Why is accessing a non static method from static method is Bad design

Question

I am accessing non static method from static Method. Below is the code. I read its bad design source. Why this bad design please help me to understand. If so how can one achieve it.

#include<iostream>

class Base 
{
public :
    static Base* initialiser;
    void BaseMethod() 
    {
        std::cout<<"Non static method Invoked"<<std::endl;
    }

    static  Base* GetInstance() 
    {
        if (!initialiser)
            initialiser = new Base();

        return initialiser;
    }
};

Base* Base::initialiser = nullptr;

class Service 
{
public: 
    void ServiceMethod()
    {
        Base::GetInstance()->BaseMethod();
    }
};

int main()
{
     Service obj;
     obj.ServiceMethod();
}

I don't see anything wrong with your code. In any case most most design questions are either matters of opinion or dependent on circumstances. — john, Jul 25 '19 at 06:17
Singletons and global state in general is arguably bad design. Calling a non-static member function from a static one is a minute cosmetic detail, not a design issue at all. — n. m. could be an AI, Jul 25 '19 at 06:20
Your `GetInstance` method should use a function level static variable to store and initialise the singleton, this is simpler, less code and safer — Alan Birtles, Jul 25 '19 at 06:32

score 2 · Accepted Answer · answered Jul 25 '19 at 07:24

Why is accessing a non static method from static method is Bad design

It is not per se, as you are actually using a static member from a static methods

Yet this code snipped is too damn rigid, too damn over-engineered. Which means more likely to be buggy, to be a mess once a new requirement come into play.

defect : This code isn't thread safe. You should instantiate initializer not in GetInstance,rather using Base* Base::initialiser{new Base()};, which is guaranteed to be thread-safe from c++11.
defect : A class such like this should be derived with extreme caution. Or add final to prevent this possibility.
design : This code has still zero line of functionality. You are still plumbing. You want to reconsider if this is the best design for the problem being solved.
design : The purpose is to provide a singleton. We dev like to enforce unnecessary constraints such as uniqueness. When the time comes to support two instances in a system designed for a singleton, we have to refactor a whole lot of things.

score 0 · Answer 2 · edited Jun 20 '20 at 09:12

Since the question was for a general case, here is a quote from Wikipedia:

Static methods are meant to be relevant to all the instances of a class rather than to any specific instance.

A static method can be invoked even if no instances of the class exist yet.

Thus, you should consider static methods to be in class namespace meant for operations on a class rather than on instances/objects of the class.

In your case of making singleton, your are not accessing non-static method from a static one, but you are initializing an instance of that (static) object initialiser within the static method.

Why is accessing a non static method from static method is Bad design

2 Answers2