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fstream 存取文件以及get()和getline()的区别

2024-07-24 02:48:43   221人阅读

取自C++编程思想的源码
require.h 一些小的内联函数
   1:  #ifndef REQUIRE_H
   2:  #define REQUIRE_H
   3:  #include <cstdio>
   4:  #include <cstdlib>
   5:  #include <fstream>
   6:   
   7:  inline void require(bool requirement, 
   8:    const char* msg = "Requirement failed") {
   9:    using namespace std;
  10:    if (!requirement) {
  11:      fputs(msg, stderr);
  12:      fputs("\n", stderr);
  13:      exit(1);
  14:    }
  15:  }
  16:   
  17:  inline void requireArgs(int argc, int args, 
  18:    const char* msg = "Must use %d arguments") {
  19:    using namespace std;
  20:     if (argc != args + 1) {
  21:       fprintf(stderr, msg, args);
  22:       fputs("\n", stderr);
  23:       exit(1);
  24:     }
  25:  }
  26:   
  27:  inline void requireMinArgs(int argc, int minArgs,
  28:    const char* msg = 
  29:      "Must use at least %d arguments") {
  30:    using namespace std;
  31:    if(argc < minArgs + 1) {
  32:      fprintf(stderr, msg, minArgs);
  33:      fputs("\n", stderr);
  34:      exit(1);
  35:    }
  36:  }
  37:    
  38:  inline void assure(std::ifstream& in, 
  39:    const char* filename = "") {
  40:    using namespace std;
  41:    if(!in) {
  42:      fprintf(stderr,
  43:        "Could not open file %s\n", filename);
  44:      exit(1);
  45:    }
  46:  }
  47:   
  48:  inline void assure(std::ofstream& in, 
  49:    const char* filename = "") {
  50:    using namespace std;
  51:    if(!in) {
  52:      fprintf(stderr,
  53:        "Could not open file %s\n", filename);
  54:      exit(1);
  55:    }
  56:  }
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每次创建ifstream和ofstream都会有assure()函数来确保文件成功打开。

get()或者读取sz-1个字符或者遇到文件为’\n’然后在buf尾部加0终结符。get()会把文件内遇到终结符留在输入流中，所以需要使用get()将终结符扔掉。也可以使用ignore()函数来做这个事情，第一个参数是要扔掉的字符数，默认为1，第二个参数是要扔掉的字符，默认是EOF。

getline()函数自动把输入流中的’\n’取消掉了。所以下次可以直接读取输入流中的数据，一般使用geiline().

get和getline都在读取后在buf的尾部加了一个字符串结尾终结符‘0’。

   1:  #include "../require.h"
   2:  #include <fstream>  
   3:  #include <iostream>
   4:  using namespace std;
   5:   
   6:  int main() {
   7:    const int sz = 100; // Buffer size;
   8:    char buf[sz];
   9:    {
  10:      ifstream in("Strfile.cpp"); // Read
  11:      assure(in, "Strfile.cpp"); // Verify open
  12:      ofstream out("Strfile.out"); // Write
  13:      assure(out, "Strfile.out");
  14:      int i = 1; // Line counter
  15:   
  16:      // A less-convenient approach for line input:
  17:      while(in.get(buf, sz)) { // Leaves \n in input
  18:        in.get(); // Throw away next character (\n)
  19:        cout << buf << endl; // Must add \n
  20:        // File output just like standard I/O:
  21:        out << i++ << ": " << buf << endl;
  22:      }
  23:    } // Destructors close in & out
  24:   
  25:    ifstream in("Strfile.out");
  26:    assure(in, "Strfile.out");
  27:    // More convenient line input:
  28:    while(in.getline(buf, sz)) { // Removes \n
  29:      char* cp = buf;
  30:      while(*cp != ‘:‘)
  31:        cp++;
  32:      cp += 2; // Past ": "
  33:      cout << cp << endl; // Must still add \n
  34:    }
  35:  } ///:~
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fstream 存取文件以及get()和getline()的区别


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