Re: [boost] Boost.Exception and constexpr

Subject: Re: [boost] Boost.Exception and constexpr
From: Marshall Clow (mclow.lists_at_[hidden])
Date: 2013-01-10 14:16:08

On Jan 10, 2013, at 9:06 AM, Vicente J. Botet Escriba <vicente.botet_at_[hidden]> wrote:

> 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 function is no longer valid.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?

Here's my test case (using a locally modified Boost::Array):
     constexpr boost::array<int, 10> arr_std {{ 0,1,2,3,4,5,6,7,8,9 }};
     int whatever [ arr.at(4) ];

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists_at_[hidden]>

A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait).
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