Trouble with LNK 2019 unresolved externals - c++

Question

Okay, as mentioned, I am having two unresolved external errors. There are no intellisense errors and such, just this two linker errors. Here is the code.
include "stdafx.h"
include < iostream >
using namespace std;
class circle; //forward declaration
class square
{
public:
square create_square(circle user_circle);
};
class circle
{
public:
friend square square::create_square(circle user_circle)
};
square square::create_square(circle user_circle)
{
square user_square(user_circle.get_circumference());
return user_square;
}
// function call
user_square = user_square.create_square(user_circle);
This isn't the whole program, but the error seems to be pointing to this batch of code
Error 1 error LNK2019: unresolved external symbol
"public: __thiscall circle::~circle(void)" (??1circle##QAE#XZ) referenced in function
"public: class square __thiscall square::create_square(class circle)"
(?create_square#square##QAE?AV1#Vcircle###Z)
C:\Users\John\documents\visual studio 2010\Projects\PROG5\PROG5\PROG5.obj
Error 2 error LNK2019: unresolved external symbol
"public: __thiscall square::~square(void)" (??1square##QAE#XZ) referenced in function
"public: class square __thiscall square::create_square(class circle)"
(?create_square#square##QAE?AV1#Vcircle###Z)
C:\Users\John\documents\visual studio 2010\Projects\PROG5\PROG5\PROG5.obj
I'm really stumped, and I don't want to just randomly change the code because my logic should be correct. A circle object gets passed to the create_square function, the function gets the diameter of the circle and makes that the perimeter of the square. As for the pointless exercise, this is for a class. Hopefully someone can help, thank you.

Answer 1

You declared a circle destructor, and a square destructor - but you didn't implement them.
Additionally, you should prefer const references over pass by value:
square create_square(circle const & user_circle);
You get the error at this point because you're passing a circle by value - which means a temporary will be created and destructed - and because you return a square by value, which means a square will be constructed and destructed.

Answer 2

square square::create_square(circle user_circle)
{
square user_square(user_circle.get_circumference());
return user_square;
} // Destruction point of user_circle
// function call
user_square = user_square.create_square(user_circle);
The problem is user_circle is passed by value to square::create_square. So, the passed parameter is copied to the receiving argument. Up on the return of the function, object needs to be destructed but since in the class declaration, you have provided just the destructor declaration but not definition which the linker is complaining about.