quote:Original post by Rayno
Has anyone seen this new (about a year old) D Language? I was wondering if anyone had any first impressions on it.

quote:
Comparing structs
The C++ Way
While C++ defines struct assignment in a simple, convenient manner:

struct A x, y;
...
x = y;


it does not for struct comparisons. Hence, to compare two struct instances for equality:

#include <string.h>

struct A x, y;

inline bool operator==(const A& x, const A& y)
{
return (memcmp(&x, &y, sizeof(struct A)) == 0);
}
...
if (x == y)
...


Note that the operator overload must be done for every struct needing to be compared, and the implementation of that overloaded operator is free of any language help with type checking. The C++ way has an additional problem in that just inspecting the (x == y) does not give a clue what is actually happening, you have to go and find the particular overloaded operator==() that applies to verify what it really does.

There''s a nasty bug lurking in the memcmp() implementation of operator==(). The layout of a struct, due to alignment, can have ''holes'' in it. C++ does not guarantee those holes are assigned any values, and so two different struct instances can have the same value for each member, but compare different because the holes contain different garbage.

To address this, the operator==() can be implemented to do a memberwise compare. Unfortunately, this is unreliable because (1) if a member is added to the struct definition one may forget to add it to operator==(), and (2) floating point nan values compare unequal even if their bit patterns match.

There just is no robust solution in C++.
The D Way
D does it the obvious, straightforward way:

A x, y;
...
if (x == y)
...