比较多个字符而不使用字符串比较函数

Question

我正在尝试使用可以在 mode 参数中传递到我的 File 类构造函数中的参数来创建一个 switch 语句。但是，由于模式最多可以有 2 个字符（'r'、'r+'、'w' 或 'w+'），我不知道如何创建一个 switch 语句来获取这是为我做的。

有人对如何实现这个有什么建议吗？目前我在下面看到的是 'r+' 和 'w+' 情况下的错误，因为它们不是字符。

File::File(const char *name, const char *mode) {
  int fileMode = 0;    //Initializing mode to 0
  fileName = name;    //Setting the file name of this File object to the name we pass in.

  switch(*mode){
    case 'r':
      fileMode = O_RDONLY | O_CREAT;
      canWeRead = true;
      canWeWrite = false;
      break;

    // Open a text file for update (that is, for both reading and writing)
    case 'r+':            
      fileMode = O_RDWR | O_CREAT;
      canWeRead = true;
      canWeWrite = true;
      break;

    case 'w':
      fileMode = O_WRONLY | O_CREAT;
      canWeRead = false;
      canWeWrite = true;
      break;

    // Open a text file for update (reading and writing),
    // first truncating the file to zero length if it exists
    // or creating the file if it does not exist.
    case 'w+':           
      fileMode = O_RDWR | O_CREAT | O_TRUNC;
      canWeRead = true;
      canWeWrite = true;
      break;
    
    default:    //We should never reach the default case, so I assert(0)
      assert(0);
  }

  fileBuffer = (char*)malloc(bufsiz); //Create buffer with malloc
  //Free is called in the File class destructor.

  assert(fileBuffer != NULL); //If fileBuffer == NULL, we did not allocate a buffer.

  fileDescriptor = open(name, fileMode);
  /*On success, open() returns the new file descriptor (a nonnegative integer).
    On error, -1 is returned and errno is set to indicate the error.*/

  assert(fileDescriptor >= 0);  //If we dont get a positive nonnegative int, we have a problem.

}

File::~File() {
    free(fileBuffer);                      //Free the memory we have for the buffer
    int rc = close(this->fileDescriptor);  //We want to close the FileDescriptor of the current File object.
    assert(rc == 0);                        //close returns 0 on success. So if we dont get 0, we have a problem.
}

Answer 1

您不能使用多个值以防万一。

1 - 您可以使用多个开关：

switch(mode[0]){
    case 'r':
        switch(mode[1]){
            case '+':
                switch(mode[2]){
                    case 0:
                        fileMode = O_RDWR | O_CREAT;
                        canWeRead = true;
                        canWeWrite = true;
                        break;
                    default:
                        assert(0);
                }
            case 0:
                fileMode = O_RDONLY | O_CREAT;
                canWeRead = true;
                canWeWrite = false;
                break;
        }
        break;
    case 'w':
        switch(mode[1]){
            case '+':
                switch(mode[2]){
                    case 0:
                        fileMode = O_RDWR | O_CREAT | O_TRUNC;
                        canWeRead = true;
                        canWeWrite = true;
                        break;
                    default:
                        assert(0);
                }
            case 0:
                fileMode = O_WRONLY | O_CREAT;
                canWeRead = false;
                canWeWrite = true;
                break;
        }
        break;
    default:
        assert(0);
}

2 - 您可以创建 mode 的类型 char[4] 并进行丑陋的函数调用。喜欢File("filename", {'r', '+'})（这种方式会受到字节序差异的影响）

switch(*(uint32_t *)mode){
    // r
    case 0x72000000:
        fileMode = O_RDONLY | O_CREAT;
        canWeRead = true;
        canWeWrite = false;
        break;
    // r+
    case 0x722b0000:
        fileMode = O_RDWR | O_CREAT;
        canWeRead = true;
        canWeWrite = true;
        break;
    // w
    case 0x77000000:
        fileMode = O_WRONLY | O_CREAT;
        canWeRead = false;
        canWeWrite = true;
        break;
    // w+
    case 0x772b0000:
        fileMode = O_RDWR | O_CREAT | O_TRUNC;
        canWeRead = true;
        canWeWrite = true;
        break;
    default:
        assert(0);
}

无论如何，对于简单的事情来说，这些方法实在是太矫枉过正了。系统调用比字符串比较要多吃数千倍的时间。

比较多个字符而不使用字符串比较函数

Compare multiple chars without using string compare functions

c++

unix

system-calls