good()   // false
cin.eof()    // false
cin.bad()    // false

So, if the buffer is not empty, things left in the buffer

getline¶

Way to get a whole line:

getline : read a whole line back to \n(read up to \n) but not including '\n' in the string.

Function prototype:

istream& getline (istream& is, string& str, char delim);

• is : input stream object, like cin, iss
• str : string variable to store the line
• delim : delimiter, default is '\n'

Example:

getline(cin, name, '\n');

cin.ignore()

Function prototype:

// Situation 1: ignore one character (default)
cin.ignore();

// Situation 2: ignore specified(n) number of characters, or until specified delimiter is found
cin.ignore(streamsize n, int delim);