Forward declaration of struct with constexpr constructor - c++

Question

I would like to add global instances of a struct class to my program that server special meaning. A MWE that works is
// Example program
#include <iostream>
constexpr int x_max = 3;
constexpr int y_max = 4;
typedef struct Position {
constexpr Position(int x, int y) : x(x), y(y) {};
int x;
int y;
inline bool operator!=(const Position &rhs) const {
return (x != rhs.x || y != rhs.y);
}
inline bool inside_grid(const Position &empty) const {
return (x >= 0 && x < x_max && y >= 0 && y < y_max && *this != empty);
}
} Position;
constexpr Position empty = Position(1,1);
int main()
{
Position p1 = Position(2,3);
Position p2 = Position(1,1);
std::cout << "p1.inside_grid(empty) = " << p1.inside_grid(empty) << " and p2.inside_grid(empty) = " << p2.inside_grid(empty) << std::endl;
}
The global constant empty would have to be passed to every call of method inside_grid which made me think if I could declare the global at the beginning of the program and modify the inside_grid method to not take any parameters like so:
// Example program
#include <iostream>
struct Position;
constexpr Position empty = Point(1, 1);
constexpr int x_max = 3;
constexpr int y_max = 4;
typedef struct Position {
constexpr Position(int x, int y) : x(x), y(y) {};
int x;
int y;
inline bool operator!=(const Position &rhs) const {
return (x != rhs.x || y != rhs.y);
}
inline bool inside_grid() const {
return (x >= 0 && x < x_max && y >= 0 && y < y_max && *this != empty);
}
} Position;
int main()
{
Position p1 = Position(2,3);
Position p2 = Position(1,1);
std::cout << "p1.inside_grid() = " << p1.inside_grid() << " and p2.inside_grid() = " << p2.inside_grid() << std::endl;
}
The problem is that this won't compile due to errors that I cannot really understand:
error: variable 'constexpr const Position empty' has initializer but incomplete type
error: invalid use of incomplete type 'struct Position'
Can this problem be resolved?

Answer 1

You can forward declare your inside_grid() method and define it later, after empty is created:
// Example program
#include <iostream>
struct Position;
constexpr int x_max = 3;
constexpr int y_max = 4;
typedef struct Position {
constexpr Position(int x, int y) : x(x), y(y) {};
int x;
int y;
inline bool operator!=(const Position &rhs) const {
return (x != rhs.x || y != rhs.y);
}
bool inside_grid() const;
} Position;
constexpr Position empty = Position(1, 1);
inline bool Position::inside_grid() const {
return (x >= 0 && x < x_max && y >= 0 && y < y_max && *this != empty);
}
int main()
{
Position p1 = Position(2,3);
Position p2 = Position(1,1);
std::cout << "p1.inside_grid() = " << p1.inside_grid() << " and p2.inside_grid() = " << p2.inside_grid() << std::endl;
}