hashable.h File Reference

Unified hashing concept suite with semantic extension and ADL support. More...

#include <concepts>
#include <functional>
#include <type_traits>
#include <utility>

Go to the source code of this file.

Classes
struct	jh::hash< T, std::enable_if_t< jh::concepts::has_std_hash< T > > >
	Case 1: std::hash<T> is valid. More...
struct	jh::hash< T, std::enable_if_t<!jh::concepts::has_std_hash< T > &&jh::concepts::has_adl_hash< T > > >
	Case 2: ADL-discovered hash(T). More...
struct	jh::hash< T, std::enable_if_t<!jh::concepts::has_std_hash< T > &&!jh::concepts::has_adl_hash< T > &&jh::concepts::has_mbr_hash< T > > >
	Case 3: Member hash(). More...

Namespaces
namespace	jh::concepts
	Behavioral concept namespace of the JH Toolkit.

Concepts
concept	jh::concepts::has_std_hash
	Checks whether std::hash<T> is valid and callable.
concept	jh::concepts::has_adl_hash
	Checks whether a free (ADL-discoverable) hash() function exists.
concept	jh::concepts::has_mbr_hash
	Checks whether a type defines a hash() member function.
concept	jh::concepts::extended_hashable
	Concept for types that can be hashed through any supported mechanism.

Detailed Description

Unified hashing concept suite with semantic extension and ADL support.

Copyright

Copyright 2025 JeongHan-Bae <mastropseudo@gmail.com>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

Full license: GitHub

Author

JeongHan-Bae <mastropseudo@gmail.com>

This header defines the duck-typed hashability model used by the JH framework. It generalizes hashing semantics beyond std::hash<T> by allowing user-defined and semantically meaningful hash strategies.

Why Allow Custom Hashing?

While std::hash<T> provides a universal entry point for associative containers, it cannot describe semantic-specific hash behaviors such as:

• Lazy evaluation — deferred hash computation with caching (e.g. jh::immutable_str).
• Algorithm selection — e.g. jh::pod::string_view uses c_hash::fnv1a64 by default, but may switch algorithms via an enum parameter.
• Semantic integrity — certain domain-specific hashes carry meaning beyond raw bytes (e.g. type-level discriminators, persistent keys, etc.).

Therefore, the JH framework permits three resolution layers:

1. Standard hash — std::hash<T>{} (highest precedence)
2. ADL-discovered free hash() — found via argument-dependent lookup
3. Member function hash() — t.hash() if provided

This priority chain ensures full interoperability with the STL, while supporting domain-specific customization and lazy semantics without breaking standard containers.

Version

1.3.x

Date

2025