C ++中的全局变量

Question

I'm writing c++ project, which contains several classes. I created .h file named Position.h, with one array and one function:

class Position
{
public:
    Coord positions[25];

public:
    void setPos(int index, double x, double y)
    {
        positions[index].x = x;
        positions[index].y = y;
    }
};

I want to set values in this array from another classes, so every class in this project will see the same values. I included "Position.h" in other classes, but i can't access the "positions" array.

Anyone can help me plz??

Answer 1

positions is a member variable associated with a class instance, and therefore not a global. You can make it similar to a global by making it static . Doing so, it will become a class-scoped variable, and not bound to an instance.

You will need to define it in a single implementation file.

An even better alternative would be having an std::vector<Coord> .

Answer 2

Just chnage the statement :

Coord positions[25]; 

to

static Coord positions[25]; 

also change void setPos to

static void setPos

while accesing the array ,access it as:

Position::positions[any value]

But before accessing the array,make sure you call the function setPos

Answer 3

As suggested by others, you can make the members static .

You can also create an instance of the Position class as a global variable, and use that:

Position globalPosition;

void function_using_position()
{
    globalPosition.setPos(0, 1, 2);
}

int main()
{
    function_using_position();
}

Or make it a local variable, and pass it around as a reference:

void function_using_position(Position &position)
{
    position.setPos(0, 1, 2);
}

int main()
{
    Position localPosition;

    function_using_position(localPosition);
}