Re: C++0x static_assert(0, "") within a non-instantiated template
okay?
Greg Herlihy ha scritto:
On Jun 23, 9:54 am, nore...@this.is.invalid (Niels Dekker - no return
address) wrote:
And as Alberto Ganesh Barbati showed us that the expression sizeof(T)
never evaluates to zero (by definition), doing
BOOST_STATIC_ASSERT(sizeof(T) == 0) is ill-formed, just like
BOOST_STATIC_ASSERT(false).
The two static asserts are evaluated in different contexts - the latter,
being non-dependent is evaluated in the context of the template definition;
while the former, being type-dependent, is evaluated in the context of a
specialization.
So I guess the question is: is the Committee is willing to make an
exception for static_assert, and allow its first argument to be false,
within an uninstantiated template?
There is no exception needed. The language of the Standard is unambiguous:
the compiler must defer the resolution of a type-dependent expression until
the template has been specialized [??14.6/8]. But even without that rule, the
compiler has no alternative. Because even if the static assert evaluates to
false for every type T (which is not true, consider T=void which evaluates
to no value at all), the compiler still needs to identify the integer on the
left-hand side of the expression. After all, how could the compiler claim
that it had tested some number against zero for equality and yet not be able
specify exactly what that value (that did not compare equal to zero) was?
It seems that my statement has been a source of confusion rather than
clarification. The reference I cited is about objects, not types. So,
it's not true that sizeof(T) > 0 for *every* type. It's true only for
*complete* types (those that can be instantiated to object). The
compiler cannot "optimize" the expression sizeof(T) == 0 assuming that
it always evaluates to false, because it might be ill-formed for some T
(for example: void). This is enough to force the compiler to treat the
expression as type-dependent.
Ganesh
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