When compiling Qt program, error: undefined reference to `GameModel::~GameModel()’ is reported.
This is because Qt does not automatically generate the class destructor, so we need to write it ourselves, even if it is an empty function. After we write GameModel::~GameModel() by hand, the problem disappears when we compile it again.

There are two ways to write destructors:
Method 1:
in .cpp file:

Method 2:
in .h file.
in Destructor of

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