Thread: How to declare an operator that return another class object?

For example (not tested):

class a {
private:
int x
public:
void set_a(int y) {x=y;}
};

class b {
private:
int var;
public:
a operator-(const b &oper2) {
a result;
result.set_a(var - oper2.var);
return(result);
}

Obviously this won't work since the class b didn't know anything about class
a. And thus it cannot return an object with type a. How to make it possible?
Thanks.

You are right, what you have written is compilable.
However, I have written a similar programme(not quotable since it is too long). But when compiling the following error message occurs:

syntax error before `operator'
parse error at null character
In function `int main()':
no match for `b & - b &'

Dunno why

Oh, sxxt!

I now know why.

If you try the following, you will know what I mean:

code:
--------------------------------------------------------------------------------
class b {
private:
int var;
public:
a operator-(const b &oper2) {
a result;
result.set_a(var - oper2.var);
return(result);
}
};

class a {
private:
int x;
public:
void set_a(int y) {x=y;}
};


int main() {
b b1;
b b2;

a a1 = b2 - b1;
return 0;
}
--------------------------------------------------------------------------------

But another problem arises. What if I want use class a in class b, as well as class b in class a? Which class should I put first?

I'm a bit new to this OO stuff, but;

why not declare a friend operator function after declaring class a & class b and overload it?

such as;
class a{
~
};

class b{
~
public:
a friend operator-(&a,&b);
b friend operator-(&b,&a);
}

a operator-(&a,&b){
//manupulate a
//manipulate b

return a;
}

b operator-(&a,&b){
//manipulate a or b

return b;
}

that way you can return either object dependant on the order of the operands.

If you want to return either a or b dependant upon some decision taken within the operator function, then i'm not sure what you are trying to do, but i think you might be able do that by making a and b derived classes from a common base class, then you could specify the return type to be the base class?