Re: [boost] Boost.Exception and constexpr

Subject: Re: [boost] Boost.Exception and constexpr
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2013-01-10 09:46:36

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function is no longer valid.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
./constexpr_test.cpp:19:23: note: in template argument for type ‘int’
./constexpr_test.cpp:19:26: error: invalid type in declaration before ‘;’ token
./constexpr_test.cpp:20:12: error: ‘a’ is not a class, namespace, or enumeration

Not exactly the error that is expected, I'd say.