c++文件操作

#include <iostream>
#include <fstream>
#include <algorithm>
#include <string>
#include <vector>
using namespace std;

void ex1_xtra2()
{
    //输入一个文件名称
    string file_name;
    cout << "Please enter a file to be opened: (try input.txt or text.txt) ";
    cin >> file_name;

    //判断是否为空
    if ( ! cin || file_name.empty() )
    {
        cerr << "oops! unable to read file name
"; return;
    }

    //建立一个读取的ifile，由于读取 input.txt中的内容
　　
    ifstream ifile( file_name.c_str() );

    if ( ! ifile )
    {
        cerr << "oops! unable to open input file: "
             << file_name << endl;
        return;
    }
    else
    {
        cerr << "!! ok: opened " << file_name << " for input
";
    }

    //新建一个input.txt.sort文件，用于保存排好序的文字
    //类型是输出ofile
    file_name += ".sort";
    ofstream ofile( file_name.c_str() );

    if ( ! ofile )
    {
        cerr << "oops! unable to open output file: "
             << file_name << endl;
        return;
    }
    else
    {
        cerr << "!! ok: opened " << file_name << " for output
";
    }


    string word;
    vector< string > text;
    //读取内容到word至文件末尾
    while ( ifile >> word )
    {
        //插入到容器text中
        text.push_back( word );
    }

    if ( text.empty() )
    {
        cerr << "bummer! input file is empty: bailing out
";
        return;
    }
    else
    {
        cerr << "!! ok: read " << text.size() << " strings from input
";
    }
    //排序操作
    sort( text.begin(), text.end() );

    int cnt = 0;
    for ( vector<string>::iterator iter = text.begin();
            iter != text.end(); ++iter )
    {
        //排列格式
        cnt += iter->size() + 1;
        if ( cnt > 40 )
        {
            ofile << '
'; 
            cnt = 0;
        }
        //输出到ofile中，即input.txt.sort文件中
        ofile << *iter << ' ';
    }

    cout << "ok: wrote sorted strings into " << file_name << endl;
}

int main(void)
{
    ex1_xtra2();
    return 0;
}

　　intput.txt

Alice Emma has long flowing red hair.  
Her Daddy says when the wind blows 
through her hair, it looks almost alive, 
like a fiery bird in flight.  A beautiful 
fiery bird, he tells her, magical but untamed.  
``Daddy, shush, there is no such thing,'' she 
tells him, at the same time wanting him to 
tell her more.  Shyly, she asks, ``I mean, 
Daddy, is there?''

　　input.txt.sort

A Alice Daddy Daddy, Emma Her Shyly, 
``Daddy, ``I a alive, almost asks, at beautiful 
bird bird, blows but fiery fiery flight. 
flowing hair, hair. has he her her her, him 
him, in is is it like long looks magical 
mean, more. no red same says she she shush, 
such tell tells tells the the there there?'' 
thing,'' through time to untamed. wanting when 
wind 

　　c_str()函数返回一个指向正规C字符串的指针常量, 内容与本string串相同. 

解决的方法有两种：

1.用c_str()函数，下面详细介绍。

2.包含头文件"string"

 

下面我们进入正题，请出我们的今天的主角  c_str()    他是一个函数哦。。。不要忘记了括号。。

语法: 
const char *c_str();
c_str()函数返回一个指向正规C字符串的指针常量, 内容与本string串相同. 
这是为了与c语言兼容，在c语言中没有string类型，故必须通过string类对象的成员函数c_str()把string 对象转换成c中的字符串样式。
注意：一定要使用strcpy()函数 等来操作方法c_str()返回的指针 
比如：最好不要这样: 
char* c; 
string s="1234"; 
c = s.c_str(); //c最后指向的内容是垃圾，因为s对象被析构，其内容被处理，同时，编译器也将报错——将一个const char *赋与一个char *。

应该这样用： 
char c[20]; 
string s="1234"; 
strcpy(c,s.c_str()); 
这样才不会出错，c_str()返回的是一个临时指针，不能对其进行操作

再举个例子
c_str() 以 char* 形式传回 string 内含字符串
如果一个函数要求char*参数，可以使用c_str()方法： 
string s = "Hello World!";
printf("%s", s.c_str()); //输出 "Hello World!"