How to dynamically update members that depend on other members? - c++

Question

Suppose I have a class MyClass:
class MyClass{
private:
vector<vector<float>> _vec;
public:
MyClass(vector<vector<float>> arg){
_vec = arg;
// shape is a 2-element vector where shape[0] = #elems in _vec
// and shape[1] = #elems in _vec[0]
shape.push_back(_vec.size());
shape.push_back(_vec.begin()->size());
}
vector<int> shape;
};
Now suppose I have a member function func of MyClass:
MyClass MyClass::func(){
MyClass res(this->_vec);
// delete the first element of `res._vec`
res._vec.erase(res._vec.begin());
return res;
}
Now, if I do:
MyClass A = {{1, 2}, {3, 4}}; // A.shape[0] = 2, A.shape[1] = 2
MyClass B = A.func(); // B._vec is {{3, 4}} but B.shape[0] is still 2
Here, B._vec changes, but B.shape is not "updated" accordingly.
How can I dynamically update MyClass.shape as and when MyClass._vec changes? One way to do this is to instead make a member function MyClass.shape() which whenever called checks the current MyClass._vec. But is there a better way of doing this?

Answer 1

How can I dynamically update MyClass.shape as and when MyClass._vec changes?
Write a function that updates shape, and call it whenever you change _vec.
But is there a better way of doing this?
Don't use shape member at all, but instead call _vec.size() and _vec.begin()->size() when you need those values. This way there is nothing to update, no redundant duplicate waste of memory, and the user of the class cannot break the apparently missing invariant.