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)

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
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();
}

安平2009@原创
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