Downcast helper functions

When you create a new derived class object from a base class object, the base class object must contain an object of the same derived class. Otherwise, the new derived class object will not be valid. Be careful to use the correct constructor and input type to ensure the constructed object is valid. You can use these helper functions for this. Each function performs type-safe downcasting, accepting a base class object and returning a new derived class object. The new object will be empty if the base class object does not contain the derived class or is empty. Use the isValid function on the new object to check its validity.

Here are the base classes whose derived classes can use the corresponding helper functions.

Every helper function follows the same signature pattern.

DerivedClassName dci = helper_function_name<BaseClassName>(bci);

Where:

• DerivedClassName is the name of the derived class (such as Point).
• dci is the variable to receive the derived class instance.
• helper_function_name is the name of the helper functions (such as geometry_cast).
• BaseClassName is the name of the base class (such as Geometry).
• bci is the variable containing the base class instance.

For example, casting StretchParameters-derived class to StretchParameters is always valid.

MinMaxStretchParameters minMaxStretchParameters(minValues, maxValues);
StretchParameters stretchParameters = stretch_parameters_cast<StretchParameters>(minMaxStretchParameters);
QASSERT(stretchParameters.isValid());

But casting from StretchParameters to StretchParameters-derived class is valid only if stretchParametersType is the same type as target type.

StretchParameters stretchParameters = stretchRenderer->stretchParameters();
MinMaxStretchParameters minMaxStretchParameters = stretch_parameters_cast<MinMaxStretchParameters>(stretchParameters);
QASSERT(minMaxStretchParameters.isValid());

StretchParameters invalidStretchParameters = invalidStretchRenderer->stretchParameters();
MinMaxStretchParameters invalidMinMaxStretchParameters = stretch_parameters_cast<MinMaxStretchParameters>(invalidStretchParameters);
QASSERT(!invalidMinMaxStretchParameters.isValid());

The following classes can be used with the downcast helper functions.