Subject: Re: [boost] review request: addition to type_traits library of has_operator_xxx
From: Edward Diener (eldiener_at_[hidden])
Date: 2010-11-16 19:29:20
On 11/16/2010 6:49 PM, Frédéric Bron wrote:
> I would like to propose to your review the following addition to the
> type_traits library, available at the following addresses:
> https://svn.boost.org/trac/boost/browser/sandbox/type_traits
> http://dl.free.fr/lRm4VL6WP/type_traits.tar.bz2
>
> The purpose of the addition is to add type traits to detect if unary
> and binary operators can be applied to given types.
> For example, is "x<y" or "!x" or "x+y" meaningful?
> If required, the return type of such an expression is checked to know
> if it is convertible to a given type.
> Default behaviour is to not check the return type.
>
> The following traits are added:
>
> // binary operators:
> template< typename LHS, typename RHS=LHS, typename RET=void>
> == has_operator_equal_to
> != has_operator_not_equal_to
> > has_operator_greater
> >= has_operator_greater_equal
> < has_operator_less
> <= has_operator_less_equal
> + has_operator_plus
> - has_operator_minus
> * has_operator_multiplies
> / has_operator_divides
> % has_operator_modulus
> && has_operator_logical_and
> || has_operator_logical_or
> & has_operator_bit_and
> | has_operator_bit_or
> ^ has_operator_bit_xor
>
> // unary operators:
> template< typename RHS, typename RET=void>
> + has_operator_unary_plus
> - has_operator_unary_minus
> ! has_operator_logical_not
>
> This new version reflects the discussions we had on the list:
> http://thread.gmane.org/gmane.comp.lib.boost.devel/194625
> In particular about the check or not of the return type.
> All operators are now included and not only comparison binary operators.
>
> Example:
>
> has_operator_less<LHS, RHS, RET>::value_type is the type bool.
> has_operator_less<int> inherits from true_type.
> has_operator_less<int, int, std::string> inherits from false_type.
> has_operator_unary_minus<int, long> inherits from true_type.
> has_operator_unary_minus<double, int> inherits from true_type.
> has_operator_unary_minus<int, std::string> inherits from false_type.
>
> Documentation is accessible at libs/type_traits/doc/html/index.html in
> the archive.
Bravo !
I would very much like to see this happen also, as I can use this
functionality in another library I would like to put in the sandbox.
Without trying to make more work along your lines, would it be easy
enough for you to add the left shift ( << ) and right shift ( >> )
binary operators, and/or the incrementable ( ++x and x++ ) and
decrementable ( --x and x-- ) unary operators, or are any of these
especially different or much more difficult cases ?