Re: Can I get some code executed while pre-compile?
"Igor Tandetnik" <itandetnik@mvps.org> wrote in message
news:uqGUiMaFIHA.3940@TK2MSFTNGP05.phx.gbl...
Giovanni Dicanio <giovanni.dicanio@invalid.it> wrote:
For example, there was a discussion also about the countof()
template, I stop at countof made using the preprocess, i.e.
#define countof( a ) ( sizeof(a) / ( sizeof( (a)[0] ) )
but there is some template metaprogramming "magic" that allows to
write a countof which gives a compiler error if you pass a pointer to
it (instead of array).
Something like this, I guess:
template <typename T, size_t N>
char (& countof_helper(T (&)[N]) )[N];
or
template <typename T, size_t N>
size_t countof(T (&)[N])
{
return N;
}
which needs no macro (but isn't an "integral constant").
#define countof(a) sizeof(countof_helper(a))
countof_helper is a function taking a reference to an array of any type
and any size, and returning a reference to an array of char of the same
size. The function doesn't need to be implemented, just declared. sizeof
only cares about the type of the expression, but doesn't actually evaluate
the expression.
Passing anything other than an array to countof_helper would fail template
parameter deduction, and you would get a compiler error.
--
With best wishes,
Igor Tandetnik
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