Re: [boost] VC8 type traits documentation

From: narechk (narechk_at_[hidden])
Date: 2005-07-13 12:29:09

It appears the support for compiler intrinsic type traits in Beta2 is 
implemented only partially:

typedef int T;
__is_pod(T);
__has_trivial_constructor(T);
__has_trivial_copy(T);
__has_trivial_assign(T);
__has_trivial_destructor(T);
__has_nothrow_constructor(T);
__has_nothrow_copy(T);
__has_nothrow_assign(T);

Ditto for the rest of fundamental types. All of the above return false, 
which AFAIK is incorrect! [§3.9, ¶10; §9, ¶4]

struct A
{
     A() {}
     A(const A&) {}
     const A& operator=(const A&) {}
};

typedef A T;
__is_pod(T); // false
__has_trivial_constructor(T); // false
__has_trivial_copy(T); // false
__has_trivial_assign(T); // false
__has_trivial_destructor(T); // true
__has_nothrow_constructor(T); // false, too conservative
__has_nothrow_copy(T); // false, too conservative
__has_nothrow_assign(T); // false, too conservative

The compiler is too conservative regarding nothrow, it should be able to 
deduce that constructor, copy and assign can never throw.

struct B
{
     B() throw() {}
     B(const B&) throw() {}
     const B& operator=(const B&) throw() {}
};

typedef B T;
__is_pod(T); // false
__has_trivial_constructor(T); // false
__has_trivial_copy(T); // false
__has_trivial_assign(T); // false
__has_trivial_destructor(T); // true
__has_nothrow_constructor(T); // true
__has_nothrow_copy(T); // true
__has_nothrow_assign(T); // true

It seems the compiler needs an explicit nothrow statement in order for 
ctor, copy and assign to be qualified as nothrow. Presumably the result 
of nothrow deduction is not fed to the intrinsics correctly in this version.

Can someone comment on the above.

Thanks,

- NK