Unresolved external

[gaunt ermine]

I'm getting an unresolved external error, but can't figure out why. I'm implementing a singleton according to this site https://refactoring.guru/design-patterns/singleton/cpp/example. My header contains this

class GameLoop {
private:
    static GameLoop* instance;

public:
    GameLoop(GameLoop& other) = delete;
    void operator=(const GameLoop& other) = delete;

    static GameLoop* GetInstance();

And my cpp contains this

#include "GameLoop.h"

GameLoop* GameLoop::instance = nullptr;

GameLoop* GameLoop::GetInstance() {
    if (instance == nullptr)
        instance = new GameLoop();

    return instance;
}

I can't see what the problem with GetInstance is though.

[grave zealot]

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[cloud snow]

some of these functions aren't implemented yet, so you get a linker error

[gaunt ermine]

I mean it says about defining it outside the class, but I don't understand. It's defined in the header and is implemented in the body.

[cloud snow]

well, firs of all, you can't call the constructor of GameObject, that is an abstract class

oh wait

GameLoop is a normal class

and second

GameLoop& GameLoop::GetInstance()
{
  static GameLoop instance; // a global variable with thread-safe creation visible only in this scope
  return instance;
}

this is a much better way of creating a singleton

[gaunt ermine]

how does that work? To me that looks like you've made a static local variable, which doesn't make sense.

[cloud snow]

this is a feature in C++11, a global variable created in a thread safe way

a.k.a. static local variable

[gaunt ermine]

That's... this hurts my little C# centric brain.

[cloud snow]

correct

C# does not have this

[gaunt ermine]

Ok, well I guess I'll do that then. No idea how that works at all but oh well. Why does what I have not work though?

[cloud snow]

what is the build error

exactly

[gaunt ermine]

error LNK2019: unresolved external symbol "private: __cdecl GameLoop::GameLoop(void)" (??0GameLoop@@AEAA@XZ) referenced in function "public: static class GameLoop * __cdecl GameLoop::GetInstance(void)" (?GetInstance@GameLoop@@SAPEAV1@XZ)```

[cloud snow]

you class GameLoop has not implemented its default constructor

implement it now

[gaunt ermine]

I thought that was implicit.

[cloud snow]

the compiler wont always generate a default constructor for you

[gaunt ermine]

Wha-?! Only sometimes?!

[cloud snow]

there are rules which decide when the compielr wil generate it automatically, and when it won't because if physically can't

just implement it

[gaunt ermine]

It works and that's dumb.

[cloud snow]

the full list of reasons can be seen here
https://en.cppreference.com/w/cpp/language/default_constructor

Deleted implicitly-declared default constructor

here

[gaunt ermine]

So if you delete the copy constructor, it will also delete the main constructor. The more you know.

[cloud snow]

yes

[gaunt ermine]

Wait... oh ye I see it now. I did declare it, just down in private where I wasn't looking at it...

[cloud snow]

because, if you explicity decided to delete the copy constructor, the compiler is like
"hmmmmmm, this guy decided that the copy constructor I made is bad, so maybe they are doing some fancy stuff, so to be safe I better NOT generate the default constructor "

[cloud snow]

[gaunt ermine]

High stress work is making me stupid 😩

Thank you. Even if the problem was just me blinding my own code, this has been informative.

!solved.

!solved