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Macros

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 requires no operation to be performed in order to be properly destroyed, and that it is possible to use memcpy() in order to create a valid independent copy of an object.
• Q_RELOCATABLE_TYPE specifies that Type has a constructor and/or a destructor, but it can still be relocated in memory by 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 standard C++ type traits; use this macro to tune the behavior. For instance many types would be candidates for Q_RELOCATABLE_TYPE despite not being trivially-copyable.

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