How to declare a class in C++ - c++

Question

I am new to c++ and am stuck on the syntax of declaring classes.
From what I have gathered you should store all declarations in a header file, I'll call it declarations.h;
#pragma once
void incptr(int* value);
void incref(int& value);
class Player
{
public:
int x, y;
int speed;
void Move(int xa, int ya)
{
x += xa * speed;
y += ya * speed;
}
void printinfo()
{
std::cout << x << y << speed << std::endl;
}
};
Now Player is a class which I want to store in a cpp file called functions.cpp
I want to move the above Player class into the below file functions.cpp
#include "common.h"
void incptr(int* value)
{
(*value)++;
}
void incref(int& value)
{
value++;
}
common.h contains;
#pragma once
#include <iostream>
#include <string>
#include "declarations.h"
What I think is happening is when i write the Player class in the header file, its being declared in that file, well, already there. If i move the Player class into functions.cpp I need to leave a declaration. I'm not sure what the compiler expects as a declaration when it comes to classes.
I have tried;
class Player();
functions::Player();
void Player::Move(int xa, int ya);
Also a few other variations but these make the most sense to me.
Sorry if this is a bit messy, still trying to get a hold on the language. Thanks in advance for you help!
Edit: Sorry I missed the main function;
#include "common.h"
int main()
{
Player player = Player();
player.x = 5;
player.y = 6;
player.speed = 2;
player.Move(5, 5);
player.printinfo();
std::cin.get();
}

Answer 1

A declaration for a class is just as simple as
class Player; // Note there are no parentheses here.
This form is most commonly used when you have circular dependencies between two classes. It is more common to define a class in a header file but put the definitions of member functions in a .cpp file. For your purposes a we can make a header file named player.h:
class Player
{
public:
int x, y;
int speed;
void Move(int xa, int ya);
void printinfo();
};
Note that this declaration does not contain the bodies of the member functions because these are really definitions. You can then put the function definitions in another file. Call it player.cpp:
void Player::Move(int xa, int ya)
{
x += xa * speed;
y += ya * speed;
}
void Player::printinfo()
{
std::cout << x << y << speed << std::endl;
}
Note how we have to now specify that each of these functions is a member of the Player class with the Player:: syntax.
Now assuming you also have a main.cpp file with your main() function, you can compile your code like this:
g++ main.cpp player.cpp
For this simple example, you will be fine defining your functions inside of the class declaration. Note that this makes the functions "inline" which is another topic that you should read about.