Introduction to the usage of Qt QVariant

1. Overview of QVariant

QVariant is a powerful variant class in Qt. It provides a general way to store the values ​​of Qt objects and other classes. It can store any type of value in a pointer-like manner.

QVariant can be used to store values ​​of known types in the QVariant conversion system, including basic Qt types such as int, QString, QPoint, etc. It can also be used to store custom types. You only need to implement the QDataStream insertion and extraction operators of that type.

Since QVariant is a generic type, almost any data type can be stored without losing semantics. As a variant type, it supports features such as self-description, value type checking and dynamic conversion, and is safe and exploitable.

2. Use of QVariant

1. How to use QVariant

QVariant var = 123;
int intValue = var.toInt(); // 将QVariant 转换为int类型
QString strValue = var.toString(); // 将QVariant 转换为QString类型

2. QVariant storage type judgment

You can use the QVariant::type() function to return the type of value stored by QVariant. Common types can also be judged using the corresponding functions, for example:

QVariant var = "Hello, World";
if(var.type() == QVariant::String){
    qDebug() << "The QVariant stored a string value!";
}

3. Type conversion of QVariant

QVariant also supports type cast operators for explicit conversions. If a unified operation cannot be performed, the conversion fails. In this case, the canConvert() function can be used for conversion:

QVariant var = 999;
if(var.canConvert()){
    QString str = var.value();
}

3. QVariant stores custom types

QVariant can store custom types. You only need to implement the QDataStream insertion and extraction operators of the type, and then call the Q_DECLARE_METATYPE() macro to add metadata to the type:

class MyCustomType {
public:
    MyCustomType() {}
    MyCustomType(int i, QString str) : m_i(i), m_str(str) {}

    int m_i;
    QString m_str;
};

Q_DECLARE_METATYPE(MyCustomType);

QDataStream& operator<<(QDataStream &out, const MyCustomType &val) {
    out << val.m_i << val.m_str;
    return out;
}

QDataStream& operator>>(QDataStream &in, MyCustomType &val) {
    in >> val.m_i >> val.m_str;
    return in;
}

4. QVariant implements template function

QVariant can also use template functions to achieve arbitrary type conversion:

template
inline QVariant toVariant(const T &value){
    return QVariant::fromValue(value);
}

Instructions:

int intValue = 123;
QVariant var1 = toVariant(intValue);
QVariant var2 = toVariant(QString("Hello, World"));

5. Performance analysis of QVariant

There are some performance issues to be aware of when using QVariant. QVariant is different from other basic data types, so do not use it for high-performance applications or too frequent tasks.

The storage of QVariant requires a certain amount of memory, so you should be careful when storing certain types. Theoretically, there is no limit to the size of the values ​​a QVariant can store, but if the value stored is too large, move and copy operations can incur significant overhead.

The use of QVariant itself can also have performance impacts, including type checking, value extraction and conversion, etc. Therefore, in scenarios with high performance requirements, you can use primitive basic data types instead of QVariant, or use QVariant to store small data structures with stable types.

6. QVariant Summary

QVariant is a very practical class in Qt and is widely used in various modules of Qt. It can be used to store any type of data and provides convenient operation methods. However, you need to pay attention to some performance issues during use. Overall, QVariant is a very convenient and flexible class that deserves to be mastered by developers.

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