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Macros

Q_DECLARE_TYPEINFO(Type, Flags)

Macro Documentation

Q_DECLARE_TYPEINFO(Type, Flags)

You can use this macro to specify information about a custom type Type. With accurate type information, Qt's generic containers can choose appropriate storage methods and algorithms.

Flags can be one of the following:

Q_PRIMITIVE_TYPE specifies that Type can be created by zero-initializing its storage, requires no operation to be properly destroyed, and for which memcpy()ing creates a valid independent copy of the object.
Q_RELOCATABLE_TYPE specifies that Type has a constructor and/or a destructor but can be moved in memory using memcpy().
Q_MOVABLE_TYPE is the same as Q_RELOCATABLE_TYPE. Prefer to use Q_RELOCATABLE_TYPE in new code. Note: despite the name, this has nothing to do with move constructors or C++ move semantics.
Q_COMPLEX_TYPE (the default) specifies that Type has constructors and/or a destructor and that it may not be moved in memory.

Example of a "primitive" type:

 struct Point2D
 {
     int x;
     int y;
 };

 Q_DECLARE_TYPEINFO(Point2D, Q_PRIMITIVE_TYPE);

An example of a non-POD "primitive" type is QUuid: Even though QUuid has constructors (and therefore isn't POD), every bit pattern still represents a valid object, and memcpy() can be used to create a valid independent copy of a QUuid object.

Example of a relocatable type:

 class Point2D
 {
 public:
     Point2D() { data = new int[2]; }
     Point2D(const Point2D &other) { ... }
     ~Point2D() { delete[] data; }

     Point2D &operator=(const Point2D &other) { ... }

     int x() const { return data[0]; }
     int y() const { return data[1]; }

 private:
     int *data;
 };

 Q_DECLARE_TYPEINFO(Point2D, Q_RELOCATABLE_TYPE);

Qt will try to detect the class of a type using std::is_trivial_v<T> to identify primitive types and it will require both std::is_trivially_copyable_v<T> and std::is_trivially_destructible_v<T> to identify relocatable types. Use this macro to tune the behavior. For instance many types would be candidates for Q_RELOCATABLE_TYPE despite not being trivially-copyable.