Question

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A Singleton that is not globally accessible

Do you know a good design pattern that would make sure that only one instance of an object is created without making this object global in c++?
That’s what singleton do but I really need it NOT to be global for code access security reasons.

Thanks for your help!

Answer 1

I think you want something like this (note: copy-pasted from an answer I already wrote and forgot about):

#include <stdexcept>

// inherit from this class (privately) to ensure only
// a single instance of the derived class is created
template <typename D> // CRTP (to give each instantiation its own flag)
class single_instance
{
protected: // protected constructors to ensure this is used as a mixin
    single_instance()
    {
        if (mConstructed)
            throw std::runtime_error("already created");

        mConstructed = true;
    }

    ~single_instance()
    {
        mConstructed = false;
    }

private:
    // private and not defined in order to
    // force the derived class be noncopyable
    single_instance(const single_instance&);
    single_instance& operator=(const single_instance&);

    static bool mConstructed;
};

template <typename T>
bool single_instance<T>::mConstructed = false;

Now you get an exception if the class is constructed more than once:

class my_class : private single_instance<my_class>
{
public:
    // usual interface (nonycopyable)
};

int main()
{
    my_class a; // okay
    my_class b; // exception
}

There’s no way to enforce the single-instance policy at compile-time in C++, though.

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(Also good on you for noticing that singletons are dumb. Globally accessible and singly-creatable are two different concepts, and should only be combined by coincidence, not by design.)