My getters are giving me a nonstandard and cannot convert error - c++

Question

I'm learning as I'm going but while writing these getters for a stat page for a character, I'm getting an error that it is nonstandard, it cant convert the const, and it tells me to add a & "to create a pointer to a member"
I have tried making them pointers with *, I have tried not making them const, making them public, adding any headers that my be missing.
These are the only lines of many that are giving the errors. It produces about 30 errors.
inline const double& getX() const { return this->getX; }
inline const double& getY() const { return this->getY; }
inline const std::string& getName() const { return this->name; }
inline const int& getLevel() const { return this->level; }
inline const int& GetExpNext() const { return this->expNext; }
inline const int& getHP() const { return this->hp; }
inline const int& getStamina() const { return this->stamina; }
inline const int& getDamageMin() const { return this->getDamageMin; }
inline const int& getDamageMax() const { return this->getDamageMax; }
inline const int& getDefense() const { return this->getDefense; }
These are some of the repeating errors.
Error C3867 'Player::getX': non-standard syntax; use '&' to create a pointer to member
Error C2440 'return': cannot convert from 'const double &(__thiscall Player::* )(void) const' to 'const double &'
Error C3867 'Player::getY': non-standard syntax; use '&' to create a pointer to member
Error C2440 'return': cannot convert from 'const double &(__thiscall Player::* )(void) const' to 'const double &'
Error C3867 'Player::getDamageMin': non-standard syntax; use '&' to create a pointer to member
Error C2440 'return': cannot convert from 'const int &(__thiscall Player::* )(void) const' to 'const int &'
Error C3867 'Player::getDamageMax': non-standard syntax; use '&' to create a pointer to member
Error C2440 'return': cannot convert from 'const int &(__thiscall Player::* )(void) const' to 'const int &'
Error C3867 'Player::getDefense': non-standard syntax; use '&' to create a pointer to member
Error C2440 'return': cannot convert from 'const int &(__thiscall Player::* )(void) const' to 'const int &'
Error C3867 'Player::getX': non-standard syntax; use '&' to create a pointer to member
Error C2440 'return': cannot convert from 'const double &(__thiscall Player::* )(void) const' to 'const double &'
Error C3867 'Player::getY': non-standard syntax; use '&' to create a pointer to member

Answer 1

It's hard to be sure because you've written only posted the lines with the errors instead of posting all the relevant code. but it seems you have written code like this
class Player
{
public:
inline const double& getX() const { return this->getX; }
private:
double x;
};
when you should have written code like this
class Player
{
public:
inline const double& getX() const { return this->x; }
private:
double x;
};
Note x not getX.
And then as has already been pointed out in the comments, inline, this and use of references are all redundant or bad in this case. So you can write the even simpler
class Player
{
public:
double getX() const { return x; }
private:
double x;
};

Answer 2

Just explaining the errors in more detail, as the true answer already is given; you are getting two of these for the same issue:
inline const double& getX() const { return this->getX; }
// ^ ^ (!)
Have you noticed that the two identifiers are the same? At the time you are trying to return getX, the function is already declared and known. And you now are trying to return exactly this function.
Error C3867 'Player::getX': non-standard syntax; use '&' to create a pointer to member
With member functions, you can do two things: Either call them or get their address to form a member function pointer. Unlike free-standing functions and static member functions, non-static member functions are not converted automatically to pointers; for these, you need to explicitly specify the address-of operator &:
void f() { }
class C { public: void f() { } };
void (*pf0)() = &f; // explicitly taking address
void (*pf1)() = f; // works for free standing functions
void (C::*pf0)() = &C::f // ONLY can explicitly take address
//void (C::*pf1)() = C::f // gives you the error you saw already
Well, syntax for function pointers, especially member FP, is really uggly. Usually you are better off defining aliases (typedef or using) or, in above case, you could have used auto.
Error C2440 'return': cannot convert from 'const double &(__thiscall Player::* )(void) const' to 'const double &'
Assuming you already fixed the first error (by adding the required &), then you have a type mismatch between what you declared as return type and what you actually are returning; the former a reference to double, the latter a member function pointer. So you can either adjust the return value (impossible in this case: you'd have to return a pointer to a function returning a pointer to a function returning a pointer to ...) or select the correct member as shown already.
If you wonder about __thiscall: It just is the calling convention, which we usually don't have to specify explicitly (unless we need a non-default one – which usually is the case if e. g. coding against WinAPI).