Compile time equality test

[Alan Woodland]

Hi,

Can anyone think of a good way of being able to spot the difference
between the two expressions below at compile time:

class foo {
public:
foo operator+(const foo& rhs) const;
};

int main() {
foo a,b,c;

//expr1:
a = b + c;

//expr2:
a = b + b;

//both expr1 and expr2 use the same operator+

return 0;
}

i.e. is there any clever (standard) trick I can use to get the same
behaviour as if I'd overloaded operator+ (or any other binary operator)
on it's two operands being the same object? The compiler must be know,
but I can't work out how to make it tell me. I know there's no way I
could hope to detect aliasing via pointers etc. but this one case would
be really helpful to spot at compile time.

If there were a way to make foo have a template parameter of type int
that gets incremented every time an instance is created (but is supplied
by default somehow) it would be trivial to provide several versions of
operator+, but I can't work out how to do that.

Thanks,
Alan

 

[Jim Langston]

Hi,

Can anyone think of a good way of being able to spot the difference
between the two expressions below at compile time:

class foo {
public:
foo operator+(const foo& rhs) const;
};

int main() {
foo a,b,c;

//expr1:
a = b + c;

//expr2:
a = b + b;

//both expr1 and expr2 use the same operator+

return 0;
}

i.e. is there any clever (standard) trick I can use to get the same
behaviour as if I'd overloaded operator+ (or any other binary operator)
on it's two operands being the same object? The compiler must be know,
but I can't work out how to make it tell me. I know there's no way I
could hope to detect aliasing via pointers etc. but this one case would
be really helpful to spot at compile time.

If there were a way to make foo have a template parameter of type int
that gets incremented every time an instance is created (but is supplied
by default somehow) it would be trivial to provide several versions of
operator+, but I can't work out how to do that.

Well, all I know is that the *this pointer of both would be the same. I.E.
In operator+
if ( this == &rhs )
would evaluate to true if it was the same instance, but that's run time, not
compile time.