Summary of qvariant use in QT

background

QVariant It can store all kinds of data types , Include Qt All built-in types in 、C++ All basic data types in , And custom types .

Constructors

QVariant variant_bool(false);
QVariant variant_string(QString("hello"));
QVariant variant_double(3.14);
//... Custom types support direct construction 

Key member function

①．type： Get the currently saved data type

QVariant variant;
qDebug() << variant.type();

variant.setValue(3.14);
qDebug() << variant.type();

②．value： Get the value saved to the specified type

QVariant variant(QString("hello"));
qDebug() << variant.value<QString>();

③．canConvert： Determine whether the stored type can be converted to the specified type

QVariant variant(3.14);
qDebug() << variant.type();
qDebug() << variant.canConvert<QString>();

④．convert： Convert the storage to type to the specified type

QVariant variant(QString("hello"));
qDebug() << variant.type();

qDebug() << variant.convert(QVariant::Int);
qDebug() << variant;

⑤．isValid： Determine whether a valid value is stored

QVariant variant;
qDebug() << variant.isValid();

variant = QString("hello");
qDebug() << variant.isValid();

Basic data type reading

①． Save the data

QVariant variant_int(12);

QVariant variant_string = QString("hello");

QVariant variant_bool;
variant_bool.setValue(false);

②． data fetch

QVariant variant_int(12);
QVariant variant_string = QString("hello");
QVariant variant_bool;
variant_bool.setValue(false);

qDebug() << variant_int.toInt();
qDebug() << variant_string.toString();
qDebug() << variant_bool.value<bool>();

Custom data type reading

①． Register as meta type

struct MyStruct
{
    
  int    id;
  QString name;
};
Q_DECLARE_METATYPE(MyStruct)
    
QVariant variant;
{
    
  MyStruct ms;
  ms.id = 1;
  ms.name = "hello";
  variant.setValue<MyStruct>(ms);
}

MyStruct ms = variant.value<MyStruct>();
qDebug() << ms.id << ms.name;

②． Custom type conversion

struct MyStruct
{
    
  int    id;
  QString name;

  MyStruct() = default;
  MyStruct(const QVariant & variant)
  {
    
    *this = variant.value<MyStruct>();
  }

  operator QVariant() const
{
    
    return QVariant::fromValue(*this);
  }
  
};
Q_DECLARE_METATYPE(MyStruct)
       
QVariant variant;
{
    
  MyStruct ms;
  ms.id = 1;
  ms.name = "hello";
    
  variant = ms;
}

MyStruct ms = variant;
qDebug() << ms.id << ms.name;

Pointer type access

MyStruct * ms = new MyStruct;
ms->id = 1;
ms->name = "hello";

QVariant variant = QVariant::fromValue(static_cast<void*>(ms));

MyStruct *  myStruct = static_cast<MyStruct*>(variant.value<void*>());
  
qDebug() << myStruct->id << myStruct->name;

QVariantMap

①． summary

stay QVariantMap The header file has the following definitions ：typedef QMap<QString,QVariant> QVariantMap, namely QVariantMap Is a type alias .

②． Code example

QVariantMap config;
config["id"] = 10;
config["name"] = QString("hello");

qDebug() << config["id"].toInt();
qDebug() << config["name"].toString();