Question

文件以二进制模式打开，第一个变体给出异常，第二个变量不给出异常。如何使用ifstream直接读取我的mhead对象？请帮我。这是我的代码：

class mhead {   

public:

  long length;  
  void readlong(std::ifstream *fp);
}  

void mhead::readlong(std::ifstream *fp)    
{
    //this one is not work
    fp->read((char*)this->length,sizeof(this->length));     

    //this is working
    long other;
    fp->read((char*)other,sizeof(other));
}
}

Answer 1

试试这个：

fp->read(&this->length,sizeof(this->length));

写(char *)this->length表示：

获取一些你刚才编的号码
写入该内存位置
希望最好的

Answer 2

如果读取成功readlong则返回true。

class mhead
{   
  public:
    long length;  

    bool readlong(std::istream &is)    
    {
      is.read(reinterpret_cast<char *>( &this->length ), sizeof(long) );
      return ( is.gcount() == sizeof(long) )
    };
}

或（我建议这个）：

istream & operator >> ( istream &is, mhead &_arg )
{
  long temp = 0;
  is.read(reinterpret_cast<char *>( &temp ), sizeof(long) );
  if ( is.gcount() == sizeof(long) )
    _arg.length = temp;
  return is;
}

如何用ifstream读入对象成员？

2 个答案: