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Subject: Re: [boost] Boost.Exception and constexpr
From: Peter Dimov (lists_at_[hidden])
Date: 2013-01-10 06:11:57

Vicente J. Botet Escriba wrote:
> I don't see a problem with throw. The problem is with throw_exception. We
> can add a basic_throw_exception that behaves as the old throw_exception
> that can be made constexpr

I don't think that this will work. According to 7.1.5,

The definition of a constexpr function shall satisfy the following
— it shall not be virtual (10.3);
— its return type shall be a literal type;
— each of its parameter types shall be a literal type;
— its function-body shall be = delete, = default, or a compound-statement
that contains only
    â€” null statements,
    â€” static_assert-declarations
    â€” typedef declarations and alias-declarations that do not define classes
or enumerations,
    â€” using-declarations,
    â€” using-directives,
    â€” and exactly one return statement;
— every constructor call and implicit conversion used in initializing the
return value (6.6.3, 8.5) shall be one of those allowed in a constant
expression (5.19).

You can't have an "if" in a constexpr function, you can't have it return
void, you can't call a non-constexpr function from a constexpr function.
None of the code you've suggested would compile, as far as I can see. I've
never used constexpr though, so there may be something that I'm missing.

You could throw by using the ternary operator and a throw expression. But
you can't have a throw_exception function. For one, its return type is void,
for another, its argument is not a literal. And I don't think you can do an
assert, either.

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