本文共 3046 字，大约阅读时间需要 10 分钟。
(Owed by: 春夜喜雨 http://blog.csdn.net/chunyexiyu)
几个编程经常碰到的数字：
1024 = 2^10
2048 = 2^11
4096 = 2^12
8192 = 2^13
16384 = 2^14
32768 = 2^15
65536 = 2^16
而QT中写文件的缺省缓存大小就是这些数字中的一个：16384 （16k）
| QFile file(strFile); if (file.open(QIODevice::WriteOnly|QIODevice::Text|QIODevice::Truncate)) { QTextStream stream(&file); stream << << endl; … file.close(); } |
这其中涉及两个qt类：QFile与QTextStream
QFile中的buffer控制在read时指定，缺省使用buffer，不使用时openmode指定Unbuffered
| enum OpenModeFlag { NotOpen = 0x0000, ReadOnly = 0x0001, WriteOnly = 0x0002, ReadWrite = ReadOnly | WriteOnly, Append = 0x0004, Truncate = 0x0008, Text = 0x0010, Unbuffered = 0x0020 }; |
QFile的buffer size宏使用的：QIODEVICE_BUFFERSIZE
定义参见qiodevice_p.h
| #ifndef QIODEVICE_BUFFERSIZE #define QIODEVICE_BUFFERSIZE Q_INT64_C(16384) #endif |
QTextStream的buffer size宏使用的：QTEXTSTREAM_BUFFERSIZE，未提供禁用buffer的处理，buffer总是有效的。
定义参见 qtextstream.cpp
| static const int QTEXTSTREAM_BUFFERSIZE = 16384;
inline void QTextStreamPrivate::write(const QString &data) { if (string) { // ### What about seek()?? string->append(data); } else { writeBuffer += data; if (writeBuffer.size() > QTEXTSTREAM_BUFFERSIZE) flushWriteBuffer(); } } |
QTextStream的文件格式转换缺省使用的是toLocal8Bit，如果未定义QT_NO_TEXTCODEC并且设置了setCodec，则使用转换格式：
| #ifndef QT_NO_TEXTCODEC if (!codec) codec = QTextCodec::codecForLocale(); #if defined (QTEXTSTREAM_DEBUG) qDebug("QTextStreamPrivate::flushWriteBuffer(), using %s codec (%s generating BOM)", codec->name().constData(), writeConverterState.flags & QTextCodec::IgnoreHeader ? "not" : ""); #endif
// convert from unicode to raw data QByteArray data = codec->fromUnicode(writeBuffer.data(), writeBuffer.size(), &writeConverterState); #else QByteArray data = writeBuffer.toLocal8Bit(); #endif |
CodeC在setCodec方法中设置：
| /*! Sets the codec for this stream to \a codec. The codec is used for decoding any data that is read from the assigned device, and for encoding any data that is written. By default, QTextCodec::codecForLocale() is used, and automatic unicode detection is enabled.
If QTextStream operates on a string, this function does nothing.
\warning If you call this function while the text stream is reading from an open sequential socket, the internal buffer may still contain text decoded using the old codec.
\sa codec(), setAutoDetectUnicode(), setLocale() */ void QTextStream::setCodec(QTextCodec *codec) { Q_D(QTextStream); qint64 seekPos = -1; if (!d->readBuffer.isEmpty()) { if (!d->device->isSequential()) { seekPos = pos(); } } d->codec = codec; if (seekPos >=0 && !d->readBuffer.isEmpty()) seek(seekPos); }
/*! Sets the codec for this stream to the QTextCodec for the encoding specified by \a codecName. Common values for \c codecName include "ISO 8859-1", "UTF-8", and "UTF-16". If the encoding isn't recognized, nothing happens.
Example:
\snippet code/src_corelib_io_qtextstream.cpp 10
\sa QTextCodec::codecForName(), setLocale() */ void QTextStream::setCodec(const char *codecName) { QTextCodec *codec = QTextCodec::codecForName(codecName); if (codec) setCodec(codec); } |
(Owed by: 春夜喜雨 http://blog.csdn.net/chunyexiyu)
转载地址：https://chunyexiyu.blog.csdn.net/article/details/78680683 如侵犯您的版权，请留言回复原文章的地址，我们会给您删除此文章，给您带来不便请您谅解！
发表评论
最新留言
关于作者
