silly little question on variable scope

[Bit byte]

What's wrong with this:

//file1.h

Class A {
public:
A(){i=1;}
~A(){;}
void doThis(void){;}
void doThat(void){;}

private:
int i;
};


//file2.h
#include "file1.h"

A a;

Class B {
public:
B(){
//a = new A(); //will not compile
//a.doThis();
}
~B(){;}
};


Anyone know why I can't use a (type A) in class B ?

 

[Axter]

Because you declared A as a concrete type instead of as a pointer.
You need a pointer to use new A()

A *a = new A;

But in general, you should avoid using pointers if you don't have to.
If you don't need to use the new operator to create an instance of an
object, than don't use it, and declare it as a concrete object instead.

 

[Puppet_Sock]

What's wrong with this:

//file1.h

Class A {
public:
A(){i=1;}
~A(){;}
void doThis(void){;}
void doThat(void){;}

private:
int i;
};


//file2.h
#include "file1.h"

A a;

Class B {
public:
B(){
//a = new A(); //will not compile
//a.doThis();
}
~B(){;}
};


Anyone know why I can't use a (type A) in class B ?

You can. Your problems don't arise from that.

First, you've got a variable at file scope in an h file.
You don't usually want that. If you include file1.h in
more than one .cpp file, you get trouble. If you want
an instance of class A at file scope, you need to
declare it in the .cpp file where you want it, not in
a header file.

Next, as somebody pointed out, you've got a call
to new and you are putting the result into a non-pointer.
Calls to new return pointers to allocated memory.
Read up on pointers.

Next, you are doing something pretty whack even
if you do change the variable a to a pointer, and
even if you do have only one instance of a at
file scope. You are assigning a pointer to a "newed"
instance to the variable a. But the variable a is
outside the scope of the class B that is doing it.
So, the second time you do this, you will wind
up with a resource leak.

Probably what you want is something like so. First,
drop the variable a entirely. Then add a pointer as
a member of class B. Then add the appropriate
thing to the dtor of B to avoid the memory leak.

#include "file1.h"

Class B {
public:
B(){
pa = new A(); //will not compile
pa->doThis();
}
~B(){delete pa;}
private:
A * pa;
};

Socks