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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 usingmemcpy()
.Q_MOVABLE_TYPE
is the same asQ_RELOCATABLE_TYPE
. Prefer to useQ_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.
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