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ios_base类会存储输出格式状态的信息（比如 输出间隔，小数点位数等） ios继承于ios_base ostream继承于ios

格式设置函数

hex()十六进制

oct()八进制

dec()十进制

abc()一万进制（骗你的）

使用的时候可以直接这样： cout << hex; 这样就设置输出为16进制的了

程序示例

#include 
   
    int main(){  using namespace std;  cout << "Enter an integer: ";  int n;  cin >> n;  cout << "n    n*n\n";  cout << n << "    " << n * n << "  (decimal)\n";  cout << hex;  cout << n << "    ";  cout << n * n << "  (hexadecimal)\n";  cout << oct << n << "    " << n * n << "  (octal)\n";  dec(cout);  cout << n << "    " << n * n << "  (decimal)\n";  return 0;}
   

调整字段宽度

width（）返回当前字段宽度

width(int i ) 设置指端宽度（i个空格）并且返回以前的字段宽度

只影响下一个项目 也就是说 输出

cout.width(12)

cout << 12 << 22 << '#';

这时候 宽度只有在12的前面显示。然后就1222#了  它不会一直持续保持宽度 一次性的

width() 是成员函数

程序示例

#include 
   
    int main(){  using std::cout;  int w = cout.width(30);  cout << "default field width = " << w << ":\n";  cout.width(5);  cout << "N" << ':';  cout.width(8);  cout << "N * N" << ":\n";  for(long i = 1; i <= 100; i *= 10)  {    cout.width(5);    cout << i << ':';    cout.width(8);    cout << i * i << ":\n";  }  return 0;}
   

字符填充

fill() 参数为指定用哪个字符填充未被使用的部分（只能是字符，不能使字符串）

#include 
   
    int main(){  using std::cout;  cout.fill('*');  const char* staff[2] = {"waldo Whipsnade", "Wilmarie Wooper"};  long bonus[2] = {900, 1350};  for (int i = 0; i < 2; i++)  {    cout << staff[i] << ": $";    cout.width(7);    cout << bonus[i] << "\n";  }  return 0;}
   

fill()一直有效 不是一次性的

 

设置显示位数 precision（） 是整体位数不是小数点位数哦

#include 
   
    int main(){  using std::cout;  float price1 = 20.40;  float price2 = 1.9 + 8.0 / 9.0;  cout << "\"Furry Friends\" is $" << price1 << "!\n";  cout << "\"Furry Friends\" is $" << price2 << "!\n";  cout.precision(2);  cout << "\"Furry Friends\" is $" << price1 << "!\n";  cout << "\"Furry Friends\" is $" << price2 << "!\n";  return 0;}
   

setf()设置末尾的0和小数点

定义了多个空值显示的多个常量

cout.setf(ios_base::showpoint（静态常量）) 显示6位数（包含整数部分）并显示末尾的0

#include 
   
    int main(){  using std::cout;  using std::ios_base;  float price1 = 20.40;  float price2 = 1.9 + 8.0 / 9.0;  cout.setf(ios_base::showpoint);  cout << "\"Furry Friends\" is $" << price1 << "!\n";  cout << "\"Furry Friends\" is $" << price2 << "!\n";  cout.precision(2);  cout << "\"Furry Friends\" is $" << price1 << "!\n";  cout << "\"Furry Friends\" is $" << price2 << "!\n";  return 0;}
   

第三行因为设置了位数 所以只能显示最前面的两位

每一位有特定的定义 不用去记哪一位的开关是啥作用的

setf()有两个原型

第一个 fmtflags setf(fmtflags)

程序示例

#include 
   
    int main(){  using std::cout;  using std::endl;  using std::ios_base;  int temperature = 63;  cout << "Today's water temperature: ";// 显示+号 10进制  cout.setf(ios_base::showpos);  cout << temperature << endl;  cout << "Por our programming freinds, that's\n";// 显示十六进制  cout << std::hex << temperature << endl;// 十六进制大写并且显示C风格标识  cout.setf(ios_base::uppercase);  cout.setf(ios_base::showbase);  cout << "or\n";  cout << temperature << endl;  cout << "How " << true << "!  oops -- How ";// 显示布尔值  cout.setf(ios_base::boolalpha);  cout << true << "!\n";  return 0;}
   

hex() 等于cout.setf(ios_base::hex, ios_base::basefield)

程序示例

#include 
   
    #include 
    
     int main(){  using namespace std;// 左对齐  cout.setf(ios_base::left, ios_base::adjustfield);// 显示+号  cout.setf(ios_base::showpos);// 显示小数点  cout.setf(ios_base::showpoint);// 三位数  cout.precision(3);// 科学计数法   ios_base::fmtflags old = cout.setf(ios_base::scientific, ios_base::floatfield);  cout << "Left Justification:\n";  long n;  for(n = 1; n <= 41; n += 10)  {    cout.width(4);    cout << n << "|";    cout.width(12);    cout << sqrt(double(n)) << "|\n";  }//符号或前缀左对齐，值右对齐  cout.setf(ios_base::internal, ios_base::adjustfield);// 科学计数法  cout.setf(old, ios_base::floatfield);    cout << "Internal Justification:\n";  for ( n = 1; n <41; n +=10)  {    cout.width(4);    cout << n << "|";    cout.width(12);    cout << sqrt(double(n)) << "|\n";  }// 右对齐  cout.setf(ios_base::right, ios_base::adjustfield);// 定点计数 非科学计数  cout.setf(ios_base::fixed, ios_base::floatfield);  cout << "Right Justification:\n";    for ( n = 1; n <41; n +=10)  {    cout.width(4);    cout << n << "|";    cout.width(12);    cout << sqrt(double(n)) << "|\n";  }  return 0;}
    
   

结果

可以使用void unsetf(fmtflags mask)消除setf()的效果 mask位模式 指定哪个位恢复为0

浮点数默认的位模式是 cout.setf(0, ios_base::floatfield) 将floatfield中的两个位都设置成0

或者用cout.unsetf(ios_base::floatfield)

使用头文件 iomanip

3个常用的控制符

setprecision() 设置精度，接受1个整形参数定位精度数

setfill()接受一个char类型字符指定填充符

setw()接受一个指定字段宽度的整数参数

#include 
   
    #include 
    
     #include 
     
      int main(){  using namespace std;  cout << fixed << right;// 设置宽度  cout << setw(6) << "N" << setw(14) << "square root"       << setw(15) << "fourth root\n";  double root;  for (int n = 10; n <= 100; n += 10)  {    root = sqrt(double(n));// . ' ' 填充字符    cout << setw(6) << setfill('.') << n << setfill(' ')// 指定精度         << setw(12) << setprecision(3) << root         << setw(14) << setprecision(4) << sqrt(root)         << endl;  }  return 0;}
     
    
   

完结  卧槽 最后一小节这么多内容的么

#include 
   
    #include 
    
     #include 
     
      #include 
      
       const int LIM = 20;struct planet{  char name[LIM];  double population;  double g;};const char* file = "planets.dat";inline void eatline() {while (std::cin.get() != '\n') continue;}int main(){  using namespace std;  planet pl;  cout << fixed;  fstream finout;// 读写  finout.open(file, ios_base::in | ios_base::out | ios_base::binary);  int ct = 0;  if (finout.is_open())  {// 输出流指针移到文件首    finout.seekg(0);    cout << "Here are the current contents of the "         << file << " file:\n";// 如果有内容就先读取出来    while (finout.read((char*) &pl, sizeof pl))    {      cout << ct++ << ": " << setw(LIM) << pl.name << ": "           << setprecision(0) << setw(12) << pl.population           << setprecision(2) << setw(12) << pl.g << endl;    }// 判断是否到文件尾    if (finout.eof())      finout.clear();    else{      cerr << "Error in reading " << file << ".\n";      exit(EXIT_FAILURE);    }  }  else  {    cerr << file << " could not be opened -- bye.\n";    exit(EXIT_FAILURE);  }  cout << "Enter the record number you wish to change: ";  long rec;  cin >> rec;  eatline();  if (rec < 0 || rec >= ct)  {    cerr << "Invalid record number -- bye\n";    exit(EXIT_FAILURE);  }// 将输出指针移动到指定位置  streampos place = rec * sizeof pl;  finout.seekg(place);  if (finout.fail())  {    cerr << "Error on attempted seek\n";    exit(EXIT_FAILURE);  }// 读取出指定位置的内容  finout.read((char*) &pl, sizeof pl);  cout << "Your selection:\n";  cout << rec << ": " << setw(LIM) <
       
        << ": "       << setprecision(0) << setw(12) << pl.population       << setprecision(2) << setw(6) << pl.g << endl;  if(finout.eof())    finout.clear();  cout << "Enter planet name: ";  cin.get(pl.name, LIM);  eatline();  cout << "Enter planetary population: ";  cin >> pl.population;  cout << "Enter planet's acceleration of gravity: ";  cin >> pl.g;// 将数据写入指定位置  finout.seekp(place);  finout.write((char *) &pl, sizeof pl) << flush;  if(finout.fail())  {    cerr << "Error on attempted write\n";    exit(EXIT_FAILURE);  }  ct = 0;// 输出指针移动到文件首 然后把文件全部读取出来  finout.seekg(0);  cout << "Here are the new contents of the " << file       << " file:\n";  while(finout.read((char *) &pl, sizeof pl))  {    cout << ct++ << ": " << setw(LIM) << pl.name << ": "         << setprecision(0) << setw(12) << pl.population         << setprecision(2) << setw(6) << pl.g << endl;  }  finout.close();  cout << "Done.\n";  return 0;}
       
      
     
    
   

 

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