【C++ Primer】【学习笔记】【第八章】标准IO库之：字符串流

╰+哭是因爲堅強的太久メ 2022-08-06 15:04 170阅读 0赞

**一、stringstream对象的使用**

**1、sstream头文件定义了三种类型的字符串流：**

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      类型 
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      说明 
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   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:463.7999755859375px"> 从istream派生而来，提供读string的功能；</td> 
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      ostringstream 
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      从ostream派生而来，提供写string的功能； 
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      stringstream 
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      从iostream派生而来，提供读写string的功能。 
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**2、与iostream类型相比，stringstream增加了如下几个操作：**

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      说明 
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      stringstream strm; 
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      创建自由的 
     <span style="font-family:微软雅黑; font-size:14px">stringstream对象；</span> 
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     <span style="font-family:微软雅黑; font-size:14px">stringstream strm(s);</span> 
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     <span style="font-family:微软雅黑; font-size:14px">创建存储s副本的</span> 
     <span style="font-family:微软雅黑; font-size:14px">stringstream对象，其中s是string类型的对象；</span> 
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   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:279px"> strm.str();</td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:279px"> 返回strm中存储的string类型对象；<br style="background-color:inherit"> </td> 
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      strm.str(s); 
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      将string类型的s复制给strm，返回void。 
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**3、应用：逐行读取并逐个处理每行中的单词：**

string line, word;
    while (getline(cin, line))
    {
        istringstream stream(line);
        while (stream >> word)
        {
            //...
        }
    }

**4、应用：数值型数据转换为字符串型数据**

int val1 = 512, val2 = 1024;
    ostringstream format_message;
    format_message << "val1: " << val1 << "\n"
                   << "val2: " << val2 << "\n";

**5、应用：字符串型数据转换为数值型数据**

基于上面的处理：

istringstream input_istring(format_message.str());
    string dump;
    input_istring >> dump >> val1 >> dump >> val2;
    cout << val1 << " " << val2 << endl;

注：使用输入操作符读取string时，空白符会被忽略。

**习题8.15：读入数据至istringstream对象并输出**

#include <iostream>
    #include <sstream>
    #include <stdexcept>
    using namespace std;
    istream& read_stream(istream& in)
    {
        int ival;
        while (in >> ival, !in.eof())
        {
            if (in.bad())
            {
                throw runtime_error("IO stream corrupted!");
            }
            if (in.fail())
            {
                cout << "Read fail, try again." << endl;
                in.clear();
                in.ignore(2, ' ');
                continue;
            }
            // ival is ok, process it
            cout << ival << " ";
        }
        cout << endl;
        in.clear();
        return in;
    }
    int main()
    {
        string line;
        cout << "Enter a line of text: " << endl;
        getline(cin, line);
        line += " ";
        istringstream isstr(line);
        read_stream(isstr);
        return 0;
    }

[chapter8]$ ./a.out 
    Enter a line of text: 
    123 456 789 
    123 456 789

**习题8.16：将文件存储到vector<string>后按单词从容器中读取**

/// 1. 读取文件并按行存储到vector<string>当中
    /// 2. 使用istringstream从vector<string>中读取行中的每个单词
    
    #include <iostream>
    #include <fstream>
    #include <sstream>
    #include <stdexcept>
    #include <string>
    #include <vector>
    using namespace std;
    
    int file_to_vector(const string &filename, vector<string> &svec)
    {
        ifstream infile(filename.c_str());
        if (!infile)
        {
            cout << "Error: open file(" << filename << ") fail." << endl;
            return 1;
        }
    
        string s;
        while (getline(infile, s))
        {
            svec.push_back(s);
        }
    
        infile.close();
       
        if (infile.eof())
        {
            cout << "OK: read to eof." << endl;
            return 0;
        }
    
        if (infile.bad())
        {
            cout << "Error: system fail." << endl;
            return 2;
        }
    
        if (infile.fail())
        {
            cout << "Error: read fail." << endl;
            return 3;
        }
    
        return 0;
    }
    
    int main()
    {
        string filename;
        cout << "Enter filename: " << endl;
        cin >> filename;
    
        vector<string> svec;
        if (0 != file_to_vector(filename.c_str(), svec))
        {
            cout << "Error: file_to_vector() return fail." << endl;
            return -1;
        }
    
        cout << "vector's size: " << svec.size() << endl;
        cout << "vector:" << endl;
        for (vector<string>::iterator it = svec.begin(); it != svec.end(); ++it)
        {
            cout << *it << endl;
        }
    
        string word;
        int line_num = 0;
        istringstream isstream;
        for (vector<string>::iterator iter = svec.begin(); iter != svec.end(); ++iter)
        {
            cout << "line-" << ++line_num << ": " << endl;
            isstream.str(*iter);
            while (isstream >> word)
            {
                cout << word << endl;
            }
            isstream.clear();
        }
    
        return 0;
    }

[chapter8]$ ./a.out
    Enter filename:
    testfile
    OK: read to eof.
    vector's size: 3
    vector:
    line1: How are you.
    line2: How old are you.
    line3: I'm OK.
    line-1:
    line1:
    How
    are
    you.
    line-2:
    line2:
    How
    old
    are
    you.
    line-3:
    line3:
    I'm
    OK.