[concept] "not" and "or" concept constraints (N2081)
Hello all, How can I use Boost.ConceptCheck to implement "not" and "or" concept constraints as they are defined in N2081? (If this is obvious from the documentation, please just point me to the link.) For example, how can I program the following code from N2081 using Boost.ConceptCheck: // "not" `!` concept constraint for some concept C (not C++ code). template<Regular T> where !C<T> void f(T x) { ... } // "or" `||` concept constraint (not C++ code). template<Regular T> where Integral<T> || Floating<T> T cos(T x) { ... } If I understand it right, I can program the "and" concept constraints in Boost.ConceptCheck just by specifying multiple concepts as in: // "and" `&&` concept constraint. template<typename T> BOOST_CONCEPT_REQUIRES( ((boost::Integral<T>)) ((boost:: Arithmetic<T>)), T) f(T x) { ... } But how do I use BOOST_CONCEPT_REQUIRES(), etc to program "not" and "or" concept constraints? Thank you very much. -- Lorenzo
At Fri, 28 May 2010 11:31:09 -0400, Lorenzo Caminiti wrote:
Hello all,
How can I use Boost.ConceptCheck to implement "not" and "or" concept constraints as they are defined in N2081?
A. You can't, not with Boost.ConceptCheck. B. Those are a really bad idea anyway. It's been proven mathematically that the use of “or” constraints would lead to a combinatorial explosion at typechecking time.
(If this is obvious from the documentation, please just point me to the link.)
For example, how can I program the following code from N2081 using Boost.ConceptCheck:
// "not" `!` concept constraint for some concept C (not C++ code). template<Regular T> where !C<T> void f(T x) { ... }
// "or" `||` concept constraint (not C++ code). template<Regular T> where Integral<T> || Floating<T> T cos(T x) { ... }
If I understand it right, I can program the "and" concept constraints in Boost.ConceptCheck just by specifying multiple concepts as in:
// "and" `&&` concept constraint. template<typename T> BOOST_CONCEPT_REQUIRES( ((boost::Integral<T>)) ((boost:: Arithmetic<T>)), T) f(T x) { ... }
But how do I use BOOST_CONCEPT_REQUIRES(), etc to program "not" and "or" concept constraints?
Boost.ConceptCheck generates errors aggressively, which means you can't use BOOST_CONCEPT_REQUIRES in an overloading context where one overload's requirements might not be satisfied. For this use case you need to use type traits, MPL, and enable_if. -- Dave Abrahams Meet me at BoostCon: http://www.boostcon.com BoostPro Computing http://www.boostpro.com
On Fri, May 28, 2010 at 12:02 PM, David Abrahams <dave@boostpro.com> wrote:
At Fri, 28 May 2010 11:31:09 -0400, Lorenzo Caminiti wrote:
Hello all,
How can I use Boost.ConceptCheck to implement "not" and "or" concept constraints as they are defined in N2081?
A. You can't, not with Boost.ConceptCheck.
B. Those are a really bad idea anyway. It's been proven mathematically that the use of “or” constraints would lead to a combinatorial explosion at typechecking time.
Wow, I'll make sure to stay away from "not"/"or" concept constraints then.
(If this is obvious from the documentation, please just point me to the link.)
For example, how can I program the following code from N2081 using Boost.ConceptCheck:
// "not" `!` concept constraint for some concept C (not C++ code). template<Regular T> where !C<T> void f(T x) { ... }
// "or" `||` concept constraint (not C++ code). template<Regular T> where Integral<T> || Floating<T> T cos(T x) { ... }
If I understand it right, I can program the "and" concept constraints in Boost.ConceptCheck just by specifying multiple concepts as in:
// "and" `&&` concept constraint. template<typename T> BOOST_CONCEPT_REQUIRES( ((boost::Integral<T>)) ((boost:: Arithmetic<T>)), T) f(T x) { ... }
But how do I use BOOST_CONCEPT_REQUIRES(), etc to program "not" and "or" concept constraints?
Boost.ConceptCheck generates errors aggressively, which means you can't use BOOST_CONCEPT_REQUIRES in an overloading context where one overload's requirements might not be satisfied. For this use case you need to use type traits, MPL, and enable_if.
Yes, I see. Thanks. -- Lorenzo
participants (3)
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David Abrahams -
Lorenzo Caminiti -
Steven Watanabe