Re: [boost] Boost.Exception and constexpr

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 
constraints:
— 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
    — null statements,
    — static_assert-declarations
    — typedef declarations and alias-declarations that do not define classes 
or     enumerations,
    — using-declarations,
    — using-directives,
    — 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.