使用constexpr构造函数向前声明结构

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;
}