Why isn't my class working?

When I try to compile this:

Code:

---

class my_class;
class my_class
{
  my_class ply;
  char board[10];
};

---

Dev-C++ returns the error:

Quote:

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Originally Posted by Dev-C++

field `ply' has incomplete type

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Why? And how can I fix it?

Code:

---

class my_class;
class my_class
{
  my_class *ply;
  char board[10];
};

---

? I think it needs to be of pointer type.

If you're interested I get this error on visual studio -

Quote:

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error C2460: 'my_class::ply' : uses 'my_class', which is being defined

---

Thanks, that worked.

But why a pointer to work?

Quote:

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Originally Posted by Cool-August

When I try to compile this:

Code:

---

class my_class;
class my_class
{
  my_class ply;
  char board[10];
};

---

Dev-C++ returns the error:

Why? And how can I fix it?

---

You can't do something like this.

Think about it this way:

sizeof(my_class) = sizeof(ply) + sizeof(board);
sizeof(my_class) = sizeof(my_class) + sizeof(char[10]);

You need to make ply a pointer (or a reference but that would be very difficult to use); essentially my_class objects would have infinite size in your original code because they'd be 10 characters larger than themselves.

Imagine trying to make a semi truck+trailer, whose trailer was large enough to hold an identical semi truck+trailer plus some additional cargo. Impossible, no?

You would get into even more craziness than that, though -- for example, when you construct a my_class object, it would automatically construct another my_class object for ply, which would construct another my_class object, which would keep going on infinitely.

>> But why a pointer to work?

I won't be able to explain (coherently), nearly as good as someone else, so I'll leave it to them.

I see.

BTW: twomers, you posted late.

Quote:

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Originally Posted by Cool-August

BTW: twomers, you posted late.

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You're right, but I got distracted by something shiny

Quote:

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Originally Posted by twomers

You're right, but I got distracted by something shiny

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Your sig?

Quote:

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Originally Posted by Cool-August

Your sig?

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You're just jealous. But yes :rolleyes:

>> You're just jealous. But yes

Why would I be jealous of "Site down for exciting upgrades!"?

I meant the colors! Honestly :rolleyes: Site's been updated to use MySQL, isn't that exciting?

Quote:

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Originally Posted by twomers

I meant the colors! Honestly Site's been updated to use MySQL, isn't that exciting?

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Why don't you say your upgrading to MySQL on the website?

And my sigs got colors ... kind'a ... sort'a ... see?

You can use a pointer because the size of a pointer is always known at compile time. You also have to call new on the pointer to call the constructor. Creating an object within itself would cause the constructor to recurse into itself infinitely ( as Cat pointed out ).