What's wrong with this constructor? - c++

Question

class Ball
{
public:
int i,j,iplus,jplus;
Ball(int i , int j , int iplus , int jplus)
{ iplus=1;
jplus=1;
i=400;
j=300;
}
};
int main()
{
Ball toop(Ball);
}
I get the following error:
request for member ‘jplus’ in ‘toop’, which is of non-class type ‘Ball(Ball)’

Answer 1

Ball toop(Ball);
should be something like:
Ball toop(10,20,30,40);
Accidentally(as most users), you have created is called as the Most Vexing Parse in C++.
It does not create a object as you think it does, instead it declares a function which takes Ball as an input parameter and returns Ball.

Answer 2

Ball toop(Ball); //function declaration
It declares a function toop which takes one argument of type Ball, and returns Ball. It doesn't define an object.
I think you wanted to write something like this:
Ball toop(1,2,3,4); //object definition
Apart from that you should be using member-initialization-list in the constructor as:
Ball(int i , int j , int iplus , int jplus)
: i(i), j(j), iplus(iplus), jplus(jplus)
{ //^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ its called member-initialization-list
}
Here, the syntax i(i) looks weird, but it means the other outer i is initialized with the inner i. The inner i is the constructor parameter-name, and the outer i is the member of the class, as shown below:
: i (i), j(j), iplus(iplus), jplus(jplus)
^ ^
| parameter-name
class-member-name
Similar explanation is for others as well : j(j), iplus(iplus), jplus(jplus)

Answer 3

Ball toop(Ball);
declares a function.
You didn't show the rest of your code, but it says
toop.jplus
somewhere else. That doesn't fly, because toop is a function pointer (non-class type). A function type doesn't support the member access operator (.) hence the error message.
Ball toop(1,2,3,4);
int y = toop.jplus;
would work. Although I'm mighty interested what you are trying to achieve in the constructor :)
I suggest:
class Ball
{
public:
int i,j,iplus,jplus;
Ball() : jplus(1), iplus(1), i(400), j(300)
{
}
};
int main()
{
Ball toop; // **not** toop()!
int test = toop.j; // 300
}

Answer 4

You forgot to post your full testcase. The code you posted is perfectly valid.
What your main presumably really looks like is:
int main() {
Ball toop(Ball);
toop.jplus = 2; // or something
}
Except when you tried to create toop, you actually just declared a function called toop that takes and returns objects of type Ball. I'm not quite sure why you've done that.
To use your constructor and create an object, use code that looks like this:
Ball toop(1,2,3,4);
And please, next time, post an actual testcase that reproduces the problem. You were lucky that:
the error message gave the issue away
I'm very good

Answer 5

Ball toop(Ball);
is wrong because you declared it with 4 parameters, so you should pass the four parameters. Like Your constructor is taking 4 integers as parameter so pass 4 integer values
Like
Ball toop(0,0,0,0);
Or
int x=0;
Ball toop(x,x,x,x);