Question

我想将一个struct类的全局实例添加到我的程序中，该服务器具有特殊含义。有效的MWE是

// 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;
}

全局常量empty必须传递给方法inside_grid的每次调用，这让我想到如果我可以在程序开头声明全局并修改inside_grid没有采取任何参数的方法：

 // 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;
}

问题是由于我无法理解的错误，这将无法编译： error: variable 'constexpr const Position empty' has initializer but incomplete type error: invalid use of incomplete type 'struct Position'

可以解决这个问题吗？

Answer 1

您可以转发声明您的inside_grid()方法，并在创建empty后稍后进行定义：

 // 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;
}

使用constexpr构造函数转发struct的结构

1 个答案: