分配给嵌套的QVariantMap [英] Assigning to nested QVariantMap

问题描述

#include <QtCore/QCoreApplication>
#include <QVariant>
#include <QtDebug>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    QVariantMap map;
    map["foo"] = QVariant(QVariantMap());
    map["baz"] = "asdf";
    qvariant_cast<QVariantMap>(map["foo"])["bar"] = "a";

    qDebug() << qvariant_cast<QVariantMap>(map["foo"])["bar"].toString();
    qDebug() << map["baz"].toString();

    return a.exec();
}

我试图分配到嵌套QVariantMap中的QVariant。第一个qDebug（）不输出任何内容，但第二个输出asdf如预期。如何将嵌套变量映射中的bar键赋值为？

I am trying to assign to a QVariant within a nested QVariantMap. The first qDebug() outputs nothing, but the second outputs "asdf" as expected. How would I assign the "bar" key in the nested variable map to a value?

推荐答案

问题是qvariant_cast doesn' t返回对其正在操作的QVariant的内部的引用;它返回一个副本。因此，如果您使用新的子映射覆盖顶级地图中的foo元素，代码将正常工作：

The issue is that qvariant_cast doesn't return a reference to the internals of the QVariant that it is operating on; it returns a copy. As such, if you overwrite the "foo" element in your top-level map with a new child map, the code will work properly:

#include <QtCore/QCoreApplication>
#include <QVariant>
#include <QtDebug>

int main(int argc, char** argv)
{
    QCoreApplication a(argc, argv);
    QVariantMap map;
    map["foo"] = QVariant(QVariantMap());
    map["baz"] = "asdf";

    QVariantMap newMap;
    newMap["bar"] = "a";
    map["foo"] = QVariant(newMap);

    qDebug() << qvariant_cast<QVariantMap>(map["foo"])["bar"].toString();
    qDebug() << map["baz"].toString();

    return a.exec();
}

大概，您想要修改现有地图而不是覆盖它。您可以通过复制现有地图，添加新数据（这将导致深度复制），然后将地图重新​​写入以实现这一点：

Presumably, you want to modify the existing map instead of overwritting it. You can accomplish this by copying the existing map, adding the new data (which will result in a deep copy), and then writing the map back in:

QVariantMap existingMap = qvariant_cast<QVariantMap>(map["foo"]);
existingMap["bar"] = "a";
map["foo"] = QVariant(existingMap);

如果您要考虑存储大量数据，您可以重新考虑使用QVariant 。

If you're considering storing a large amount of data, you may wish to reconsider your use of QVariant.

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