CODEWITHSUNDEEP
c++pointers
user1683333
user1683333

Reputation:

C++ passing "this" instance to property in class

I have this code

void Field::tick() {
    this->snake.tick(this);
}

where this->snake is a class property of the Snake class. The Snake class takes this parameters in the tick() method:

void tick(Field field);

Of course, in the header file of Snake, I imported Field.h.

Now the problem is upon passing an instance of this in the field class to the tick() method in Snake, I get the following error:

c:/Users/x/Documents/NetBeansProjects/snake/Snake.h:12:15: fout: Field has not been declared
c:/Users/x/Documents/NetBeansProjects/snake/Field.cpp: In memberfunction void Field::tick():
c:/Users/x/Documents/NetBeansProjects/snake/Field.cpp:14:27: fout: no matching function for call to Snake::tick(Field&)
c:/Users/x/Documents/NetBeansProjects/snake/Field.cpp:14:27: note: candidate is:
c:/Users/x/Documents/NetBeansProjects/snake/Snake.h:12:10: note: void Snake::tick(int)
c:/Users/x/Documents/NetBeansProjects/snake/Snake.h:12:10: note:   no known conversion for argument 1 from Field to int

Any suggestions?

Upvotes: 0

Views: 172

Answers (1)

barak manos
barak manos

Reputation: 30146

In file Snake.h, declare class Field before the declaration of class Snake:

class Field;

class Snake
{
    ...
    void tick(Field* field);
    ...
};

This should fix the compilation error, because you are using a pointer of class Field in class Snake. So the compiler only needs to know that such class exists, but it doesn't need to know anything about the contents of that class. If you were using an instance, then you would get a compilation error, because the compiler would need to know the size of that instance.

Therefore, the alternative option of declaring class Snake before the declaration of class Field will not compile, because you are using an instance of class Field in class Snake, and the compiler needs to know the size of that instance:

class Snake;

class Field
{
    ...
    Snake snake;
    ...
};

Upvotes: 1

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