Before this PR, some classes do not have default initial values for their own member variables.
For such a class, each constructor should have its member initializer list, and those list tend to be the same in every constructor.
Sometimes those those classes have some uninitialized values at their constructors, which would potentially and unexpectedly cause undefined behaviours.
To avoid duplicating boilerplates as well as missing initialization, I would like to introduce default initial values for member variables where they are missing.
Before this PR, some classes do not have default initial values for their own member variables.
For such a class, each constructor should have its member initializer list, and those list tend to be the same in every constructor.
Sometimes those those classes have some uninitialized values at their constructors, which would potentially and unexpectedly cause undefined behaviours.
To avoid duplicating boilerplates as well as missing initialization, I would like to introduce default initial values for member variables where they are missing.
Related items: #164.