Re: [boost] Boost.Exception and constexpr

Subject: Re: [boost] Boost.Exception and constexpr
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2013-01-10 12:06:54

Le 10/01/13 15:46, Andrey Semashev a écrit :
> On Thu, Jan 10, 2013 at 6:22 PM, Sebastian Redl
> <sebastian.redl_at_[hidden]> wrote:
>> On 10.01.2013 14:49, Andrey Semashev wrote:
>>> I must be missing something crucial. My understanding was that constexpr
>>> functions should be evaluated at compile time, am I wrong?
>> Yes.
>> A constexpr function *can* be evaluated at compile time if all its arguments
>> are constant expressions and evaluation using those arguments doesn't hit
>> anything that isn't allowed (like a throw). If that is the case, the result
>> itself may be used as a constant expression. Otherwise, it's a normal
>> function call.
> Ok, thank you for clearing this.
>
>> A constexpr function only *must* be evaluated at compile time if the result
>> has to be a constant expression. In that case, it is a compile time error if
>> it cannot be evaluated.
> So, this means that my argument is still valid to some point. If
> array::at() is used in a constant expression and the check fails, the
> function is no longer valid. Changing my previous code sample to this:
>
> template< typename T, unsigned int n >
> struct array
> {
>      constexpr T at(unsigned int i) const
>      {
>          return i < n ? T() : throw 0;
>      }
> };
>
> template< int m >
> struct check
> {
>      static int get() { return m; }
> };
>
> int main(int, char*[])
> {
>      array< int, 5 > arr;
>      check< arr.at(10) > a;
>      return a::get();
> }
>
> produces the following error:
>
> ./constexpr_test.cpp: In function ‘int main(int, char**)’:
> ./constexpr_test.cpp:19:21:   in constexpr expansion of ‘arr.array<T,
> n>::at [with T = int, unsigned int n = 5u](10u)’
> ./constexpr_test.cpp:6:36: error: expression ‘<throw-expression>’ is
> not a constant-expression
Have you tried declaring arr as a constexpr?

Vicente