C++ generic class error, what's the problem?

问题

Why the following code doesn't compile?

namespace mtm {
    template<class T>
    class Matrix {
    private:
    public:
        class AccessIllegalElement;

    };

    Matrix::AccessIllegalElement{};
}

I'm trying to implement the inner class for handling errors

Error I get:

'Matrix' is not a class, namespace, or enumeration

Plus, if inside AccessIllegalElement I want to write a function that prints the illegal index what is preferable?

1) to define a function that takes one parameter

2) to give every class object a member called index to save that data

回答1:

Matrix is a template, not a class. You need to let the compiler know the template arguments of this template when you declare/define inner items:

template <typename T>
class Matrix<T>::AccessIllegalElement {};

来源：https://stackoverflow.com/questions/62359649/c-generic-class-error-whats-the-problem