Question

如何在Qt中使用自定义数据类型

Answer 1

这里我们使用下面这个struct来做说明（这里不管是struct还是class都一样）：
struct Player
{
int number;
QString firstName;
QString lastName;
};
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QVariant
为了能在QVariant中使用自定义数据类型做，需要使用Q_DECLARE_METATYPE()来向Qt的元系统声明这个自定义类型。如下列所示：
struct Player
{
...
};
Q_DECLARE_METATYPE(Player);
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在作为QVariant传递自定义数据类型时，需要使用QVariant::fromValue()或者qVariantFromValue：
Player player;
object->setProperty("property", QVariant::fromValue(player));
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为了更方便一点，你可以在自定义类型中定义一个QVariant() 类型转换符：
struct Player
{
...
operator QVariant() const
{
return QVariant::fromValue(*this);
}
};
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这样我们便可以像下面这样使用了：
Player player;
object->setProperty("property", player);
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信号和槽
对于直接连接类型（默认情况下就是直接连接）而言，使用自定义数据类型做信号参数不需要做其他其他处理，就像内置数据类型一样：
connect(sender, SIGNAL(playerCreated(const Player&)), receiver, SLOT(addPlayer(const Player&)));
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但在跨线程时如果你还这么做，编译器就会给出警告了：
QObject::connect: Cannot queue arguments of type 'Player'
(Make sure 'Player' is registered using qRegisterMetaType().)
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这时我们需要先注册Player：
qRegisterMetaType<Player>("Player");
qRegisterMetaType<Player>( ); （上面那个是错误的，除非名字刚好和类名一样）
connect(sender, SIGNAL(playerCreated(const Player&)), receiver, SLOT(addPlayer(const Player&)));
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QDebug
最好是能这样：
qDebug() << player;
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而不是这样：
qDebug() << "Player(" << player.number << "," << player.firstName << "," << player.lastName << ")";
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怎么做呢？我们需要对QDebug<<操作符重载一下：
inline QDebug operator<<(QDebug debug, const Player& player)
{
debug.nospace() << "Player("
<< player.number << ","
<< player.firstName << ","
<< player.lastName << ")";
return debug.space();
}
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QDataStream
跟上面的QDebug很像，我们也需要重载一下<<操作符：
inline QDataStream& operator<<(QDataStream& out, const Player& player)
{
out << player.number;
out << player.firstName;
out << player.lastName;
return out;
}
inline QDataStream& operator>>(QDataStream& in, Player& player)
{
in >> player.number;
in >> player.firstName;
in >> player.lastName;
return in;
}
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QSettings
QSettings 用QVariant保存键值，用QDataStream序列化自定义数据。（参考后面的variantToString函数）
为了能在QSettings中使用自定义数据类型，需要让Qt的元系统知道有此类型，就像上面介绍QVariant部分一样，另外还要提供相应的QDataStream操作符，还必须注册这个流操作符：
qRegisterMetaTypeStreamOperators<Player>("Player");
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如此处理之后我们就可以像下面这样使用了：
QSettings settings;
Player player;
settings.setValue("key", player);
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QSettings settings;
Player player = value("key").value<Player>();
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参考：
QString QSettingsPrivate::variantToString(const QVariant &v)
{
QString result;
switch (v.type()) {
case QVariant::Invalid:
result = QLatin1String("@Invalid()");
break;
case QVariant::ByteArray: {
QByteArray a = v.toByteArray();
result = QLatin1String("@ByteArray(");
result += QString::fromLatin1(a.constData(), a.size());
result += QLatin1Char(')');
break;
}
case QVariant::String:
case QVariant::LongLong:
case QVariant::ULongLong:
case QVariant::Int:
case QVariant::UInt:
case QVariant::Bool:
case QVariant::Double:
case QVariant::KeySequence: {
result = v.toString();
if (result.startsWith(QLatin1Char('@')))
result.prepend(QLatin1Char('@'));
break;
}
#ifndef QT_NO_GEOM_VARIANT
case QVariant::Rect: {
QRect r = qvariant_cast<QRect>(v);
result += QLatin1String("@Rect(");
result += QString::number(r.x());
result += QLatin1Char(' ');
result += QString::number(r.y());
result += QLatin1Char(' ');
result += QString::number(r.width());
result += QLatin1Char(' ');
result += QString::number(r.height());
result += QLatin1Char(')');
break;
}
case QVariant::Size: {
QSize s = qvariant_cast<QSize>(v);
result += QLatin1String("@Size(");
result += QString::number(s.width());
result += QLatin1Char(' ');
result += QString::number(s.height());
result += QLatin1Char(')');
break;
}
case QVariant::Point: {
QPoint p = qvariant_cast<QPoint>(v);
result += QLatin1String("@Point(");
result += QString::number(p.x());
result += QLatin1Char(' ');
result += QString::number(p.y());
result += QLatin1Char(')');
break;
}
#endif // !QT_NO_GEOM_VARIANT
default: {
#ifndef QT_NO_DATASTREAM
QByteArray a;
{
QDataStream s(&a, QIODevice::WriteOnly);
s.setVersion(QDataStream::Qt_4_0);
s << v;
}
result = QLatin1String("@Variant(");
result += QString::fromLatin1(a.constData(), a.size());
result += QLatin1Char(')');
#else
Q_ASSERT(!"QSettings: Cannot save custom types without QDataStream support");
#endif
break;
}
}
return result;
}

qsetting为了让保存的ini文件能和ascii兼容，所以
我们将ini文件中的键值读入到 QVariant 中，需要两个步骤：
因为文件内的一些字符被转义了，比如 "\x1234\t\0"等，所以需要 unescape
从 unescape 后的字符串构造出 QVariant
当将QVariant写入文件时：
将 QVariant 转换成字符串
处理字符串中的特殊字符，即 escape

If you store types that QVariant can't convert to QString (e.g., QPoint, QRect, and QSize), Qt uses an @-based syntax to encode the type. For example:
pos = @Point(100 100)
To minimize compatibility issues, any @ that doesn't appear at the first position in the value or that isn't followed by a Qt type (Point, Rect, Size, etc.) is treated as a normal character.
不能转化为 QString 的QVariant 会以@-based 编码，base为(Point, Rect, Size, etc.)
Although backslash is a special character in INI files, most Windows applications don't escape backslashes (\) in file paths:
windir = C:\Windows
QSettings always treats backslash as a special character and provides no API for reading or writing such entries.
反斜杠不能用在键值中
The INI file format has severe restrictions on the syntax of a key. Qt works around this by using % as an escape character in keys. In addition, if you save a top-level setting (a key with no slashes in it, e.g., "someKey"), it will appear in the INI file's "General" section. To avoid overwriting other keys, if you save something using the a key such as "General/someKey", the key will be located in the "%General" section, not in the "General" section.
key用%转义。顶层的 key会在General段中，General/someKey会在"%General"段中。

Following the philosophy that we should be liberal in what we accept and conservative in what we generate, QSettings will accept Latin-1 encoded INI files, but generate pure ASCII files, where non-ASCII values are encoded using standard INI escape sequences. To make the INI files more readable (but potentially less compatible), call setIniCodec().
意思是生成的ini是ascii的，non-ASCII全部被转移，latin-1也不例外