文件流的练习

 

文件流的练习

RReadStream 与 RWriteStream 是用于操作流的基类，对于文件的操作流来说有 RFileReadStream 和 RFileWriteStream

对于流来说可以通过 >> 与 << 符号分别用于读出及写入，如果用 << 或 >> 一定要记得类型的匹配，例如：

TInt aId=10; aStream<<aId; 这样就会报错，报 \Symbian\8.0a\S60_2nd_FP2_SC\EPOC32\INCLUDE\s32strm.inl(290) : error C2228: left of '.ExternalizeL' must have class/struct/union type         \Symbian\8.0a\S60_2nd_FP2_SC\EPOC32\INCLUDE\s32strm.inl(333) : see reference to function template instantiation 'void __cdecl DoExternalizeL(const int &,class RWriteStream &,class Externalize::Member)' being compiled \Symbian\8.0a\S60_2nd_FP2_SC\EPOC32\INCLUDE\s32strm.inl(293) : error C2228: left of '.InternalizeL' must have class/struct/union type         \Symbian\8.0a\S60_2nd_FP2_SC\EPOC32\INCLUDE\s32strm.inl(336) : see reference to function template instantiation 'void __cdecl DoInternalizeL(int &,class RReadStream &,class Internalize::Member)' being compiled \Symbian\8.0a\S60_2nd_FP2_SC\EPOC32\INCLUDE\s32strm.inl(290) : error C2664: 'ExternalizeL' : cannot convert parameter 1 from 'class RWriteStream' to 'class RFileWriteStream &'         A reference that is not to 'const' cannot be bound to a non-lvalue         \Symbian\8.0a\S60_2nd_FP2_SC\EPOC32\INCLUDE\s32strm.inl(333) : see reference to function template instantiation 'void __cdecl DoExternalizeL(const struct TUserGroup &,class RWriteStream &,class Externalize::Member)' being compiled 这种系统错误，导致无法查错，如果改成 TInt32　就不会报错，或把　<< 换成 writeInt16 ，这样也不会出错，编译器就知道该怎么操作了

所以对于这种操作最好是通过 WriteXXX 或 ReadXXX 来操作（XXX 代表一个总的，可以换成 Int32 或 Int16 或 Real16)

1. 文件流要通过 Open 或 Replace 去创建，创建完后要通过 PushL 方法压入栈中，这个 PushL 是流对像提供的，
   用完后要 CommitL 提交更新，然后执行 Pop() 和 Release() 去释放空间
2. 如果要把一个类对像流入或流出，这个类一定要实现 ExternalizeL 及 InternalizeL 方法，这两个方法的定义
   

   void ExternalizeL(RWriteStream& aStream) const; void InternalizeL(RReadStream& aStream);

以下是练习代码

class TUserGroup { public:     TInt16 iGroupId;     TBuf16<20> iGroupName;     void ExternalizeL(RWriteStream& aStream) const     {                         aStream.WriteInt16L(iGroupId);         aStream<<iGroupName;     }     void InternalizeL(RReadStream& aStream)     {         iGroupId = aStream.ReadInt16L();         aStream>>iGroupName;     } }; void useStream() {     TFileName filename;     _LIT(KFilename,"c:\\a.txt");     filename.Copy(KFilename);     RFs rf;     User::LeaveIfError(rf.Connect());     RFileWriteStream stream;     stream.Replace(rf,filename,EFileWrite);     stream.PushL();     TInt16 aId = 10;     _LIT(KGroupName,"groupName");     _LIT(KGroup2,"beijin");     TBuf16<20> gn(KGroupName);     stream.WriteInt16L(aId);     stream<<gn;     TUserGroup ug;     ug.iGroupId = 50;     gn.Zero();     gn.Copy(KGroup2);     ug.iGroupName = gn;     stream<<ug;     // 提交更改并释放资源     stream.CommitL();     stream.Pop();     stream.Release();     // 将数据清空     aId = 0;     gn.Zero();     // 读取数据     RFileReadStream rStream;     rStream.Open(rf,filename,EFileRead);     _LIT(KRead,"read data:\n");      console->Printf(KRead);     aId = rStream.ReadInt16L();     rStream>>gn;     _LIT(Kfmt,"id=%d\ngroupname=%S\n");     console->Printf(Kfmt,aId,&gn);     rStream>>ug;     console->Printf(Kfmt,ug.iGroupId,&ug.iGroupName);     rStream.Pop();     rStream.Release();     rf.Close(); }

 

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安平2009@原创
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