Re: [boost] [variant] Please vote for behavior (Was: Basic rvalue and C++11 features seupport)

Subject: Re: [boost] [variant] Please vote for behavior (Was: Basic rvalue and C++11 features seupport)
From: Louis Dionne (louis.dionne92_at_[hidden])
Date: 2013-01-21 11:55:37

> II: Set operand.p_ to NULL, add BOOST_ASSERT in get operations
>   + good performance
>   + provides noexcept guarantee for move constructor
>   + optimization will be used even if varinat has no type with trivial
> default constructor
>   + easy to implement
>   - triggers an assert when user tries to reuse moved object
>   - adds an empty state to the recursive_wrapper

My vote goes to II.

    - Using moved-from objects should rarely happen, if ever.
    - It is not even a breaking change since move from variants was not
      supported in the past.

Moreover, I am explicitly against proposition III:

    - The state of the moved-from variant is not explicitly set by the
      programmer. I tend to prefer explicit to implicit.
    - The type held in the moved-from variant depends on the types in the
      variant, which could make it harder to use variants in generic code.
    - It is simpler to have a single, well-documented behavior. It introduces
      less complexity.

Question:
Would it be possible to assign to moved-from variant?
If so, proposition III is equivalent to proposition II with the following,
unless I am mistaken:

    typedef variant<int, recursive_wrapper<foo> > V;
    V v1(std::move(v2)); // using v2 now triggers an assertion
                         // (except when trying to set its content)
    v2 = int(); // this is explicit and equivalent to III

Just my 2 cents.

Louis Dionne