Implementation of a generic singleton template on C++. It allows turning any class to singleton by one line.

Singleton.hpp

#pragma once

/// <summary>
/// The singleton`s implementation to Insert into any other class.
/// </summary>
/// <typeparam name="T">The class which will be turned into Singleton.</typeparam>
/// <param name="Action">Actions to be taken in the default constructor.</typeparam>
#define Singleton(T, Action)                \
    protected:                              \
        T() { (Action)(); }                 \
        T(T const&) = delete;               \
        T& operator=(T const&) = delete;    \
        ~T() = default;                     \
    public:                                 \
        static T* Instance()                \
        {                                   \
            static T instance;              \
            return &instance;               \
        }                                   \
    private:                                \



/*
 * How to use it?
 */

class A
{
    Singleton(A, [this](){ this->a = 5; })

    int a;
    
public:
    int Foo() { return this->a; }
}

auto instance = A::Instance();
auto value = instance->Foo();